Anatomy of the Cat

by Jacob Reighard; H. S. Jennings

THE MUSCLES.

I. THE MUSCLES OF THE SKIN. ([Fig. 62].)

M. cutaneus maximus ([Fig. 62], b).—This is a very large, thin muscle which covers almost the whole side of the body. It arises from the outer surface of the latissimus dorsi ([Fig. 68], m) near its ventral end and from the bicipital arch ([Fig. 65], t′) in the axilla; from the linea alba for a considerable distance (two or three inches) caudad of the base of the xiphoid process, and from the thorax over a line joining the axilla and the base of the xiphoid. Sometimes a few fibres take origin from the fascia which covers the pectoantibrachialis on the ventral side of the arm.

From their origin the fibres diverge. The cranial ones curve about the base of the forelimb and are inserted into the skin at or near the middle line caudad of the first thoracic vertebra. The most cranial fibres of all are inserted about one to one and a half centimeters from the middorsal line; thence the line of insertion approaches the middorsal line to reach it at about the eighth or ninth thoracic vertebra. The middle fibres run parallel to the middorsal line in the lumbar and sacral regions as far as the root of the tail, a small bundle passing onto the dorsal side of the tail, another onto the ventral side. The fibres of the caudal portion pass onto the thigh, the ventral ones running in the fold of skin which stretches from thigh to abdomen, and are finally lost in the fascia along a line connecting the knee and the root of the tail. A strong fascia connects the adjacent borders of the cutaneus and platysma and lies over the scapular region.

[Fig. 62].—Muscles of the Skin.

a, a′, a″, M. platysma; b, M. cutaneus maximus; c, M. orbicularis oris. Between the platysma and the cutaneus maximus are seen the following deeper muscles: d, M. acromio-trapezius; e, M. levator scapulæ ventralis; f, M. spino-deltoideus; g, M. acromio-deltoideus; h, M. clavobrachialis; i, caput laterale of M. triceps brachii; j, caput longum of M. triceps brachii; k, M. xiphi-humeralis; l, M. pectoralis minor. 1, external jugular vein.]

This muscle lies immediately beneath the integument. It covers the first layer of body muscles.

Action.—Moves the skin.

M. platysma ([Figs. 62] and [64], a, a′, a″).—This muscle forms a thin layer of fibres covering the sides of the neck and face, in close relation with the integument. Several more or less distinct portions may be distinguished.

(1) Most of the fibres (a) arise from the middorsal line, from the occiput to the first thoracic vertebra, in a narrow fascia common to the muscles of the two sides. The most cranial fibres of this region arise as a small bundle from the external occipital crest, beneath the levator auris longus ([Fig. 63], g, g′).

From this origin in the middle line the fibres pass craniolaterad. The most cranial fibres curve about the ventral side of the ear and pass toward the caudal angle of the eye, where they unite with fibres of the zygomaticus ([Fig. 64], d) or corrugator supercilii lateralis ([Fig. 64], k), or pass to the lower eyelid. Caudad of these the fibres cover the side of the face and become lost among the facial muscles, some passing to the lower eyelid, some to the fibrous pad which supports the vibrissæ, some to the angle of the mouth, some to the lower lip. The most ventral fibres meet the fibres of the opposite muscle just ventrad of the symphysis of the mandible.

The ventral free border of the platysma is separated on the ventral side of the neck from the border of the opposite muscle by a wedge-shaped area having its point at the symphysis menti.

The fibres of this portion of the muscle are interrupted by an attachment to the skin, along a line passing from the base of the ear to about the middle of the coracoid border of the scapula, The dorsal (a) and ventral (a′) portions of the muscle, separated by this line of attachment, are sometimes described as separate muscles (the supercervicocutaneus and cervicofacial, respectively, of Strauss-Durckheim).

(2) A band of fibres one or two centimeters across (a″) arises in the fascia of the side of the neck just craniad of the middle of the coracoid border of the scapula, and passes caudoventrad toward the manubrium, its fibres crossing the fibres of the first part of the platysma at right angles. These fibres become lost in the fascia ventrad of the manubrium, or pass across the middle line to intermingle with the corresponding fibres of the opposite side. This portion of the platysma is sometimes absent.

The platysma is everywhere subcutaneous, except at its dorsocranial angle, where a small bundle of fibres is covered by the levator longus auris. It covers the deeper muscles of the neck and head. Closely attached to its inner surface are the submentalis and depressor conchæ, whose fibres bridge over the ventral interval between the borders of the platysmas of opposite sides.

Action.—Moves the skin of the face and neck.

II. THE MUSCLES OF THE HEAD.

A. Superficial Muscles.

—The most superficial layer of muscles on the face and head is formed by differentiation of the fibres of the platysma. The muscles thus formed are not clearly distinct from each other; in this region sets of fibres differing in direction and in origin or insertion receive separate names even though the different sets of fibres are closely interwoven.

In the quadrangle on the dorsal surface of the head enclosed between the two eyes and the two ears, a thin superficial sheet of fibres is found, in which a number of different sets may be distinguished ([Fig. 63]). These have received the following names.

M. intermedius scutulorum ([Fig. 63], a).—This consists of a broad thin sheet of transverse fibres between the two external ears. The fibres are attached at either end to the scutiform cartilage (1) of the two ears, and pass without interruption across the middle line. At its cranial edge this muscle is continuous with the corrugator supercilii medialis (b); at its lateral edge with the frontoscutularis; at its caudal edge with the levator auris longus (g).

Relations.—Outer surface with the integument. Inner surface with the galea aponeurotica (to which the muscle is closely united), the epicranius muscle (h) and the temporal muscle (n).

Action.—Draws the two ears dorsad, toward the middle line.

[Fig. 63].—Muscles on the Dorsal Surface of the Head.

On the right side are shown the superficial muscles; on the left side the superficial muscles have mostly been removed, exposing the deeper muscles and the bone. a, M. intermedius scutulorum; b, M. corrugator supercilii medialis; c, M. orbicularis oculi; d, M. corrugator supercilii lateralis; e, cranial end fibres of M. platysma; f, M. adductor auris superior; g, g′, M. levator auris longus (g, cranial portion; g′, caudal portion); h, h′, M. epicranius (h, M. occipitalis; h′ M. frontalis); i, M. transversus auriculæ; k, M. auricularis superior (cut on the left side); l, M. abductor auris brevis; m, M. abductor auris longus; n, M. temporalis; o, cut origin of M. frontoscutularis; p, p′, M. levator labii superioris alæque nasi (p′, the origin from the maxillary bone); q, angular head of M. quadratus labii superioris. 1, scutiform cartilage; 2, external ear; 3, bones of the skull.

M. corrugator supercilii medialis (b).—This consists of a thin sheet of scattered fibres lying craniad of the last and intermingling with it. The fibres take origin near the middle line, pass laterad, then curve craniad, converging, to be inserted into the whole extent of the upper eyelid, especially near the caudal angle. Here the fibres unite with those of the orbicularis oculi (c).

This muscle is continuous caudad with the intermedius scutulorum (a), craniad with the orbicularis oculi (c); laterad with the corrugator supercilii lateralis (d). Toward the medial side the fibres lose themselves in a tendinous sheet that joins the galea aponeurotica.

Relations.—Outer surface with the integument. Inner surface with the frontoscutularis and the skull.

Action.—Raises the upper eyelid.

M. orbicularis oculi ([Fig. 63], c; [Fig. 64], s).—This consists of two thin bands of muscle-fibres which lie one in either eyelid parallel to its border and unite at the angle of the eye.

Origin by short tendon-fibres from a tubercle on the surface of the frontal process of the maxillary bone just dorsad of the orbital end of the lachrymal canal and between the two parts of the quadratus labii superioris ([Fig. 63], p and q).

The muscle splits into two parts which pass into the two eyelids. At the outer angle of the eye the two bands unite by the intervention of tendon-fibres between the muscle-fibre bundles of the two.

Relations.—Outer surface with the integument. Inner surface with the inner membrane of the eyelid.

Action.—Closes the eye.

M. corrugator supercilii lateralis ([Fig. 63], d; [Fig. 64], k).—This consists of a number of scattered fibres which arise from among the fibres of the frontoscutularis, and from the tendon lying just craniad of the external opening of the ear, to which are united also parts of the zygomaticus ([Fig. 64], d) and submentalis ([Fig. 64], c). The fibres pass craniad, converging so as to form a narrow band which is inserted at the caudolateral angle of the eye, where it unites with the orbicularis oculi ([Fig. 64], s). This muscle is continuous on the medial side with the corrugator supercilii medialis ([Fig. 64], j) and the frontoauricularis, on the lateral side with the platysma.

Relations.—Outer surface with the integument. Inner surface with the frontal bone and the frontoscutularis.

Action.—Pulls the angle of the eye caudad; at the same time pulls the external ear craniad.

M. frontoauricularis.—A few of the fibres which are attached along the upper eyelid sometimes pass dorsocaudad, mingled with fibres of the corrugatores supercilii medialis and lateralis and the frontoscutularis, to the craniomedial angle of the auricular cartilage, where they unite with the fibres of the adductor auris superior. These fibres are sometimes distinguished as the frontoauricularis muscle.

M. levator auris longus ([Fig. 63], g and g′). (Part of the auricularis posterior of man.)—This lies on the caudal half of the dorsal surface of the head, forming a laterocaudal continuation of the intermedius scutulorum (a).

Origin from the middle line of the neck dorsad of the atlas, and from the sagittal crest for about one centimeter craniad of the external occipital tubercle. The fibres form a broad thin sheet which passes craniolaterad as far as the caudal end of the scutiform cartilage (1). Here the muscle divides; the major portion is attached to the scutiform cartilage (1), its fibres intermingling with those of the intermedius scutulorum (a). The caudal portion of the muscle (g′) passes onto the surface of the auricle, extending one or two centimeters distad; here it is inserted on an oblique line which lies directly craniad of the insertion of the transversus auriculæ (i).

The caudal portion of this muscle (g′) having origin above the atlas and insertion on the auricle, is sometimes considered a separate muscle, the supercervicoauricular or cervicoauricular. The cranial portion (g) with origin on the sagittal crest and insertion on the scutiform cartilage might be distinguished as the occipitoscutularis.

The levator auris longus is continuous at its cranial end medially with M. epicranius (h); laterally with M. intermedius scutulorum (a).

Relations.—Outer surface with the integument. Inner surface with the temporal muscle (n), the auricularis superior (k), the abductor longus auris (m), a narrow strand of the platysma, and the clavotrapezius.

Action.—Pulls the external ear dorsocaudad.

The four following muscles lie partly or entirely beneath those already described; they are doubtless formed as differentiations of the inner layers of the platysma.

M. auricularis superior (or attollens auris) ([Fig. 63], k).—This muscle forms a band about one centimeter broad lying just beneath the levator longus (g).

Origin on the sagittal crest for about one centimeter craniad of the interparietal bone. The muscle passes laterad onto the surface of the auricle and is inserted into the auricular cartilage a little caudad of the middle of its convex surface. At its lateral end the cranial margin is closely united to the under surface of the levator auris longus (g).

Relations.—Outer surface at the medial end with the epicranius (h), the abductor auris longus (m), and levator auris longus (g); at the lateral end with the skin. Inner surface with the temporal muscle (n) and the auricular cartilage.

Action.—Pulls the external ear dorsad.

M. abductor auris longus ([Fig. 63], m; [Fig. 64], q). (Part of the auricularis posterior of man.)

Origin on the sagittal crest dorsad of the interparietal bone, caudad of that of the auricularis superior (k), which it partly covers.

The muscle passes laterad as a flat band 8 to 10 millimeters wide over the caudal surface of the concha of the ear, and is inserted ([Fig. 64], q) on the lateral surface of the eminentia conchæ, caudad of the antitragus.

Relations.—Outer surface with the levator auris longus (g) and the integument. Inner surface with the auricularis superior (k), the abductor auris brevis (l), and the concha.

Action.—Pulls the external ear caudad.

M. abductor auris brevis ([Fig. 63], l).

Origin by a tendon from the lambdoidal crest for one or two centimeters laterad of the middle.

The muscle passes lateroventrad lying beneath the abductor longus (m) as a flat band 6 to 8 millimeters wide which is inserted into the medial surface of the most proximal portion of the concha, just distad of its junction with the cartilaginous auditory meatus.

Relations.—Outer surface with the abductor longus (m), a small strand of the platysma and the concha. Inner surface with the temporal muscle (n).

Action.—Pulls the concha caudad.

M. epicranius (or occipitofrontalis) ([Fig. 63], h and h′).

Origin on the sagittal crest or suture, just craniad of the origin of the levator longus auris, to the inner surface of which this muscle is closely united. From the origin the fibres pass craniad forming a band (h) about 8 or 10 millimeters in width, the two muscles lying close to one another and partly united in the middle line. About two centimeters craniad of their origin the fibres of both muscles end in a tendinous sheet, the galea aponeurotica, which covers the surface of the skull in the region between the ears and eyes, and is formed by the inner surface of the intermedius scutulorum (a) and other muscles of this region. The galea aponeurotica passes craniad onto the surface of the nose, where it gives origin again to a thin sheet of muscle-fibres (h′) which are inserted into the integument near the cranial ends of the nasal bones.

M. epicranius is thus formed of two muscular portions (h and h′), connected by a long tendinous sheet. The caudal portion is frequently distinguished as the occipital muscle (M. occipitalis, h), the cranial portion, on the nose, as the frontal muscle (M. frontalis, h′).

Relations.—Outer surface of the occipitalis (h) with the intermedius scutulorum (a), the levator auris longus (g), and the integument; inner surface with the auricularis superior (h), the abductor auris longus, and the bone. Outer surface of the frontalis (h′) with the integument; inner surface with the bones of the skull.

Action.—Moves the integument of the dorsal surface of the head and of the nose.

Ventrad of the external ear the following three differentiated portions of the platysma may be distinguished.

M. zygomaticus (major) ([Fig. 64], d).—A slender band connecting the angle of the mouth with the scutiform cartilage of the ear.

Fig. 64.—Superficial Muscles on the Lateral Surface of the Head and Neck.

a, a′, a″, M. platysma; b, M. depressor conchæ; c, M. submentalis; d, M. zygomaticus (b, c, and d lie beneath the platysma and are seen through it); e, M. zygomaticus minor; f′, M. caninus; f, g, parts of M. quadratus labii superioris (f, angular head; g, M. levator labii superioris alæque nasi); h, M. myrtiformis; i, M. orbicularis oris; j, M. corrugator supercilii medialis; k, M. corrugator supercilii lateralis; l, M. adductor auris superior; m, M. helicis; n, M. antitragicus; o, M. adductor auris inferior; p, M. conchæus externus; q, M. abductor auris longus; r, parts of M. auricularis externus; s, M. orbicularis oculi.

At the angle of the mouth the fibres arise from among those of the orbicularis oris (i), from both the upper and the lower lip. They pass thence dorsocaudad across the zygomatic arch as a band 8 or 10 millimeters wide which is attached to a tendinous aponeurosis lying just craniad of the external opening of the ear. To this aponeurosis are attached also, wholly or partly, the corrugator supercilii lateralis (k) and the submentalis (c). The aponeurosis is united with the caudoventral angle of the scutiform cartilage and with the ventral edge of the cranial margin of the auricular cartilage.

This muscle is united at its cranial end with the orbicularis oris (i); throughout its length more or less with the platysma (a, a′); at its caudodorsal end with the corrugator supercilii lateralis (k) on its medial side, and with the submentalis (c) on its lateral side.

Relations.—Outer surface with the integument and the platysma (a′). Inner surface with the masseter, temporal, frontoscutularis, and adductor auris inferior (o).

Action.—Draws the angle of the mouth dorsocaudad; the external ear ventrocraniad.

M. submentalis ([Fig. 64], c).—A flat band, similar to the last, and closely connected for some distance to its caudal border.

It arises as scattered fibres near the ventral middle line at about the level of the larynx, the fibres of the opposite muscles crossing the middle line and intermingling. The fibres pass dorsad, converging so as to form a narrower band, which unites dorsad with the zygomaticus (d), to be inserted with it into the tendinous aponeurosis above [described].

Relations.—Outer surface with the integument and platysma (a′). Inner surface with the muscles of the hyoid, the digastricus, the parotid gland, the masseter, the zygomatic arch and the temporal muscle.

Action.—Draws the external ear ventrad.

M. depressor conchæ ([Fig. 64], b).—A thin band of fibres, caudad of the last and parallel with it. The muscle arises as scattered fibres on the ventral surface of the neck, one or two centimeters craniad of the manubrium, the fibres of the muscles of opposite sides crossing the middle line and interdigitating. They pass dorsad, gathering together to form a narrow band which is inserted into the summit of the antitragus.

Relations.—Outer surface with the skin at the ventral end, and with the platysma (a′) further dorsad. Inner surface with the deep muscles of the neck and with the parotid gland.

Action.—Draws the external ear ventrad.

Lying deeper than the muscles thus far described, but closely connected with a number of them, are the following:

M. frontoscutularis. Origin ([Fig. 63], o) on the frontal bone, along the supraorbital margin, from the craniomedial angle of the eye caudad to the zygomatic process of the frontal.

The fibres form a large muscle which passes dorsocaudad to the scutiform cartilage and is mostly attached along its ventrolateral border. Some of the outer fibres, however, pass distad of the scutiform cartilage toward the cartilage of the ear, thus joining the adductor auris superior (f).

The outer surface of this muscle is closely connected with fibres of the intermedius scutulorum (a), the corrugatores supercilii medialis (b) and lateralis (d), and the frontoauricularis, when this exists. The outer surface is covered near the origin by the orbicularis oculi (c), farther dorsad by the muscles just mentioned and the integument. The inner surface is in relation with the frontal bone and the adductor auris inferior ([Fig. 64], o).

Action.—Pulls the ear craniad.

M. adductor auris inferior. ([Fig. 64], o.) (Part of the auricularis anterior of man.)—A small muscle lying beneath the frontoscutularis.

Origin on the ligament which connects the zygomatic process of the frontal with the frontal process of the malar. The fibres form a thin band about 7 millimeters wide which passes craniad, closely united to the inner surface of the frontoscutularis. At the caudal edge of the zygomaticus (d) the muscle is interrupted by a short tendinous interval; the fibres then continue to their insertion on the tip of the antitragus.

Relations.—Outer surface with the frontoscutularis, the zygomaticus (d), and the integument. Inner surface with the temporal muscle and M. antitragicus (n).

Action.—Pulls the ear craniodorsad.

The remainder of the muscles of the ear, which, with the exception of the tragicus lateralis, merely interconnect the cartilages of the external ear or parts of these cartilages, are [described] in connection with the account of the Auditory Organ.

Owing to the presence of the fibrous pad to which the whiskers are attached, the facial muscles between the eye and the mouth differ in the cat in some respects from those of related animals. This fibrous pad interrupts the muscles, frequently breaking muscles which are elsewhere single into two.

M. zygomaticus minor, or malaris ([Fig. 64], e).—This muscle is not always present; when it exists it forms a very thin flat band of fibres passing from the ventral side of the eye toward the angle of the mouth.

Origin among the fibres of the orbicularis oculi (s) in the lower eyelid. The fibres pass ventrad; the insertion varies. In some specimens the insertion is among the fibres of the orbicularis oris (i), at the angle of the mouth, beneath the zygomaticus (d). In other cases this muscle is said to be inserted into the pad on which the whiskers rest.

Relations.—Outer surface with the integument and near its insertion with the zygomaticus major (d). Inner surface with the malar bone and the masseter muscle.

Action.—Pulls the angle of the mouth (or the whiskers) dorsad.

M. orbicularis oris ([Fig. 64], i).—This forms a very thin layer of subcutaneous concentric fibres which surround the mouth, that of the upper lip being thicker than that of the lower lip. The part of the muscle in the lower lip has its fibres intermingled with those of the platysma (a′). In the median line the fibres of the upper lip are interrupted by a raphe, and caudad of this are intermingled with those of the caninus.

M. quadratus labii superioris ([Fig. 63], p and q; [Fig. 64], f and g).—This is a complex of muscle-fibres attached chiefly to the fibrous pad on which the whiskers rest. In it two parts can be distinguished more or less completely.

(1) M. levator labii superioris alæque nasi ([Fig. 63], p; [Fig. 64], g) on the sides of the nose. This arises (a) as a continuation of the frontal portion of the epicranius, and (b) in a small bundle ([Fig. 63], p) from near the dorsal end of the frontal process of the maxillary bone, in common with the next. The two slips thus formed unite craniad and are inserted partly into the integument on the outer side of the wing of the nose, partly into the skin of the upper lip, lateroventrad of the nose, and partly into the pad on which the whiskers are located, in connection with the next.

(2) The angular head (caput angulare), or levator labii superioris proprius ([Fig. 63], q; [Fig. 64], f) is much larger than the last. It takes origin from a small tubercle at the cranial border of the orbit, close to the origin of the orbicularis oculi. The fibres diverge from their origin to their insertion among the whiskers and in the fibrous pad on which the whiskers rest.

Action.—Erects the whiskers and raises the upper lip.

M. caninus or levator anguli oris ([Fig. 64], f′).

Origin in a depression on the lateral surface of the maxillary bone, just craniad of the last premolar tooth. The fibres pass craniad, diverging, toward the whiskers, and have their insertion into the pad of tissue on which the whiskers rest.

Action.—Retracts the whiskers and raises the upper lip.

M. buccinator.—A thin muscle against the mucous membrane of the upper lip, partly united with the orbicularis oris and lying beneath M. caninus.

Origin on the outer surface of the superior maxillary bone in the depression between the infraorbital foramen and the first premolar tooth. The fibres pass ventrad, diverging, and mingle with those of the deep part of the orbicularis oris.

Relations.—Outer surface with the caninus and with the orbicularis oris. Inner surface with the mucous membrane of the upper lip.

Action.—Raises the upper lip.

M. myrtiformis (probably corresponds to a part of the human M. nasalis) ([Fig. 64], h).—A thin sheet of muscle-fibres lying craniad of the levator labii superioris alæque nasi and passing from the whiskers to the nose and upper lip. It is distinguishable from the alæque nasi by the direction of the fibres.

Origin from the pad upon which the whiskers rest. The fibres diverge and have their

Insertion upon the wing of the nose and into the skin of the upper lip near the median line.

Action.—Dilator of the nares and elevator of the upper lip.

M. “moustachier” (Str.-D.) (probably corresponds to one of the Mm. incisivi).—It lies on the upper lip near the median line.

Origin from the outer surface of the premaxilla near the suture along the ventral border of the narial opening. The fibres diverge and pass caudad into the upper lip within the superficial fibres of the orbicularis oris.

Insertion.—The skin of the upper lip just craniad of the whiskers. Some of the fibres pass into the pad on which the whiskers rest.

Action.—It carries the lip craniad.

M. quadratus labii inferioris.—A thin flat band extending almost the length of the lower lip.

Origin from the alveolar border of the mandible, between the molar tooth and the canine. The fibres pass dorsad into the lower lip, where they intermingle with those of the orbicularis oris. The muscle is perhaps not constant.

Action.—Depressor of the lower lip.

Sometimes a few fibres arising from the cranial portion of the alveolar border of the mandibula pass mediad, seeming to join corresponding fibres from the opposite side. Such fibres would constitute a M. transversus menti.

B. Deep Muscles of the Head.

(a) Muscles of Mastication.

M. digastricus ([Fig. 65], b).—The digastric is a thick prismatic muscle lying mediad of the angle of the lower jaw, connecting it with the base of the skull.

Origin by fleshy fibres from the outer surface of the jugular process of the occipital bone, and by a thin tendon from the tip of the mastoid process and from the ridge between the mastoid and the jugular processes. The muscle passes craniad, becoming at the same time broader and more voluminous.

Insertion.—The ventral border of the mandible craniad of the caudal border of the molar tooth. The insertion area extends slightly onto both surfaces of the bone and is triangular, with its apex craniad.

Relations.—Outer (ventral) surface with the following, beginning at the caudal end: the rectus capitis lateralis, the cleidomastoid (h), the submaxillary gland (2), the stylohyoid (d), a large lymph-gland (3), and the integument. Inner (dorsal) surface with the jugulohyoid, the tympanic bulla, the masseter (a), the hyoglossus, and the mylohyoid (c).

Action.—Depressor of the lower jaw.

M. masseter ([Figs. 65] and [66], a).—The masseter forms the projecting mass so prominent in the cat behind and below the eye. Its outer surface is covered by a strong aponeurosis. The muscle is divided into three layers which are distinct as to origin, insertion, and direction of fibres, but which are otherwise not distinct.

The superficial layer takes

Origin by the superficial aponeurosis from the ridge which runs lengthwise of the outer surface of the malar bone, and by fleshy fibres from the surface of the malar bone ventrad of this ridge. The fibres pass obliquely caudad to their insertion.

Insertion.—The caudal half of the lateral margin of the ventral border of the mandible, by a sheet of fascia which lies on the inner surface of the muscle. The superficial fibres curve ventrad of the mandible and are inserted into a tendinous intersection between this muscle and the internal pterygoid ([Fig. 66], c).

The middle layer takes

Origin from the ventral border of the malar bone, from opposite the molar tooth to within three millimeters of the mandibular fossa. The fibres pass ventrad and have their

Insertion into the mandible at the ventral border of the external coronoid fossa by means of the aponeurosis which covers the inner surface of the muscle.

[Fig. 65].—Muscles on the Ventral Surface of the Thorax, Neck and Head.

The left side shows the first layer of muscles beneath the skin and skin-muscles; the right side shows the second layer, except in the head region. a, M. masseter; b, M. digastricus; c, M. mylohyoideus; d, M. stylohyoideus; e, M. sternohyoideus; f, M. thyreohyoideus; g, M. sternomastoideus; g′, M. sternothyreoideus; h, M. cleidomastoideus; i, M. levator scapulæ ventralis; j, M. clavotrapezius; k, M. clavobrachialis; l, M. pectoralis major; m, M. pectoantibrachialis; n, part of M. scalenus; o, M. pectoralis minor; p, M. xiphihumeralis; q, M. latissimus dorsi; r, M. epitrochlearis; s, caput longum of M. triceps brachii; t, M. biceps brachii; t′, bicipital arch; u, M. acromiodeltoideus; v, M. brachialis. 1, parotid gland; 2, submaxillary gland; 3, lymphatic glands; 4, external ear; 5, external jugular vein; 6, manubrium; 7, first rib; 8, common carotid artery; 9, molar gland.

The deep layer takes

Origin by a strong tendon from the ventral border of the zygoma just craniad of the mandibular fossa. The fibres pass cranioventrad, diverge and have their

Insertion into the external coronoid fossa.

Relations.—Outer surface with the following, beginning at the caudal border: the external ear, the parotid gland ([Fig. 65], 1), the platysma, the submentalis, the zygomaticus (major), the zygomaticus minor. Ventral border with a lymph gland caudad of the angle of the mouth ([Fig. 65], 3), and the digastric muscle (b). Inner surface with the bones.

Action.—A very powerful elevator of the lower jaw.

M. temporalis ([Fig. 63], n).—The temporal muscle is the great mass taking origin from the temporal fossa and having its insertion on both surfaces and both borders of the coronoid process of the mandible. It may be divided into two layers, deep and superficial.

The superficial layer. The temporal fascia stretches over the temporal fossa, being attached to its borders; that is, to the sagittal and lambdoidal crests, to the curved ridge which connects the sagittal crest with the zygomatic process of the frontal bone, to the caudal border of this process, to the caudal border of the malar bone, to the caudal border of the ligament connecting the malar bone and the zygomatic process of the frontal, and to the dorsal border of the zygomatic process of the temporal bone and its dorsal root. The craniolateral part of this fascia is much stronger than the remainder.

The muscle-fibres take origin from the strong craniolateral part of the inner surface of the temporal fascia, from the groove on the temporal bone dorsad of the dorsal root of its zygomatic process, from the whole inner surface of this process and of the zygomatic process of the malar bone, and sometimes also from that part of the frontal bone which lies within the temporal fossa.

Insertion.--The outer surface of the coronoid process of the mandible dorsad of the coronoid fossa, and both borders of the coronoid process. The cranial fibres may be inserted craniad of the coronoid process onto the aponeurosis covering the deep portion of the muscle. The caudal part of this portion is more or less distinct and is sometimes described as a separate head.

The deep portion.

Origin by fleshy fibres from the whole surface of the temporal fossa, except in cases where the cranial part of the fossa is occupied by the superficial portion. The fibres converge toward the coronoid process, and the ventral third of the outer surface is covered by strong fascia.

Insertion.—The whole inner surface of the coronoid process of the mandible.

Relations.—Outer surface with the external ear, the ear-muscles ([Fig. 63]), the epicranius ([Fig. 63], h), the corrugatores supercilii lateralis (d) and medialis (b). Inner surface with the skull, the pterygoideus externus ([Fig. 66], b), and the structures within the orbit.

Action.—Elevator of the lower jaw.

M. pterygoideus externus ([Fig. 66], b).—This lies on the medial side of the mandible, ventrad of the ventral portion of the temporal.

Origin.—The external pterygoid fossa ([Fig. 40], h), which lies on the lateral surface of the perpendicular plate of the palatine bone, and on the surface of the pterygoid process of the sphenoid bone dorsad of the internal pterygoid fossa. The fossa extends from the foramen rotundum to the sphenopalatine foramen. The muscle passes with parallel fibres laterad, ventrad, and caudad and has its

Insertion by a strong flat tendon into the medial surface of the mandible near its ventral border between the opening of the mandibular canal and the base of the angular process.

Relations.—Ventral surface with the masseter (a), the digastric, and the soft palate with its muscles. Dorsal surface with the temporal, and near its origin with the muscles of the eye, and other structures in the orbit.

Action.—Elevator of the lower jaw.

M. pterygoideus internus ([Fig. 66], c).—This lies caudad of the pterygoideus externus, nearly parallel to it, and is connected with it at its lateral end.

Origin from the whole surface of the internal pterygoid fossa ([Fig. 40], s).

Insertion by fleshy fibres into the ventral surface of the external pterygoid and its tendon, into the medial surface of the angular process of the mandible and caudad of the angular process for about one centimeter into a fascia common to it and the masseter muscle, and into the stylomandibular ligament.

[Fig. 66].—Obliquely Ventral View of the Deep Muscles of the Lower Jaw and Palate.

The tongue and other structures forming the floor of the mouth have been removed, showing the roof of the mouth. On the right side the masseter and pterygoideus internus have been removed, a, a′, M. masseter; b, M. pterygoideus externus; c, M. pterygoideus internus; d, d′, M. tensor veli palatini; e, e′, M. levator veli palatini; f, M. constrictor pharyngis superior. 1, aponeurosis of the velum palatinum; 2, ridges formed by the palatines and pterygoid portions of the sphenoid; 3, hamular processes; 4, free caudal border of the velum palatinum; 5, bulla tympani.

Relations.—Ventral surface with the styloglossus and the soft palate. Dorsal surface with the bulla tympani (5) and base of the skull. Cranial surface with the pterygoideus externus (b), with which it is partly united.

Action.—Assists the pterygoideus externus.

(b) Muscles of the Hyoid Bone.

M. stylohyoideus ([Fig. 65], d).—The stylohyoid is a ribbon-like muscle about three millimeters wide. It lies on the superficial surface of the digastric, caudad and ventrad of the angle of the jaw.

Origin by fleshy fibres from the outer surface of the stylohyal bone near its middle. The muscle curves over the outer surface of the digastric (b), and ends about one centimeter from the middle line in a slender flat tendon.

Insertion into the middle of the ventral surface of the body of the hyoid bone. Some of the fibres of the mylohyoid (c) are inserted into its tendon.

Relations.—Outer surface with the submaxillary gland (2) and the submentalis muscle. Inner surface with the digastric (b), the hyoglossus ([Fig. 67], h), and the geniohyoid ([Fig. 67], g).

Action.—Raises the hyoid.

M. geniohyoideus ([Fig. 67], g).—The geniohyoid is a long slender muscle which lies next to the median line between the symphysis menti and the hyoid bone, closely applied to its fellow of the opposite side and within (dorsad of) the mylohyoid ([Fig. 65], c).

Origin from the ventral half of the inner surface of the mandibula for about one millimeter from the symphysis.

Insertion by muscle-fibres into the lateral half of the ventral surface of the body of the hyoid bone.

Relations.—Outer (ventral) surface with the stylohyoid ([Fig. 65], d) and the mylohyoid ([Fig. 65], c); medial border with the muscle of the opposite side. Lateral border with the hyoglossus ([Fig. 67], h) and genioglossus ([Fig. 67], f). Inner (dorsal) surface with the genioglossus.

Action.—Draws the hyoid forward.

M. jugulohyoideus ([Fig. 67], b).—The jugulohyoid is a small flat quadrangular muscle covering the outer part of the ventral surface of the tympanic bulla.

Origin from the ventral border of the jugular process.

Insertion into the caudal side of the cartilaginous part of the stylohyoid opposite the origin of the styloglossus.

Relations.—Ventral surface with the digastric. Dorsal surface with the tympanic bulla (3).

Action.—Draws the hyoid backward and thus gives a firmer surface of origin for the styloglossus.

M. mylohyoideus ([Fig. 65], c).—The mylohyoid is a large thin muscle seen after the reflection of the digastric, filling with its fellow of the opposite side the whole angle between the rami of the lower jaw. Its fibres are transverse.

Origin from the middle of the medial surface of the body of the mandible between the caudal opening of the mandibular canal and the symphysis of the jaw.

[Fig. 67].—Muscles of Tongue, Hyoid Bone, and Pharynx.

a, M. tragicus lateralis; b, M. jugulohyoideus; c, M. pterygoideus externus; d, partially cut surface of M. pterygoideus internus; e, M. styloglossus; f, M. genioglossus; g, M. geniohyoideus; h, M. hyoglossus; i, M. glossopharyngeus; j, M. constrictor pharyngis medius; k, M. constrictor pharyngis inferior; l, M. stylopharyngeus; m, M. sternohyoideus (cut); n, M. cricothyreoideus; o, M. sternothyreoideus (cut); p, M. thyreohyoideus. 1, mandible; 1′, angular process of mandible; 2, stylomandibular ligament; 3, bulla tympani; 4, trachea; 5, œsophagus; 6, thyroid gland; 7, isthmus of the thyroid gland.

Insertion with the opposite muscle into a median raphe which extends from the symphysis of the jaw to the hyoid bone and is closely united to the external surface of the geniohyoid. Some of the fibres gain insertion into the body of the hyoid bone through their insertion into the tendon of the stylohyoid (d).

Relations.—Outer surface with the submentalis ([Fig. 64], c) and the digastric ([Fig. 65], b). Lateral edge with the masseter (a), the digastric (b), and the mandibula. Caudal border with the stylohyoid (d). Inner (dorsal) surface with the geniohyoid ([Fig. 67], g), the hyoglossus ([Fig. 67], h), and the genioglossus ([Fig. 67], f).

Action.—Raises the floor of the mouth and thus brings the hyoid forward.

M. ceratohyoideus.

Origin, the two proximal pieces of the cranial horn of the hyoid. The fibres pass caudad parallel to one another and within the middle constrictor of the pharynx.

Insertion into the whole length of the caudal cornu of the hyoid bone.

Relations.—Outer surface with the middle constrictor of the pharynx. Inner surface with the pharynx.

Action.—Draws craniad the body of the hyoid by bringing its cornua together and bending the joint between the two pieces of the cranial cornua.

The muscles connecting the hyoid and the tongue are [described] in connection with the description of the tongue.