Anatomy of the Cat

by Jacob Reighard; H. S. Jennings

II. THE PERIPHERAL NERVOUS SYSTEM.

1. Cranial Nerves. Nervi cerebrales.—

I. N. olfactorius.

—The fasciculi of the olfactory nerves arise from the olfactory bulb ([Fig. 144], a) and pass through the foramina of the cribriform plate, upon which the bulb lies, to be distributed to the olfactory mucous membrane of the nose.

II. N. opticus.

—The second nerve or optic ([Fig. 138], II) arises from the optic chiasma ([Fig. 138], c), passes through the optic foramen, and extends in an S-shaped curve to the eyeball. Its course is craniodorsad. It pierces the sclerotic and choroid coats of the eye and spreads out into the retina.

III. N. oculomotorius.

—The third or oculomotor nerve arises ([Fig. 138], III) from the pedunculus cerebri and passes into the orbit through the orbital fissure. It passes between the lateral and superior recti, sends a large branch to the superior rectus, supplies the medial rectus and the retractor oculi, passes laterad of the optic nerve and supplies the inferior rectus, and sends a long branch to the inferior oblique ([Fig. 166], f). At the point where the branch is given off to the inferior oblique muscle is the triangular reddish ciliary (or ophthalmic) ganglion, about two millimeters in diameter. It is directly attached to the inferior oblique branch of the third nerve and receives a delicate sensory filament from the ophthalmic nerve. From the ciliary ganglion proceed two short ciliary nerves which pass along each side of the optic nerve to the eyeball. These nerves penetrate the sclerotic at the sides of the eyeball to be distributed to the ball.

IV. N. trochlearis.

—The fourth nerve, the trochlearis (or patheticus), arises from the lateral border of the velum medullare anterius, as already described ([Fig. 141], n). It passes craniad, emerges through the orbital fissure in company with the oculomotor, abducens, and ophthalmic, passes dorsad of the superior rectus muscle, and reaches ([Fig. 154], l) the caudal border of the belly of the superior oblique muscle near its middle. It supplies only the superior oblique.

V. N. trigeminus.

—The fifth nerve, the trigeminus (or trifacial) arises ([Fig. 138], V) by two roots, a large sensory and a small motor root (4), as already described ([p. 347]). One of these, the motor (4), is smaller and more ventral; the other, the sensory root, is larger and dorsal. The dorsal root soon enlarges to form a large ganglion, the semilunar (or Gasserian) ganglion ([Fig. 138], k), from which three branches diverge. One branch is joined by the ventral root (4), which passes over the ventral surface of the semilunar ganglion; and the nerve thus formed is the mandibular division (1) of the fifth nerve. Of the other two branches from the ganglion, the middle and longest is the maxillary nerve (2), and the smallest is the ophthalmic (3). The mandibular nerve is thus mixed, motor and sensory, while the others are sensory.

1. N. ophthalmicus.—The ophthalmic or first division of the fifth nerve arises from the semilunar (or Gasserian) ganglion. It passes out of the cranial cavity and into the orbit by way of the orbital fissure, in company with the third, fourth, and sixth nerves and with the extension of the carotid (arterial) plexus.

It passes between the superior and medial recti along with the third nerve, crosses dorsad of the optic nerve, and divides into infratrochlear and ethmoidal branches. In the orbital fissure it gives off the frontal nerve, and while crossing the optic it gives off the long ciliary nerve.

a. N. frontalis.—The frontal nerve passes along the lateral border of the superior oblique muscle and then laterad of the pulley to near the middle of the supraorbital crest of the frontal bone. Here it passes out of the orbit and is distributed to the integument of the upper eyelid and the adjacent region at the side of the nose.

b. N. infratrochlearis.—The infratrochlear nerve passes between the superior rectus and the superior oblique in the first part of its course. It then passes ventrad of the superior oblique and ventrad of the pulley to be distributed to the integument of the upper eyelid near the inner angle.

c. N. ethmoidalis.—The ethmoidal nerve passes along with the ethmoidal artery through the ethmoidal foramen (or foramina) in the orbital plate of the frontal bone. It is finally distributed to the mucosa of the nose and to the cartilage and integument of the snout.

d. N. ciliaris longus.—The long ciliary nerve arises from the ophthalmic and passes along the optic nerve to be distributed to the eyeball. It divides into several branches before penetrating the sclerotic.

One or two small communicating branches to the ciliary ganglion are given off at about the same point as the long ciliary nerve. (For a description of this ganglion see the account of the oculomotor nerve, [page 369].)

2. N. maxillaris.—The maxillary nerve, the second division of N. trigeminus, rises from the semilunar (Gasserian) ganglion and leaves the skull by the foramen rotundum. It is the sensory nerve of the palate, upper teeth and upper lip, and of part of the forehead and cheek.

On leaving the foramen the maxillary nerve divides into three branches, the two infraorbital nerves ([Fig. 154], f) and the sphenopalatine ([Fig. 154], g). Two smaller branches are likewise given off either within or just outside of the foramen, the lachrymal nerve ([Fig. 154], j) and the zygomatic (subcutaneus malæ) ([Fig. 154], i).

a. N. lachrymalis ([Fig. 154], j; [Fig. 155], m).—The lachrymal nerve passes along the periorbita to the lachrymal gland ([Fig. 154], 11), to which it gives branches. It then continues caudad of the zygomatic process of the temporal to the integument; here it turns caudad and is distributed to the integument in the region between the eye and the external ear ([Fig. 155], m). It anastomoses with the zygomatic branch of the seventh nerve ([Fig. 155], h).

b. N. zygomaticus (subcutaneus malæ) ([Fig. 154], i).—This arises with the preceding and follows it for some distance. It passes through a foramen in the frontal process of the malar bone and is distributed to the lower eyelid and adjacent integument.

c. Nn. infraorbitales ([Fig. 154], f; [Fig. 155], l).—The infraorbital nerves are two of nearly equal size. They pass through the orbit ventrad of the eyeball to the infraorbital canal. On their course each divides once or twice and each divides again in the infraorbital foramen, so that about eight branches emerge from the infraorbital foramen and diverge to the integument and whiskers of the upper lip and to the side and wing of the nose ([Fig. 155], l). In the infraorbital canal, and before reaching it, branches are given to the molar teeth, and a branch continues in the bone to the canine, incisor, and premolar teeth.

d. N. sphenopalatinus ([Fig. 154], g).—The sphenopalatine nerve turns mediad from the infraorbitals, directing its course toward the sphenopalatine foramen. Before reaching this it gives off the greater palatine nerve (N. palatinus major), which enters the posterior palatine canal and passes to the hard palate. The sphenopalatine then usually divides into two branches which pass along side by side to enter the sphenopalatine ganglion. This is a large elongated triangular ganglion lying on the dorsal surface of the external pterygoid muscle, just laterad of the sphenopalatine foramen.

The following nerves are connected with the sphenopalatine ganglion:

a. N. palatinus minor.—This leaves the craniolateral angle of the ganglion and passes to the soft palate.

b. N. nasalis posterior.—The posterior nasal nerve enters the nasal cavity by the sphenopalatine foramen and is distributed to the mucosa of the ventral and middle parts of the nasal cavity.

c. N. canalis pterygoidii (Vidian Nerve).—This is a large nerve which leaves the caudal angle of the sphenopalatine ganglion and passes caudad. It enters the orbital fissure, lying in a groove on its ventral wall (the cranial end of this groove is sometimes converted into a canal). The groove ends caudally in a foramen which pierces the sphenoid bone between the wing and the body and lies just mediad of the foramen rotundum. The groove and foramen constitute the pterygoid canal (from which the nerve is named). After emerging from the pterygoid canal onto the ventral surface of the basisphenoid the nerve enters the tympanic bulla along with the Eustachian tube, lying on the medial side of the latter. Just after entering, on reaching the internal carotid artery, it divides into two. One of these, N. petrosus superficialis major ([p. 375]), passes into the hiatus facialis of the petrous bone and joins the facial nerve. The other, N. petrosus profundus, accompanies the internal carotid artery caudad, turning therefore out of the bulla and passing caudad along its medial side; it finally joins the superior cervical ganglion of the sympathetic system.

3. N. mandibularis.—The third division of the fifth, the mandibular nerve, takes origin by a strong root from the semilunar (or Gasserian) ganglion, and receives also after separating from the ganglion the smaller ventral root (portio minor) of the fifth nerve. The root from the ganglion is sensory; the smaller root is motor, so that the mandibular nerve is both sensory and motor. The nerve passes through the foramen ovale, and sends off at once the following branches:

a. N. auriculotemporalis ([Figs. 154] and [155], n).—This passes dorsad between the cartilaginous auditory meatus and the zygomatic process of the temporal bone ([Fig. 154], n), emerges at the caudal border of the masseter muscle ([Fig. 155], n), and divides into two chief branches. One, the auricular branch, passes along the cranial side of the external ear and is distributed to its integument. The other, the temporal branch, passes along the zygomatic arch, gives branches to the temporal muscle and to the skin, follows the zygomatic arch almost to the angle of the mouth, and anastomoses with the superior buccal branch of the facial ([Fig. 155], k).

b. N. temporalis profundus.—One or more large branches passing mediad of the zygomatic arch to the temporal muscle.

c. N. massetericus.—The masseteric nerve passes dorsocraniad to the masseter muscle.

d. N. pterygoideus.—One or more pterygoid branches pass to the pterygoid muscles. A small twig from the nerve to the internal pterygoid passes into the tympanic cavity and supplies the tensor tympani muscle.

e. N. buccinatorius ([Fig. 154], e).—This passes craniad along the dorsolateral surface of the pterygoid muscles ([Fig. 154], 8) to the angle of the mouth. Here it divides into branches to the masseter muscle, the mucosa of the mouth, and to the lips.

[Fig. 154].—Dissection showing a Number of the Cranial Nerves.

The mandible and zygomatic arch have been cut and removed; the temporal and internal pterygoid muscles are also removed. a, N. hypoglossus; b, N. lingualis; c, N. chorda tympani; d, N. alveolaris inferior (cut); e, N. buccinatorius; f, Nn. infraorbitales; g, N. sphenopalatinus; h, branch of N. oculomotorius; i, N. zygomaticus; j, N. lachrymalis; k, N. abducens; l, N. trochlearis; m, N. frontalis; n, N. auriculotemporalis. 1, cut end of zygomatic process of temporal; 2, tympanic bulla; 3, M. digastricus; 4, M. constrictor pharyngis medius; 5, M. hyoglossus; 6, M. genioglossus; 7, M. geniohyoideus; 8, M. pterygoideus externus (cut); 9, cut cranial end of zygomatic arch; 10, mandible (cut); 11, lachrymal gland.

After giving off the above branches the mandibular nerve passes three or four millimeters laterad along the root of the zygomatic process of the temporal bone, and divides into two large branches, the inferior alveolar (or dental) nerve (d) and the lingual nerve (b).

f. N. alveolaris inferior ([Fig. 154], d).—This passes toward the mandibular foramen, but before reaching it gives off a branch to the mylohyoid and digastric muscles. It then enters the foramen, passes through the mandibular canal, and gives numerous inferior dental branches to the teeth. At the mental foramen (or foramina) it passes out of the canal and divides into several mental nerves, to the chin, lower lip, and mucosa of the lower jaw.

g. N. lingualis ([Fig. 154], b).—The lingual nerve passes between the internal and external pterygoid muscles, and two or three millimeters from its origin it receives the chorda tympani nerve (c), a communicating branch from the facial (see [page 376]). It passes onto the ventral surface of the external pterygoid, thence onto the side of the tongue, beneath the mylohyoid. On the side of the tongue it divides into branches which enter the tongue and are distributed to its mucous membrane. One branch passes to the mucosa of the pharynx. Just before entering the tongue a branch is given off to the sublingual and submaxillary glands.

VI. N. abducens.

—The sixth nerve, the abducens, arises from the medulla, as already described ([Fig. 138], VI, and [page 347]). It passes into the orbit through the orbital fissure, then extends obliquely cranioventrad along the medial surface of the lateral rectus muscle ([Fig. 154], k). At about the middle of the length of the muscle it divides into two or three branches which enter at once into the lateral rectus and supply it.

VII. N. facialis

([Fig. 155]).—The seventh or facial nerve arises from the trapezium at the caudal margin of the pons, craniad of the origin of the eighth nerve, as described in the account of the brain ([Fig. 138], VII). It passes into the internal auditory meatus, traverses the facial canal through the petrous bone, and emerges at the stylomastoid foramen.

Within the facial canal the nerve bears an enlargement, the ganglion geniculi. It gives off within the canal a branch to the stapedius muscle, and the superficial petrosal and chorda tympani nerves.

The superficial petrosal (N. petrosus superficialis major) nerve passes into the hiatus facialis, through the canal which forms its continuation, and joins the sphenopalatine ganglion ([p. 373]).

[Fig. 155].—Superficial Nerves of the Face.

a, N. facialis; b, branch to digastric; c, N. auricularis posterior; d, branch to inside of ear; e, ventral ramus; f, dorsal ramus; g, temporal nerve; h, zygomatic nerve; i, nerve to stylohyoid muscle; j, inferior buccal nerve; k, superior buccal nerve; l, infraorbital branches of fifth nerve; m, lachrymal branches of fifth nerve; n, auriculotemporal branches of fifth nerve. 1, M. digastricus; 2, M. stylohyoideus; 3, M. masseter.

The chorda tympani is given off two or three millimeters before the emergence of the facial at the stylomastoid foramen. It passes into the tympanic cavity, extends across it between the malleus and incus, passing close against the tensor tympani muscle, and leaves it (by a small cleft, the canal of Huguier) between the bulla and the squamous portion of the temporal. It then passes craniad ([Fig. 154], c), ventrad of the root of the zygomatic process of the temporal, and joins the lingual nerve ([Fig. 154], b) (branch of the third division of the fifth nerve).

On emerging from the stylomastoid foramen the facial nerve ([Fig. 155], a) gives off at once a small branch (b) to the digastric muscle (1), and a larger branch, N. auricularis posterior (c), which passes dorsocaudad about the base of the ear, supplying some of its muscles. Another small branch (d) pierces the cartilaginous external ear and is distributed to its inner surface. The facial nerve then curves craniad about the proximal part of the cartilaginous external ear, and divides five or six millimeters from its emergence into two main branches, which may be designated as the dorsal (f) and ventral (e) rami. A third small branch may also rise from the point of union of the two; it is usually, however, a twig from the dorsal ramus, and passes to M. submentalis.

The dorsal ramus (f) sends two or three small branches to the cheek (including the one just mentioned), and divides into the temporal (g) and the zygomatic (h) branches. The former (g) passes along the cranial margin of the external ear, supplying the superficial muscles, and extends into the temporal region, where it lies deeper than the terminal branches of the lachrymal nerve (m) (from the fifth cranial). The zygomatic branch (h) passes across the malar bone to the caudal angle of the eye, sends branches into both eyelids, anastomoses with twigs from the lachrymal branch (m) of the fifth nerve, and passes along the medial side of the eye to the lateral surface of the nose, where it ramifies.

The ventral ramus (e) gives off a minute branch (i) to the stylohyoid muscle (2), then proceeds toward the angle of the mouth and divides into superior (k) and inferior (j) buccal branches, the former to the muscles of the upper lip and the contiguous regions, the latter to those of the lower lip and chin.

The seventh nerve thus supplies most of the muscles of the head except those of mastication, and of these it supplies the digastric.

VIII. N. acusticus.

—The eighth or auditory nerve takes origin ([Fig. 138], VIII) from the floor of the fourth ventricle, as before described ([page 347]). It passes into the internal auditory meatus and is distributed to the internal ear within the petrous bone.

IX. N. glossopharyngeus

([Fig. 156], a).—The ninth or glossopharyngeal nerve takes origin ([Fig. 138], IX) from the side of the medulla, as [described] in the account of the brain. Close to its origin its trunk bears a small enlargement, the ganglion superius. It passes along with the tenth and eleventh nerves through the jugular foramen. At its exit it has a ganglionic enlargement, the ganglion petrosum; this is connected by fine fibres with the ganglion nodosum ([Fig. 156], d) of the vagus. The glossopharyngeal ([Fig. 156], a) then passes craniad over the surface of the tympanic bulla (15) and mediad of the digastric muscle. It continues mediad of the carotid artery, and as it approaches the cranial cornu of the hyoid divides into two portions, one of which passes to the muscles and mucosa of the pharynx, and the other to the tongue, where it is the special nerve of taste.

X. N. vagus.

—The vagus nerve arises from the side of the medulla in the manner described under the Brain ([page 346] and [Fig. 138], X). It passes through the jugular foramen along with the glossopharyngeal and accessory nerves.

Cervical Portion of the Vagus ([Fig. 156], d, d′).—In the foramen, or just before entering it, it presents a ganglionic enlargement, the ganglion jugulare (or “ganglion of the root”), and a short distance beyond the foramen it forms a second ganglion, the ganglion nodosum (d) (or “ganglion of the trunk”), which lies dorsocaudad of the superior cervical sympathetic ganglion (e). The ganglia of the vagus and sympathetic are closely bound together by connective tissue, and that portion of the vagus craniad of the ganglion nodosum is interconnected by a network of nervous fibres with the sympathetic (e), hypoglossal (b), and accessory (c) nerves. From the ganglion nodosum the vagus (d′) passes caudad, closely bound up with the sympathetic; the two lie at the side of the common carotid artery. Just before entering the thorax the two separate, the vagus being the larger and lying ventrad of the sympathetic. (In rare cases the two are distinctly separated throughout their length.)

[Fig. 156].—Glossopharyngeal, Hypoglossal, Vagus, Sympathetic, and First Cervical Nerves in the Neck.

a, N. glossopharyngeus; b, N. hypoglossus; c, N. accessorius; d, ganglion nodosum of vagus nerve; d′, N. vagus; e, ganglion cervicale superius of sympathetic; f, ramus descendens of N. hypoglossus (united with first cervical); g, branch to M. thyrohyoid from ramus descendens of N. hypoglossus; h, N. laryngeus superior of N. vagus; i, united vagus and sympathetic nerves; j, first cervical nerve; k, second cervical nerve; l, N. pharyngeus from vagus. 1, M. masseter; 2, outline of external ear; 3, M. splenius; 4, M. levator scapulæ ventralis; 5, M. longus capitis; 6, trachea; 7, M. sternothyreoideus; 8, M. sternohyoideus; 9, M. thyreohyoideus; 10, M. constrictor pharyngis inferior; 11, M. constrictor pharyngis medius; 12, cut end of M. stylohyoideus; 13, M. hyoglossus; 14, M. stylopharyngeus; 15, bulla tympani; 16, M. jugulohyoideus; 17, M. styloglossus; 18, M. mylohyoideus.

Branches of the vagus in the cervical region ([Fig. 156]).

a. N. auricularis.—This leaves the ganglion jugulare, passes into the petrous bone to the facial canal, leaves the skull with the facial nerve by the stylomastoid foramen, and is distributed to the external ear.

b. N. pharyngeus (l).—This leaves the vagus craniad of the ganglion nodosum, passes ventrad, sends a small communicating branch to N. laryngeus superior (h), and is distributed to the pharyngeal muscles and cranial part of the œsophagus.

c. N. laryngeus superior (h).—This arises from the ganglion nodosum, passes ventrad, crossing the medial surface of the carotid artery, and reaches the larynx at about the level of the caudal end of M. thyreohyoideus. It passes into the larynx and supplies its mucosa.

Thoracic Portion of the Vagus ([Fig. 157]).—As the vagus (b) and sympathetic (c) separate to enter the thorax, the vagus (b) lies more ventrad. The right vagus now lies along the lateral surface of the trachea, the left one ([Fig. 157]) along that of the œsophagus. At about the level of the first rib or further craniad there arise, apparently from the vagus (but really from the sympathetic), two slender nerves which pass caudad parallel and close to the vagus, lying slightly dorsad of it. These are the cardiac nerves (i); they arise from the sympathetic farther craniad in the neck region, accompany the vagus, and seem to branch from it. In cases where vagus and sympathetic are separate throughout their courses, these nerves arise clearly from the sympathetic. The cardiac nerves pass to the heart and aid in forming a network of nerves known as the cardiac plexus (k).

The vagus usually receives also, at about the level of the first rib, one or two communicating branches from the middle cervical ganglion (d) of the sympathetic. It then passes to the roots of the lungs, crossing the lateral surface of the aortic arch on the left side, and the medial surface of the azygos vein on the right side. As it crosses the aortic arch the left vagus gives off N. laryngeus inferior (j). This curves around the caudal side of the aortic arch to the lateral surface of the trachea (o), then extends on the lateral and ventral surface of the trachea craniad into the neck region. In the neck it passes to the dorsolateral side of the trachea and reaches the larynx. It passes into the larynx between the cricoid and thyroid cartilages, and supplies the muscles of the larynx. It anastomoses with the superior laryngeal.

[Fig. 157].—Sympathetic, Vagus, and Phrenic Nerves in the Neck and Thorax.

1-13, the thirteen ribs (cut); V-VIII, the fifth to eighth cervical nerves (cut); I′, first thoracic nerve (cut). a, combined trunk of vagus and sympathetic; b, vagus; c, sympathetic; d, middle cervical ganglion of sympathetic; e, inferior cervical ganglion; e′, communicating branch to sixth, seventh, and eighth cervical nerves; f, phrenic nerve; g, loop of sympathetic about subclavian artery, between the middle and inferior cervical ganglia; h, cardiac branches from the inferior cervical ganglion; i, cardiac branch passing from sympathetic along with vagus to heart; j, inferior laryngeal nerve; k, pulmonary and cardiac plexus; l, ventral œsophageal branch of vagus; l′, similar branch from the opposite side; m, dorsal œsophageal branch of vagus; n, great splanchnic nerve; o, trachea; p, œsophagus; q, aorta; r, heart; s, root of lung.

The right inferior laryngeal nerve is given off from the right vagus much further craniad than the left; it curves around the right subclavian artery just caudad of the origin of the internal mammary artery, then passes to the trachea, and has a course and distribution like that of the left side.

On reaching the root of the lungs the vagus divides into numerous branches which form the pulmonary plexus (k) over the roots of the lungs, and extends onto the basis of the heart as the cardiac plexus. From these plexuses numerous branches pass to the heart, lungs, pulmonary artery, pericardium, etc. The cardiac branches from the sympathetic, mentioned above, are also connected with the cardiac plexus.

Caudad of the pulmonary plexus the vagus is continued on each side as two trunks, a dorsal (m) and a ventral (l) which pass along the œsophagus (p). The ventral branches of right and left vagi (l and l′) unite a short distance caudad of the root of the lung, and the single trunk extends into the abdominal cavity on the ventral surface of the œsophagus. The dorsal branches of right and left vagi unite farther back, near the diaphragm, and the single trunk thus formed (m) enters the abdominal cavity on the dorsal surface of the œsophagus. Both divisions give fine nerves to the œsophagus.

Abdominal Portion of the Vagus ([Fig. 164], [page 407]).—After passing through the diaphragm the ventral division ([Fig. 164], l) reaches the lesser curvature of the stomach (1), over which it ramifies, some branches being traceable almost to the pylorus. The network of branches thus formed is the anterior gastric plexus. Some twigs from the ventral division pass transversely across the cranial end of the stomach and join the plexus formed by the dorsal division.

The dorsal division (m) of the vagus reaches the greater curvature of the stomach, where it ramifies, forming the posterior gastric plexus. Branches from this anastomose with the anterior gastric plexus and with the cœliac plexus of the sympathetic (e).

XI. N. accessorius.

—The accessory (or spinal accessory) nerve arises ([Fig. 138], XI) by numerous rootlets from the lateral surface of the medulla and of the spinal cord as far caudad as the fifth to seventh cervical nerve. These spinal rootlets join to form a nerve which enters the cranium through the foramen magnum. After receiving the rootlets from the medulla, it leaves the cranial cavity along with the vagus and glossopharyngeal by the jugular foramen. Just outside the foramen ([Fig. 156], c) it becomes involved in a plexus of fine branches which interconnect it with the vagus, sympathetic, and hypoglossal. It then turns laterad and caudad ([Fig. 158], 1) and pierces the cleidomastoid muscle (d), to which it gives small branches. It then divides: one branch enters the sternomastoid muscle; the other passes caudad along the dorsal border of the levator scapulæ ventralis, sends branches to the clavotrapezius, and may be traced to the acromiotrapezius and spinotrapezius, which it supplies.

XII. N. hypoglossus

([Fig. 156], b).—The twelfth or hypoglossal nerve arises from the ventral side of the medulla ([Fig. 138], XII), as previously [described]. It emerges from the skull through the hypoglossal canal. At first it passes ventrad, then gradually turns craniad, following thus a curved course and passing successively laterad of the vagus and sympathetic nerves ([Fig. 156], d and e), the common carotid artery, and the cranial cornu of the hyoid bone. A short distance from its origin the nerve gives off a ramus descendens ([Fig. 156], f), which receives a communicating branch from the first cervical nerve (j), and divides into two branches, one (g) to the thyrohyoid muscle (9), the other to the sternohyoid (8) and sternothyroid muscles (7). The main nerve passes beneath the mylohyoid muscle into the tongue, at first following the lingual artery, then laterad of it, then crossing it again. It sends branches to all the muscles of the tongue, and may be traced to its tip.

The hypoglossal nerve is involved in the plexus just outside of the jugular foramen, with which the vagus, sympathetic, and accessory nerves are also connected.

2. Spinal Nerves.

—The origin and general features of the spinal nerves have been described in connection with the account of the spinal cord ([page 337]). A description of their peripheral distribution will now be given.

A. Cervical Nerves.

—There are eight pairs of cervical nerves. The first leaves the vertebral canal through the atlantal foramen. The second passes out between the arches of the atlas and axis, not through a special intervertebral foramen, so that its ganglion lies among the muscles of the back of the neck. The others emerge through the intervertebral foramina, the eighth one from between the last cervical and first thoracic vertebræ.

[Fig. 158].—Superficial Nerves of the Neck.

M. clavotrapezius has been partly removed. 1, N. accessorius; 2-4, ventral rami of second to fourth cervical nerves; 5, N. auricularis magnus; 6, N. cutaneus colli. a, M. clavotrapezius (cut); b, M. splenius; c, M. levator scapulæ ventralis; d, M. cleidomastoideus; e, M. sternomastoideus; f, parotid gland.

Dorsal Rami (Rami posteriores).—The dorsal ramus of the first nerve (N. suboccipitalis) supplies the short dorsal muscles which move the head and connect the atlas and skull. In the second nerve the dorsal ramus is much larger, forming N. occipitalis major. It sends small branches to the muscles about its origin, then turns craniad on the surface of the obliquus superior muscle, passes through the biventer cervicis and splenius, joins a small branch from the third nerve, and reaches the dorsal surface of the back of the head. It passes craniad, lying beneath the levator auris longus, emerges from between the two divisions of this muscle, and supplies the skin and cutaneous muscles between the two external ears. The dorsal rami of the other cervical nerves supply muscles and integument on the back of the neck.

Ventral Rami (rami anteriores).—These pass ventrad between the transverse processes of the vertebræ, except in the case of the first nerve ([Fig. 156], j). This passes ventrad from the atlantal foramen along the groove for the vertebral artery, through the notch in the wing of the atlas, and across the lateral surface of the longus capitis muscle (5). Here it sends a branch caudad to join the second cervical (k), then crosses the vagus (d′) and sympathetic and the carotid artery, giving off communicating branches to the vagus and sympathetic, and uniting with a branch of the descending ramus of the hypoglossal (f). At the lateral surface of the larynx it turns caudad, following the lateral border of the sternothyroid muscle (7), and is distributed to the sternohyoid (8) and sternothyroid (7).

The ventral ramus of the second nerve ([Fig. 158], 2) receives a branch from that of the first nerve, passes laterad between the levator scapulæ ventralis (c) and the cleidomastoid (d), receives a communicating branch from the third cervical (3), sends a branch to N. accessorius (1) and numerous small nerves into the sternomastoid (e) and cleidomastoid (d), then turns craniad and divides into N. auricularis magnus (5) and N. cutaneus colli (6).

N. auricularis magnus (5), the great auricular nerve, passes dorsocraniad across the lateral surface of the sternomastoid (e) to the lateral and caudal surface of the external ear and parotid gland (f), where it ramifies. The cutaneus colli (6) is the smaller, ventral, division of the second nerve; it may receive also an accession from the third. It passes to the integument over the ventral part of M. masseter and ventrad of that muscle.

The third nerve ([Fig. 158], 3) communicates with the second and supplies the levator scapulæ ventralis (c), cleidomastoid (d), sternomastoid (e), longus capitis, and other muscles of this region and aids in forming the cutaneus colli (6). The fourth (4) and fifth are distributed to the muscles and integument of the sides of the neck. A branch of the fourth supplies the integument in the hollow of the shoulder, and one from the fifth follows the vena cephalica and supplies the integument over the shoulder. The fifth by sending a branch to aid in forming the phrenic nerve ([Fig. 157], f) may be considered to enter partly into the brachial plexus.

Owing to the intercommunicating branches between the ventral roots of the first five cervical nerves, these are sometimes considered as forming a loose plexus which receives the name cervical plexus.

The sixth, seventh, and eighth cervical nerves (with a part of the fifth) become interconnected with each other and with the first thoracic to form the brachial plexus.

The Brachial Plexus

([Fig. 159]).—The brachial plexus is formed by the ventral rami of the fifth, sixth, seventh, and eighth cervical nerves and the first thoracic. Of the fifth cervical only a small part enters into the plexus, forming part of the phrenic nerve. The formation of the plexus is due to the union of the different nerves by means of strong connecting branches or ansæ. The plexus lies in the axilla, along with the axillary artery and vein; all its component nerves pass laterad in front of the first rib. Its branches supply the arm and shoulder.

The precise arrangement of the different strands is somewhat variable. The plexus is commonly made up in approximately the following manner ([Fig. 159]). From the fifth cervical nerve (V) a small branch joins a similar one from the sixth to form the phrenic nerve (a); the remainder of the fifth does not enter into the plexus. From the sixth cervical (VI) arise parts of the phrenic nerve (a), the suprascapular (b), the cranial one of the three subscapular nerves (c), the axillary (d), and the musculocutaneous (f). The sixth also gives off close to its origin a nerve (b′) which passes to the inner surface of the levator scapulæ and ramifies over the surface, supplying this muscle and extending to the rhomboideus, which it also innervates. The seventh cervical (VII) is the largest nerve entering into the plexus; it furnishes parts of one or both of the anterior thoracic nerves (k and n), of the posterior thoracic (m), the three subscapular nerves (c, e, and i), the axillary (d), musculocutaneous (f), radial (h), and median (g) nerves. The eighth cervical (VIII) supplies parts of one of the anterior thoracic nerves (k), the caudal one of the subscapular nerves (i), the radial (h), median (g), and ulnar (j) nerves. The first thoracic (1) curves craniad, so as to leave the thorax on the cranial side of the first rib; it supplies the median cutaneous nerve (l) and parts of the radial (h), ulnar (j), and median (g) nerves; sometimes also a part of one (k) of the anterior thoracic nerves.

Fig. 159.—Diagram of the Right Brachial Plexus.

V, VI, VII, VIII, the fifth to eighth cervical nerves. 1, the first thoracic nerve. a, phrenic nerve; b, suprascapular; b′, nerve to serratus anterior and levator scapulæ muscles; c, first or cranial subscapular nerve; d, axillary nerve; e, second subscapular; f, musculocutaneous; g, median; h, radial; i, third subscapular; j, ulnar; k, second anterior thoracic; l, medial cutaneous; m, posterior thoracic; n, first anterior thoracic.

The following are the nerves which arise from the brachial plexus:

A. Nerves of the Shoulder and Breast ([Fig. 160]).

1. Nn. thoracici anteriores.—There are two anterior thoracic nerves. One ([Fig. 160], h; [Fig. 159], n) arises from the seventh cervical and passes to the pectoral muscles (9) along with the anterior thoracic artery. The other ([Fig. 159], k; [Fig. 160], q and r) arises usually by two roots, one from the eighth cervical, one from the first thoracic; it passes along with the long thoracic artery to be distributed to the pectoralis muscles ([Fig. 160], 8) and sometimes also to the latissimus dorsi (7).

2. N. thoracicus posterior.—The posterior thoracic (or long thoracic) nerve (external respiratory) ([Fig. 159], m) arises from the seventh cervical nerve near its beginning. It passes within the scalenus muscle and extends caudad on the outer surface of the serratus anterior muscle, which it supplies.

3. N. suprascapularis ([Figs. 159] and [160], b).—The suprascapular nerve arises from the sixth (and sometimes the seventh) cervical. It passes laterad and gives off a branch which passes over the shoulder-joint and penetrates the clavodeltoid muscle to be distributed to the integument on the ventral surface of the upper arm. It then follows the transversa scapulæ artery into the supraspinatus fossa and accompanies the artery to be distributed to the supraspinatus and infraspinatus muscles.

4. Nn. subscapulares.—The subscapular nerves are three. The cranial one ([Figs. 159] and [160], c) arises from the sixth and seventh cervical and supplies the subscapular muscle. The middle one ([Fig. 159], e) arises from the seventh; it supplies principally the teres major. The caudal one ([Fig. 159], i) arises from the seventh and eighth cervical and supplies the latissimus dorsi. (The points of origin from the plexus vary.)

5. N. axillaris (or circumflexus) ([Fig. 159], d).—The axillary nerve arises from the sixth and seventh cervical nerves. It passes toward the shoulder-joint, then follows the posterior circumflex artery ventrad of the long head of the triceps to be distributed to the spinodeltoid and acromiodeltoid. A branch of it continues to the clavobrachial, which it supplies.

B. The Phrenic Nerve ([Fig. 157], f, [page 381]).

6. N. phrenicus.—The phrenic nerve (internal respiratory) is formed by the junction of two slender branches, one from the fifth and one from the sixth cervical nerves ([Fig. 159], a). (It is said to receive sometimes a branch from the fourth.) It passes caudad into the thorax on the ventral surface of the subclavian artery and then extends to the diaphragm lying at the side of the inferior and superior venæ cavæ. It is the motor nerve of the diaphragm.

[Fig. 160].—Blood-vessels and the more Ventral Nerves of the Axilla, Ventral View.

The pectoral and clavobrachial muscles have been cut and laid aside, their ends being shown; only a part of the nerves are exhibited. 1, M. clavobrachialis; 2, cut end of M. pectoralis major; 3, cut end of M. pectoralis minor; 4, M. biceps; 5, M. teres major; 6, M. epitrochlearis, partly cut and turned back; 7, M. latissimus dorsi, partly cut; 8, M. pectoralis minor; 9, M. pectoralis major; 10, short portion of caput mediale of M. triceps brachii. a, branches of the thyrocervical axis to clavobrachial and clavotrapezius muscles; b, suprascapular nerve; c, first subscapular nerve; d, musculocutaneous nerve; e, median nerve; f, V. axillaris; g, A. axillaris; h, first anterior thoracic nerve, accompanied by the anterior thoracic artery and vein; i, V. subscapularis; j, large muscular branch (to subscapular muscle) of the subscapular vein and brachial artery; l, A. subscapularis; l′, A. circumflexa humeri anterior; m, radial nerve; n, ulnar nerve; o, medial cutaneous nerve; p, A. thoracica longa; q, r, second anterior thoracic nerve; s, V. thoracica longa; t, A. and V. thoracicodorsalis; u, A. profunda brachii; v, branch of brachial artery accompanying medial cutaneous nerve; w, branch of A. collateralis radialis superior; x, A. collateralis radialis superior; y, V. mediana cubiti; z, A. collateralis ulnaris.

C. Nerves of the Arm.

7. N. musculocutaneus ([Fig. 160], d).—The musculocutaneous nerve or external cutaneous arises from the ventral surface of the sixth and seventh nerves ([Fig. 159], f). It passes toward the shoulder-joint and supplies the biceps ([Fig. 160], 4) and coracobrachialis muscles. It then passes distad, resting on the dorsal border of the biceps (4), and supplies the brachialis muscle. Thence it passes dorsad of the biceps to the lateral side, and reaches the integument by passing between the clavobrachialis and pectoantibrachialis muscles near the elbow; it is then distributed to the skin on the radial side of the ventral surface of the forearm, as far as the wrist ([Fig. 130], h, [page 319]).

8. N. cutaneus medialis ([Fig. 160], o).—The medial (or internal) cutaneous nerve rises from the first thoracic. It passes distad along the medial side of the biceps, and at the junction of the second and third thirds of the upper arm reaches the integument by passing between the epitrochlearis and pectoantibrachialis muscles. It then curves spirally about the dorsal border of the forearm and is distributed to the integument of the forearm on the ulnar side, extending nearly to the wrist.

9. N. medianus ([Fig. 160], e).—The median nerve is formed by the junction of three branches, one each from the seventh and eighth cervical with the brachial artery (g) passing between them, and one from the first thoracic nerve. It accompanies the brachial artery (g) lying on its medial side, and passes with it through the supracondyloid foramen; here it is connected by a branch to the musculocutaneous nerve. It passes into the forearm ([Fig. 161], b), lying at first beneath the pronator teres (5). Here it gives branches to the pronator teres and flexor muscles (flexor carpi radialis, palmaris longus, five heads of the flexor profundus digitorum, but not to the flexor carpi ulnaris). Continuing along the forearm, lying on the flexor carpi radialis, it sends a posterior interosseous nerve to the pronator quadratus muscle and branches to the deep and superficial divisions of the flexor sublimis digitorum. It thus supplies the flexor and pronator muscles (except the flexor carpi ulnaris). It then passes into the palm beneath the transverse ligament and gives rise to three principal branches. The first of these supplies the integument of the thumb, sending a branch on either side of it; and onto the radial side of the second digit. The second passes along the contiguous sides of the second and third digits and supplies their integument. The third passes along the contiguous sides of the third and fourth digits to their integument. Each sends twigs into the trilobed pad in the palm, so that there are thus seven terminal branches in the palm. From these, branches (probably) pass to the three radial Mm. lumbricales.

[Fig. 161].—Nerves and Arteries of the Forearm, seen from the Flexor Side.

Mm. palmaris longus, flexor carpi radialis, flexor carpi ulnaris, and the third and fourth heads of M. flexor profundus digitorum have been removed; also part of the pronator teres. 1, M. biceps; 2, conjoined tendon of M. brachialis and M. clavobrachialis; 3, short portion of caput mediale of M. triceps; 4, intermediate portion of caput mediale of M. triceps brachii; 5, cut ends of M. pronator teres; 6, M. brachioradialis; 7, M. extensor carpi radialis longus and brevis; 8, fifth head of M. flexor profundus digitorum; 9, second head of M. flexor profundus digitorum; 10, radial part of M. flexor sublimis digitorum; 11, common tendon of M. flexor profundus digitorum; 12, first head of M. flexor profundus digitorum; 13, cut origin of M. flexor carpi ulnaris. a, ulnar nerve; b, median nerve; c, dorsal cutaneous branch of the ulnar nerve; d, palmar branch of ulnar nerve; e, deep palmar branch; f, superficial palmar branch; g′, A. brachialis; g, A. radialis; h, A. collateralis ulnaris superior; i, A. radialis recurrens; j, A. ulnaris recurrens; k, A. ulnaris; l, A. interossea anterior; m, large branch of A. radialis, joining A. ulnaris.

10. N. radialis.—The radial (or musculospiral) nerve is formed by the junction of branches from the seventh and eighth cervical and first thoracic nerves. It immediately sends branches to the epitrochlearis, long head of the triceps, and long and intermediate portions of the medial head of the triceps. It then passes between the long and intermediate portions of the medial head of the triceps, following the profunda brachii artery, and curves about the humerus to its cranial side. It supplies there the lateral head of the triceps and the anconeus, and, lying on the brachialis muscle, divides into a superficial and a deep branch. The superficial branch is the superficial radial nerve ([Fig. 130], g); the deep one forms the dorsal (or posterior) interosseous nerve. The former is sensory and the latter motor.

The superficial radial nerve ([Fig. 130], g, [page 319]) becomes cutaneous at the junction of the second and third thirds of the upper arm, where it emerges from between the lateral head of the triceps and the brachialis muscle. It then follows the course of the vena cephalica (c) to the wrist and follows the dorsal tributary of the same vein onto the dorsum of the hand. It supplies the integument of the distal part of the ventral surface of the upper arm and that along the forearm. On the hand it is distributed to the dorsal surface in the same manner as the median nerve on the ventral surface, i.e. to the integument of the thumb on both sides and to that of the medial side of the second digit; to that of the contiguous sides of the second and third digits and of the contiguous sides of the third and fourth digits. There are thus seven terminal branches, one for each of these regions.

The dorsal interosseous nerve follows the brachialis muscle and passes onto the forearm between it and the extensor carpi radialis longus. It supplies the supinator and extensor muscles of the forearm (brachioradialis, supinator, extensor carpi radialis longus, extensor carpi radialis brevis, extensor communis digitorum, extensor digitorum lateralis, extensor indicis, extensor brevis pollicis). The dorsal interosseous and radial nerves thus supply all the extensor muscles of the forearm and upper arm and the supinators (modified extensors).

11. N. ulnaris ([Fig. 160], n).—The ulnar nerve arises from the eighth cervical and first thoracic and accompanies the brachial artery (g) and median nerve (e) through the upper arm. It does not pass through the supracondyloid foramen, but curves over the dorsal end of the medial epicondyle of the humerus within the short division of the medial head of the triceps, and reaches the dorsal border of the forearm. In the forearm ([Fig. 161], a) it lies beneath the flexor carpi ulnaris. It supplies muscular branches to the flexor carpi ulnaris and the first or ulnar head of the flexor profundus digitorum. Near the middle of the forearm it divides into two branches, dorsal cutaneous (c) and palmar (d).

The dorsal cutaneous branch (c) curves about the ulnar side of the wrist to the dorsum of the hand and divides into twigs which supply the dorsal part of the ulnar side of the fifth digit and of the contiguous sides of the fourth and fifth. The palmar branch (d) sends a twig to the integument on the flexor surface of the arm near the wrist, and then divides into deep palmar (e) and superficial palmar (f) branches.

The superficial palmar branch (f) passes into the palm and supplies the ventral portion of the integument on the ulnar side of the fifth digit, and on the contiguous sides of the fifth and fourth.

The deep palmar branch (e) passes into the palm, beneath the ligament of the pisiform bone. Just distad of the pisiform bone it curves toward the radial side and breaks up into a number of small branches which are distributed to the short muscles in the palm.

B. Thoracic Nerves.

Dorsal Rami.—The dorsal rami of the thoracic nerves are small and supply the muscles and integument of the back. Each gives off a branch directly dorsad to the spinal muscles, and a lateral branch which reaches the integument some distance from the middle line.

Ventral Rami.—The ventral ramus of the first thoracic nerve enters into the brachial plexus, as already [described]. The ventral rami of the other thoracic nerves form the intercostal nerves. Each of these passes ventrolaterad, lying close to the caudal border of a rib, in company with the intercostal artery. Branches are given to the intercostal muscles, and at about the middle of the length of the rib a large lateral branch is given off, which passes to the more superficial muscles of the thoracic wall (serrati posteriores, obliquus externus, etc.), its main branches running dorsad and ventrad. The main intercostal nerve extends ventrad to the transversus costarum and rectus abdominis, supplying these muscles.

C. Lumbar Nerves.

—There are seven lumbar nerves, one passing from the vertebral canal caudad of each lumbar vertebra.

Dorsal Rami.—The dorsal rami are similar to those of the thoracic region, sending one branch dorsad to the muscles of the vertebral column, another dorsolaterad to reach the integument at about the lateral border of the longissimus dorsi. The dorsal rami are somewhat smaller caudad.

Ventral Rami.—The last four lumbar nerves are interconnected to form the lumbar or lumbosacral plexus. The first three are distinct, and will therefore be described separately.

The first three lumbar nerves are directed strongly caudad (as well as ventrad), so that on leaving the intervertebral foramen they pass ventrad of the transverse process of the vertebra immediately succeeding. Each communicates with the sympathetic system and gives off near its origin branches to the muscles on the ventral side of the vertebræ,—the first to the crus of the diaphragm, the second and third ([Fig. 162], II and III) to the quadratus lumborum and psoas muscles. Each divides three to five centimeters from its origin into a lateral and a medial branch, the first having a more cranial, the latter a more caudal course. The first three nerves of the cat are represented in man by the iliohypogastric and the ilioinguinal nerves. There seems no good ground for applying these names to two of the three in the cat, in preference to the third, so that we shall speak of these nerves in the cat as simply the first, second, and third lumbar nerves.

1. The first lumbar nerve arises from the intervertebral foramen caudad of the first lumbar vertebra. Its lateral division passes between the transversus and obliquus internus muscles, then between the obliquus internus and externus. At about the middle of the abdomen it pierces the obliquus externus and is distributed to the integument of the middle of the ventral surface of the abdomen. The medial division passes between the obliquus internus and transversus, crosses the lateral division of the second (lying mediad of it), and extends to the rectus abdominis, which it supplies.

2. The second lumbar nerve ([Fig. 162], a) divides, like the others, into two branches. The lateral branch pierces the muscles of the abdominal wall at the lateral border of the longissimus dorsi and passes, lying just beneath the integument, ventrocaudad nearly to the pubis. It supplies the integument of the caudal half of the abdomen and of the fold between the thigh and abdomen. The medial branch passes at first almost directly caudad, lying on the medial surface of the transversus abdominis; it curves gradually ventrad, lying in the substance of the transversus, and reaches the rectus abdominis three or four centimeters craniad of the pubis.

3. The third lumbar nerve (b) is not united with the fourth, as in many animals, but remains distinct. It divides one or two centimeters from its origin. The lateral branch passes caudad to the outer surface of the abdominal muscles, and extends, lying just beneath the integument, to that portion of the abdominal wall which is partly covered by the thigh. The medial branch passes caudad on the lateral surface of the iliopsoas muscle, follows the iliolumbar artery for a short distance, and gives branches to the caudal part of the transversus and rectus abdominis muscles.

The Lumbar Plexus

([Fig. 162]).—The fourth, fifth, sixth, and seventh lumbar nerves are interconnected by short branches, forming thus the lumbar plexus. They are also connected with the sacral plexus, so that the two are often considered together as the lumbosacral plexus ([Fig. 162]). In some other animals the first three lumbar nerves form also a part of the plexus; but they are not connected with it in the cat.

The fourth lumbar nerve forms the genitofemoral (c) and part of the lateral cutaneous (d) nerves, and is connected by a short strand with the fifth. The fifth aids in forming the lateral cutaneous (d) and femoral (f) nerves, and is connected by a short branch with the sixth. The sixth lumbar is large; it forms the major part of the femoral (f) and a large part of the obturator nerve (g), and sends a large connecting branch caudad to join the seventh and thus pass into the sacral plexus. The seventh passes caudad to join the sacral plexus and aid in forming the great sciatic nerve (h); by its connection with the sixth it aids in forming also the obturator nerve (g).

4. N. genitofemoralis (or N. lumboinguinalis) (c, c′, c″).—This is a direct continuation of the fourth lumbar nerve. Its medial branch (c″) passes along the medial surface of the iliopsoas (8) and the psoas minor (9) to the external iliac artery, accompanies this, lying on the ventral surface, from its origin to the point where it gives off the profunda femoris, then accompanies the latter artery and passes onto that branch of it that spreads out under the integument of the ventral pelvic region. It crosses the spermatic cord and ramifies in the skin of the proximal part of the medial side of the thigh. The lateral branch of the genitofemoralis (c′) pierces the psoas minor (9), appearing on its ventral surface opposite the fifth lumbar vertebra. It passes caudad on the ventral surface of this muscle, crosses the iliolumbar artery, then turns caudolaterad, passes through the abdominal wall, and is distributed to the craniomedial surface of the thigh and to the adjacent abdominal wall.

This nerve is variable in origin and in distribution. Its lateral branch is sometimes lacking.

5. N. cutaneus femoris lateralis (d).—The lateral cutaneous nerve arises from the connecting strand between the fourth and fifth nerves, most of its fibres coming from the fifth. It passes caudoventrad between the iliopsoas and psoas minor and accompanies the iliolumbar artery across the ventral surface of the iliopsoas. It pierces the abdominal wall in company with the artery and passes onto the cranial border of the thigh. Its terminal branches are distributed to the integument along with the branches of the iliolumbar artery,—supplying the lateral surface of the femur and extending as far as the knee.

6. N. femoralis (f).—The femoral is a large nerve which rises by strong roots from the fifth and sixth lumbar nerves. It gives branches to the iliopsoas (8) and passes through that muscle to its ventral surface. It leaves the abdominal cavity lying on the ventral surface of the iliopsoas, and at the point where it pierces the abdominal wall it divides into three (or four) branches. One (or two) of these pass to the sartorius muscle. Another passes between the rectus femoris and vastus medialis muscles, and divides into numerous branches which supply these muscles and the vastus intermedius.

The third branch of the femoral is N. saphenus or the long saphenous nerve ([Fig. 127], g, [page 310]). This passes distad along with the femoral artery and vein, giving a few twigs to the integument. Where the long saphenous artery (f) separates from the femoral artery the nerve follows the former and passes with it along the medial side of the lower leg. At the knee and in the lower leg it gives off a number of cutaneous branches, and finally divides just distad of the middle of the lower leg into two main branches. Both of these pass to the concavity of the ankle-joint and divide into fine branches, which form a sort of plexus on the ankle and dorsal surface of the foot; branches from this can be traced almost to the toes.

7. N. obturatorius ([Fig. 162], g).—The obturator nerve arises from the connecting band between the sixth and seventh nerves. It lies at first laterad, then dorsad, of the common iliac vein, and passes almost directly caudad to the cranial border of M. obturator internus (15). Here it turns sharply laterad about the pubis, and passes through the obturator foramen. It gives twigs to the obturator externus muscle, and divides into several branches, which innervate the adductor femoris, adductor longus, pectineus, and gracilis.

[Fig. 162].—The Lumbar and Sacral Nerves, as seen in a View of the Dorsal Wall of the Abdominal Cavity, after Removal of the Viscera.

On the right side the iliopsoas, psoas minor, and quadratus lumborum have been removed. The symphysis pubis has been split and the two parts divaricated, to show the pelvic cavity; on the right side parts of the innominate bones and the levator ani and coccygeus muscles have been removed. II-VII, second to seventh lumbar nerves; 1-3, first to third sacral nerves. a, second lumbar nerve; b, lateral and medial branches of third lumbar nerve; c, N. genitofemoralis (c′, its lateral branch; c″, its medial branch); d, N. cutaneus femoralis lateralis; e, muscular branch of N. femoralis; f, N. femoralis; g, N. obturatorius; h, N. ischiadicus or great sciatic nerve; i, N. gluteus superior; j, N. gluteus inferior; k, N. cutaneus femoris posterior; l, N. pudendus; m, N. hemorrhoidalis inferior; n, part of the sympathetic (incompletely shown, merely to indicate connections with the spinal nerves); 4, M. transversus abdominis; 5, 6, cut edges of M. obliquus internus and externus; 7, dorsal border of the diaphragm; 8, M. iliopsoas (mostly removed on the right side); 9, M. psoas minor; 10, M. pyriformis; 11, M. levator ani; 11′, M. iliocaudalis; 12, M. flexor caudæ longus; 13, M. flexor caudæ brevis; 14, edge of pubic symphysis; 15, cut end of M. obturator internus; 16, cut ramus of ischium; 17, cut ramus of pubis.

D. Sacral Nerves and Sacral Plexus

([Fig. 162]).—A large band passes caudad from the sixth and seventh lumbar nerves to connect with the sacral nerves; this constitutes the so-called lumbosacral cord. The sacral nerves are three in number. Their dorsal rami pass dorsad out of the two posterior sacral foramina and the foramen caudad of the sacrum, to the muscles and skin dorsad of the sacrum. The ventral rami pass from the two anterior sacral foramina and from the foramen between the last sacral and first caudal vertebræ. The first is the largest; it joins the lumbosacral cord to form the great sciatic nerve (N. ischiadicus) (h) and the superior (i) and inferior (j) gluteal, and sends a branch caudad to connect with the second sacral. The second and third sacral nerves are small; they unite with the connecting branch from the first to form a network from which arises the pudendus (l), cutaneus femoris posterior (k), hemorrhoidalis inferior (m), and a small branch to the great sciatic (h).

The nerves arising from the sacral plexus are the following:

1. N. gluteus superior ([Fig. 162], i; [Fig. 163], j).—The superior gluteal nerve arises from the lumbosacral cord and the first sacral nerve; it passes dorsad in the notch between sacrum and ilium, caudolaterad of the first anterior sacral foramen, curves over the dorsal border of the ilium between the gluteus medius and pyriformis muscles ([Fig. 163], 7), passes between the gemellus superior (6) and gluteus minimus (5) and ventrad of the latter, and reaches the medial surface of the tensor fasciæ latæ, which it innervates. In its course it sends branches to the gluteus medius, gluteus minimus (5), and gemellus superior.

2. N. gluteus inferior ([Fig. 162], j; [Fig. 163], i).—A small nerve from the lumbosacral cord and the first sacral nerve. It passes dorsocaudad, lying on the dorsal surface of the great sciatic nerve ([Fig. 162], h). After leaving the pelvis by the great sciatic notch it divides beneath the pyriformis ([Fig. 163], 7) into two branches; one goes to M. caudofemoralis (4), the other to M. gluteus maximus (3).

3. N. ischiadicus ([Fig. 162], h; [Fig. 163], a).—The great sciatic nerve, the largest nerve in the body, arises from the lumbosacral cord (formed chiefly by the sixth and seventh lumbar nerves), the first sacral nerve and a small branch from the second sacral. It passes caudodorsad and leaves the pelvis by passing across the great sciatic notch, between M. pyriformis and M. gemellus superior. It passes across the tendon of M. obturator internus ([Fig. 163], 9) near the insertion, and here sends caudad a large muscular branch (b) beneath the biceps. This branch innervates the biceps, semitendinosus (17) and semimembranosus (16). Other twigs in this region pass to the quadratus femoris (10) and tenuissimus. The great sciatic nerve (a) now passes across the quadratus femoris (10), adductor femoris (15), and semimembranosus (16), lying beneath the biceps femoris. It thus enters the popliteal space and approaches the popliteal artery and vein. Here it gives off one or two small muscular branches to the distal part of the biceps, and sends a slender branch, N. suralis (c), along the medial surface of the biceps to the lateral surface of the lateral head of M. gastrocnemius (20). The sural nerve (c) becomes cutaneous at the distal (ventral) border of the biceps, and divides three or four centimeters proximad of the ankle into two branches. One passes over the tendon of Achilles (27) to the proximal end of the calcaneus and ramifies in this region. The other passes onto the lateral surface of the foot, and supplies the integument in this region over the tarsus and part of the metatarsus.

The great sciatic nerve now divides in the popliteal space into the peroneus communis (d) and the tibialis (e).

[Fig. 163].—Dissection of Leg, Lateral View, to show Distribution of the Great Sciatic Nerve.

The biceps, caudofemoralis, gluteus maximus, gluteus medius, tensor vaginæ femoris, and part of the peroneus longus have been removed. a, N. ischiadicus or great sciatic nerve; b, muscular branch; c, N. suralis; d, N. peroneus communis; e, N. tibialis; f, N. peroneus profundus; g, N. peroneus superficialis and its terminal branches; h, branches of N. cutaneus femoris posterior; i, N. gluteus inferior; j, N. gluteus superior; k, vena saphena parva; k′, communicating branch to V. glutea inferior. 1, M. sartorius; 2, cut origin of M. gluteus medius; 3, cut origin of M. gluteus maximus; 4, cut origin of M. caudofemoralis; 5, M. gluteus minimus; 6, M. gemellus superior; 7, M. pyriformis; 8, M. flexor caudæ longus; 9, M. obturator internus; 10, M. quadratus femoris; 11, tuberosity of the ischium; 12, great trochanter of femur; 13, M. rectus femoris; 14, M. vastus lateralis; 15, M. adductor femoris; 16, M. semimembranosus; 17, M. semitendinosus (the slender ribbon-like muscle crossing this is M. tenuissimus); 18, M. vastus intermedius; 19, M. plantaris; 20, M. gastrocnemius; 21, M. peroneus longus (cut); 22, M. tibialis anterior; 23, M. extensor longus digitorum; 24, transverse ligament; 25, M. peroneus brevis; 26, M. soleus; 27, tendon of Achilles; 28, M. extensor brevis digitorum; 29, tendon of M. peroneus tertius.

a. N. peroneus communis (d).—This is the more lateral of the two divisions of the great sciatic nerve. It passes along the medial surface of the biceps to the lateral surface of the lateral head of M. gastrocnemius (20), where it extends to a point just distad of the head of the fibula. Here it passes beneath that part of the gastrocnemius which has origin on the fascia of the shank, passes between the soleus and peroneus longus (21), then between the peroneus longus and peroneus tertius. It gives off a number of small muscular branches, then divides two or three centimeters distad of the head of the fibula into two main branches, N. peroneus superficialis (g) and N. peroneus profundus (f).

(1) N. peroneus superficialis (g).—The superficial peroneal nerve passes distad between the peroneus longus and peroneus tertius. Near the ankle it becomes superficial, passing along the concavity of the ankle across the transverse ligament which binds down the tendons of the extensor longus (23) and tibialis anterior (22), and thus reaches the dorsal side of the foot. Two or three minute twigs are given off to the ankle, then the nerve divides into four divisions. These pass distad to the toes, subdividing so as to send a nerve to each side of each toe.

(2) N. peroneus profundus (f).—The deep peroneal nerve passes between the tibialis anterior and extensor longus digitorum muscles, gives branches to these muscles, and passes distad with the tibialis anterior artery, lying on the inner surface of the tibialis anterior muscle. It passes onto the dorsum of the foot, lying on the inner surface of the tendon of the muscle. On the tarsus it divides into two branches. The lateral branch passes into the extensor brevis digitorum, while the medial one extends in the dorsal groove between metatarsals four and five to the toes; here it divides into two nerves which supply the contiguous sides of digits four and five.

b. N. tibialis (e).—The tibial nerve passes distad parallel with the peroneal nerve, but mediad of it. It passes between the lateral and medial heads of the gastrocnemius, giving off muscular branches to these, and to the plantaris and soleus. It then passes between the plantaris and the medial head of the gastrocnemius, and reaches the space between the plantaris and flexor longus hallucis, where it passes distad. It gives off, in the region just described, muscular branches to the flexor longus digitorum, flexor longus hallucis, and tibialis posterior. Below the middle of the lower leg the nerve becomes superficial ([Fig. 127], i, [page 310]), lying on the ventral surface of the flexor longus hallucis. It passes in the depression between the heel and the medial malleolus onto the plantar surface of the tarsus, sends a small branch to the plantar surface of the heel, and divides into two branches, the medial and the lateral plantar nerves.

(1) N. plantaris medialis.—The medial plantar nerve passes along the medial border of the flexor brevis digitorum and divides into two branches. One passes to the medial (second) digit, supplying both sides; the other supplies the third digit in the same way. Both send branches to the fibrous pad on the sole of the foot.

(2) N. plantaris lateralis.—The lateral plantar nerve passes laterad across the tendon of the flexor longus digitorum to its lateral edge. At about the proximal end of the metatarsi it divides into two branches. The lateral branch passes distad to the ventral surface of the fifth digit. The medial branch passes beneath the tendon of the flexor longus digitorum and transversely across the interossei; it divides into numerous tendons which innervate the interossei and other short muscles of the sole of the foot.

4. N. pudendus ([Fig. 162], l; [Fig. 112], 8, [page 265]).—This arises ([Fig. 162], l) from the sacral plexus caudad of the great sciatic nerve. It has two roots, one from the second sacral, one from the third, and it may also receive a small accession from the great sciatic nerve (h) three or four centimeters caudad of the origin of the latter. It passes at first dorsolaterad, laterad of the coccygeus muscle, then curves mediad toward the anus. It divides into two branches: one passes to the base of the penis, sends twigs into the compressor urethræ muscle, and passes onto the dorsal surface of the penis. This branch is N. dorsalis penis; it extends with the artery of the same name to the glans. The second branch of N. pudendus (N. hemorrhoidalis medius) passes to the anus and innervates the muscles and other structures about the caudal end of the rectum. In the female, branches of N. pudendus supply the urogenital sinus and adjacent structures ([Fig. 112], 8).

5. N. cutaneus femoris posterior ([Fig. 162], k; [Fig. 163], h).—This arises from the sacral plexus, its roots coming chiefly from the second and third sacral nerves, and passes caudodorsad, at first in close connection with N. pudendus. It then accompanies the posterior gluteal artery and vein, sends branches (perineal nerves) into the fat at the sides of the anus, and branches onto the lateral surface of the biceps muscle. One of the latter follows the communicating vein ([Fig. 163], k′) from V. saphena parva, and may be traced as far distad as the popliteal space.

6. N. hemorrhoidalis inferior (or posterior) ([Fig. 162], m).—This arises by two roots, from the second and third sacral nerves, and passes with the inferior hemorrhoidal artery ventrad across the lateral surface of the rectum to the urethra. Here it divides into two parts: one passes craniad to the bladder, the other caudad onto the ventrolateral surface of the rectum.

A small nerve passes from the sacral plexus, especially from the third sacral nerve, into M. levator ani ([Fig. 162], 11). Another small nerve from the same region passes to M. coccygeus, and a third to the proximal end of M. tenuissimus.

E. Nerves of the Tail. Coccygeal Nerves.

—From the intervertebral foramina of the first seven or eight caudal vertebræ spinal nerves of the usual type are given off. The dorsal rami innervate the dorsal muscles of the tail. The ventral rami are interconnected with each other and with the last sacral nerve by a longitudinal cord; they innervate the muscles and integument of the ventral side of the tail.

3. The Sympathetic Nervous System. Systema nervorum sympathicum.

The sympathetic system consists essentially of a chain of ganglia on each side of the ventral surface of the vertebral column, interconnected by longitudinal nerve-cords, and stretching from the base of the skull to the tail. The ganglia are connected to the spinal nerves by communicating branches, and numerous branches pass from them to the abdominal and thoracic viscera, and to the walls of the lymph- and blood-vessels, forming complicated plexuses.

Cervical portion ([Fig. 156], [page 379]).—The sympathetic system begins just caudad of the tympanic bulla as the superior cervical ganglion (G. cervicale superius) ([Fig. 156], e). This is a large ganglion closely applied to the ganglion nodosum (d) of the vagus, and lying on its ventrocranial side. From the superior cervical ganglion the sympathetic trunk passes caudad usually closely bound up with the vagus (d′), so that the two can scarcely be distinguished. They lie (i) at the side of the trachea (6), close against the lateral surface of the carotid artery. A few centimeters before reaching the first rib the two separate ([Fig. 157], [page 381]); the sympathetic (c) is here the smaller and more dorsal of the two. Just craniad of the first rib the sympathetic usually forms a small swelling, the middle cervical ganglion ([Fig. 157], d), beyond which it divides into two portions. The larger dorsal division passes dorsocaudad and joins a large ganglion, the inferior cervical (e), which lies at the head of the first rib, on the lateral surface of the longus colli muscle. The ventral branch (g′) passes caudad, turns laterad about the medial and caudal surface of the subclavian artery, and likewise reaches the inferior cervical ganglion.

In some cases the middle cervical ganglion (d) is lacking.

Branches of the Sympathetic in the Cervical Region.—From the cranial end of the superior cervical ganglion several nerves pass craniad. Some of these become connected immediately with the cranial nerves emerging from the jugular foramen, forming a plexus in this region. Others pass with the internal carotid artery craniad, forming a plexus about that artery; from the plexus twigs arise which pass to the fifth and sixth cranial nerves; doubtless also to the seventh and eighth. A specially large branch, the deep petrosal nerve (see [page 373]), passes from the superior cervical ganglion along with the internal carotid artery to the pterygoid canal, thence through this to the sphenopalatine ganglion. Before separating from the vagus the sympathetic gives off one or two cardiac nerves ([Fig. 157], i). These are closely bound up with the vagus (b), so that they appear usually to have origin from the vagus at about the level of the first rib. These pass with the vagus to the heart and take part in the formation of the cardiac plexus (k).

At the middle cervical ganglion (d) usually a communicating branch is given to the vagus. The branches of the inferior cervical ganglion (e) fall in the thoracic region, and are [described] in that connection.

Thoracic Portion of the Sympathetic ([Fig. 157], [page 381]).—The inferior cervical ganglion (e) lies entirely within the thoracic cavity. It has the following branches:

1. One or two small cardiac branches (h), which pass to the heart (r).

2. A large communicating branch (e′), which passes craniad from the cranial end of the ganglion (e) along the lateral surface of the longus colli muscle, and gives communicating branches to the sixth, seventh, and eighth cervical nerves (VI, VII, and VIII).

3. One or two small communicating branches to the first thoracic nerve (I′).

4. A communicating branch to the second thoracic nerve.

The arrangement of these communicating branches is somewhat variable; they may arise in various combinations, as at first united, or as more or less completely separated nerves.

The sympathetic trunk (c) continues caudad from the caudal end of the inferior cervical ganglion, lying at first on the lateral surface of the longus colli, then on the lateral surface of the centra of the vertebræ. For each vertebra there is a slight enlargement, forming one of the thoracic ganglia of the sympathetic, and from each ganglion one or two communicating branches are given off to the corresponding spinal nerve. Just caudad of the last rib (but while still in the thoracic cavity) the sympathetic gives off the large N. splanchnicus major or great splanchnic nerve (n), which passes ventrocaudad and pierces the diaphragm. The sympathetic itself then passes through the diaphragm laterad of the crus.

Abdominal Portion of the Sympathetic ([Fig. 164]).—The great splanchnic nerve (b) passes through the diaphragm (2) laterad of the crus, extends two or three centimeters caudoventrad, and joins a large ganglion lying just caudad of the cœliac artery (4), almost on the lateral surface of the superior mesenteric (5). This is the cœliac (or semilunar) ganglion (d). From it nerves branch in all directions, forming the cœliac plexus (c), a part of the large solar plexus. Less than a centimeter ventrocaudad of the cœliac ganglion, on the caudal surface of the superior mesenteric artery (5), is the somewhat smaller superior mesenteric ganglion (f); the two are connected by two strong nerve-trunks.

[Fig. 164].—Sympathetic and Vagus in the Abdomen (somewhat Schematic).

1, stomach; 2, crus and cut edge of diaphragm; 3, aorta; 4, cœliac artery; 5, superior mesenteric artery; 6, kidney; 7, inferior mesenteric artery; 8, large intestine. a, main trunk of sympathetic; b, great splanchnic nerve; c, lesser splanchnic nerves; d, cœliac (or semilunar) ganglion; e, cœliac plexus; f, superior mesenteric ganglion; g, superior mesenteric plexus, following the artery; h, aortic plexus; i, inferior mesenteric ganglion and plexus; j, suprarenal plexus on suprarenal body; k, renal plexus following renal artery; l, ventral œsophageal branch of vagus, forming anterior gastric plexus; m, dorsal œsophageal branch of vagus, forming posterior gastric plexus.

The cœliac ganglion (d) receives from the sympathetic (a), in addition to the great splanchnic nerve, two (or three) lesser splanchnic nerves (c), which arise from the main trunk of the sympathetic, the first just as it reaches the abdominal cavity, the second one or two centimeters caudad of the diaphragm.

From the cœliac and superior mesenteric ganglia a network of nerves passes in all directions to the abdominal viscera. These nerves follow especially the arteries, forming plexuses over their surfaces, and passing with them to the organs which they supply. The cœliac plexus anastomoses with the posterior gastric plexus of the vagus. Parts of the cœliac plexus following the branches of the cœliac artery may be distinguished as hepatic plexus, splenic plexus, etc. A dense plexus is formed on the surface of the suprarenal body (suprarenal plexus, j), and a renal plexus (k) follows the renal artery to the kidneys (6). The superior mesenteric plexus (g) forms a dense network over the superior mesenteric artery (5) and follows it to the intestine. A plexus passes caudad on the aorta (aortic plexus, h). This is a continuation of the cœliac and superior mesenteric plexuses; it receives a number of branches from the main sympathetic trunk, lying dorsad of the aorta. The aortic plexus contains a number of small ganglia.

From the aortic plexus the smaller inferior mesenteric plexus (i) follows the inferior mesenteric artery (7) to the large intestine; in this lies a ganglion of considerable size, the inferior mesenteric ganglion.

In the pelvic region small plexuses are formed in a similar manner, from branches given off by the main sympathetic trunk, together with branches from the plexuses of the abdominal cavity. These plexuses supply the various organs in the pelvis.

The main sympathetic trunk (a) passes into the abdomen at the side of the crus of the diaphragm, lying dorsad of the great splanchnic nerve (b). In the abdomen it lies nearer the middle line than in the thorax, so that the two trunks of right and left side are separated by but a few millimeters as they lie on the ventral surface of the centra of the lumbar vertebræ. There is a ganglion for each vertebra ([Fig. 162], n), and from each ganglion are given off one or more communicating branches to the spinal nerves, and usually a branch which passes ventrad to join one of the plexuses among the viscera. The first three branches of this kind ([Fig. 164], c) are larger and pass to the cœliac and superior mesenteric ganglia. The sympathetic may be traced into the pelvis, where it becomes smaller, and is lost as the caudal region is reached.

---