The Arterial System
The Abdominal Aorta and its Branches.
The right and left aortic arches, [Fig. 61], Aod, Aos, arising from the heart in the manner already described, form a rather long loop and approach each other in the middorsal line. Here they are united by a short, wide connective in such a way that the left arch seems continued into the cœliac artery and the right into the dorsal aorta proper. Each arch, anterior to the connective, gives off two fairly large branches, oe, to the posterior region of the œsophagus.
The cœliac artery, [Fig. 61], c, is the largest branch of the abdominal aortic system. After giving off a couple of small branches, oe, to the posterior region of the œsophagus, it gives off a large spleno-intestinal artery, si, to the spleen and small intestine.
The cœliac then breaks up into three arteries of about the same size: the gastro-hepatico-intestinal, ghi, carrying blood to the stomach, liver, and small intestine; the pancreo-intestinal, pi, leading to the pancreas and small intestine; and the gastric, ga, to the greater part of the stomach.
From the dorsal aorta proper, da, which, as has been said, seems to be the direct continuation of the right aortic arch, several arteries are given off; these will be described as they occur in an antero-posterior direction.
At about the point of union of the two aortic arches arises the most anterior of seven or eight pairs of lumbar arteries, lu 1-7; this first lumbar artery is continued cephalad for some distance as a longitudinal trunk that gives off several lateral branches to the walls of the thoracic region. The other six or seven lumbars are distributed to the dorsal body wall, and arise, at more or less regular intervals, as far caudad as the sacrum, or even back of that point.
Fig. 61. The arteries of the posterior region of the Florida alligator.—For lettering, see [pages 224-25].
The first large branch of the aorta is the unpaired mesenteric artery, m¹, which is given off in about the region of the fourth pair of lumbars; it carries blood through the mesentery to the greater part of the small intestine and also sends a small branch to the large intestine.
Posterior to the mesenteric, the aorta gives off four or five pairs of short arteries, the urogenitals, u 1-4, that lead to the nearby reproductive organs and kidneys.
About the middle region of the kidneys, a short distance anterior to the sacrum, is given off a pair of rather large arteries, called by Bronn the ischiadicæ, is¹; each ischiadica, after giving off a couple of small branches to the back, passes laterad and divides into three main branches: (1¹) to the ventral body wall, (3¹) to the anterior border and deeper region of the thigh, and (2¹) to the pelvis.
In the region of the sacrum is given off a pair of iliac arteries, il¹. Each iliac is of about the same diameter as the ischiadica and gives off, soon after leaving the aorta, an artery, ab, that apparently leads chiefly to the abdominal muscles. Distal to the origin of the abdominal, the iliac gives off a small pelvic artery, pa, which leads, as the name would indicate, to the pelvis. The iliac then passes into the thigh, where it gives off several large branches and may be called the sciatic, sc. At the knee the sciatic gives off two rather small branches: one, the fibular artery, f¹, extends down along the posterior side of the lower leg; the other is parallel to the first and may be called the tibial artery, tb, since it extends along the anterior or tibial side of the shank. These two arteries give off numerous branches to the muscles of the lower leg. After giving off the fibular and tibial arteries, the sciatic passes, as a large vessel, through the lower leg, to which it gives but few branches, and may here be called the crural artery, cr. At the tarsus it divides rather suddenly and, perhaps, variably, into four chief branches, leading to the toes.
Fig. 62. The arteries of the anterior region of the Florida alligator. The arteries of the left foreleg shown at A.—For lettering, see [pages 224-25].
A short distance caudad to the origin of the iliacs the dorsal aorta gives off a pair of small pelvic arteries, pa¹, going to the muscles of that region. Caudal to these pelvic arteries is given off the unpaired first hæmorrhoidal artery, he¹, which divides into a rectal, rt¹, and a cloacal, cl, branch.
Caudal to the first hæmorrhoidal arises the second hæmorrhoidal, he²; also unpaired, leading to the cloaca.
Posterior to the second hæmorrhoidal, the aorta continues into the tail as the large caudal artery, ca.
The Anterior Arteries.
The origin of the great arterial trunks—the pulmonary, aortic arches, primary carotid, and right subclavian has already been given and the distribution of the pulmonary arteries and aortic arches has been described, so that it now remains to describe the distribution of the right subclavian, [Fig. 62], sc.d., and the primary carotid, capr.
The right subclavian, sc.d., since it has an independent origin from the heart, instead of arising as a branch of the primary carotid, will be described first. After leaving the heart it passes cephalad and laterad and gives off the following branches in order, beginning at the heart: an œsophageal artery, oe, a small, caudally directed vessel carrying blood to the posterior region of the œsophagus. Close to the œsophageal arises another small, caudally directed vessel, the pleural artery, plu, extending to the pleura and possibly to the pericardium. From the same region as the preceding two arteries, but extending cephalad along the trachea and œsophagus, arises the much larger branch of the right subclavian, the right collateralis colli, cc, whose course and distribution will be described later.
Close to the distal side of the collateralis colli arises the very small thyroid artery, th, leading to the oval thyroid gland that lies against the ventral surface of the trachea a short distance anterior to the heart.
A short distance distal to the thyroid artery the subclavian gives off a fairly large artery, the internal mammary, im¹ (shown too large in the figure), that passes to the inner surface of the ribs near the sternum and lies parallel to the vein of the same name, described above.
A short distance distal to the internal mammary arises an artery of about the same diameter, the vertebral, v¹; it passes dorsad and caudad to the region of the thoracic vertebræ.
After giving off the five vessels just described, the subclavian artery passes into the shoulder where it divides into three main branches: (a) the subscapular, sc¹, going to the skin and muscles of the shoulder; (b) the thoracic, t¹, carrying blood to the posterior muscles of the shoulder and to the posterior region of the upper arm; (c) the brachial, br¹, which is really the continuation of the subclavian and is the chief artery of the anterior appendage.
After sending several branches to the upper arm the brachial divides, in the region of the elbow, into two main vessels, the radial, ra¹, and ulnar, ul¹, arteries, [Fig. 62], A. The radial artery, in the carpal region, divides in a complicated way into five main vessels that extend into the digits. The ulnar artery gives off several branches to the forearm, but apparently does not connect directly with the branches to the digits.
The primary carotid, capr. After leaving the heart, this very large vessel passes cephalad and laterad for some distance on the left side of the body and then gives off, from its anterior side, the large left subclavian artery, sc.s., to be described later. After giving off the subclavian artery, it makes a short loop, still farther to the left, and then turns sharply mediad to pass to the head in the median plane directly dorsal to the œsophagus. Its distribution in the cervical and cephalic region will be described later. The mate to the œsophageal branch, oe (near heart), of the right subclavian which was mentioned above is apparently sometimes given off from the primary carotid near its base (as shown in [Fig. 62]) and sometimes as a branch of the left pleural artery.
The left subclavian artery, sc.s., although it has a different origin, has the same branches as described in connection with the right subclavian. The exact order in which the first of these (the thyroid, th; the internal mammary, im¹; the collateralis colli, cc; the pleural, plu; and the vertebral, v¹) are given off is, as might be expected, subject to some variation.
The collateralis colli, cc (following Bronn’s nomenclature), whose origin was noted above, will now be discussed; since the two are alike only one need be described. After leaving the subclavian, it passes cephalad, at the side of the trachea and œsophagus, in company with the internal jugular vein, so that in this part of its course it would seem to be the internal carotid artery. It gives numerous small twigs to the trachea and œsophagus, oe. In the region of the posterior part of the huge jaw muscle it is connected directly, x, with the adjacent branch, cm (called by Bronn the common carotid) of the primary carotid, and indirectly, x¹, with a complicated group of branches from the common carotid. Cephalad to the connective x¹, which extends dorsad and is hence foreshortened in the figure, the collateralis colli gives off a small vessel, y (too large in [Fig. 62]), to the shoulder and skin; it then sends a fairly large branch, jm, into the large jaw muscle, close to which it now lies. Next a small branch, lg, is sent to the larynx. Continuing cephalad and laterad (in [Fig. 62] it is drawn farther to the side than it actually lies) for a short distance farther, it divides into three branches: (1) a short twig, mg, that goes to the musk gland on the side of the mandible and to the skin of that region; (2) a large branch, the mandibular, md, that enters the large foramen on the mesial side of the mandible and extends in the cavity of that bone throughout its entire length; (3) the lingual artery, l¹, which in turn divides, some distance cephalad, into two branches, one extending along the lateral region, the other nearer the mid-ventral surface of the tongue. It is seen, then, that the collateralis colli arteries supply directly the lower side of the head—tongue, mandible, etc.—though they may also send blood through the above-mentioned connectives to the brain and dorsal regions of the skull.
The primary carotid, capr, as was noted above, makes a curve to the left after leaving the heart and then passes back to the median plane, where it may be seen lying against the ventral side of the neck muscles and dorsal to the œsophagus; in this place it gives off a series of unpaired cervical arteries, [Fig. 62], ce, each of which almost immediately divides into an anterior and a posterior branch, that carry blood to the cervical vertebræ. At the base of the skull, in the region where it is united by the first connective, x, with the collateralis colli, as described above, the primary carotid divides into two similar branches, called by Bronn the common carotids, cm. The distribution of these two vessels is symmetrical, so that only one need be described. While the collateralis colli, as has been said, carry blood chiefly to the tongue and lower jaw, the common carotids supply the cranium and upper jaw.
Soon after its formation by the division of the primary carotid, the common carotid is joined, as noted above, with the collateralis colli of that side by the connective, x; since the common carotid and its branches all lie dorsal to the collateralis colli and its branches, the connectives x and x¹ extend in a more or less dorso-ventral direction. The two common carotids, almost completely surrounded by bone, in passing cephalad sweep first laterad, then mediad, so that they together form almost a complete ellipse, as seen in [Fig. 62]; there is, however, no apparent connection between them at the anterior region where they lie so close together.
A short distance cephalad to the connective x the common carotid is connected laterally, z, with a rather complicated plexus of vessels lying at the base of the skull; it is through this plexus that the common carotid is connected with the collateralis colli by the second connective, x¹.
The short branch z quickly divides into three parts: (1) a small anteriorly directed vessel which may be called the internal carotid, ic, since it enters the skull through the most ventral of the three foramina in the exoccipital, and probably supplies the brain, though its further course could not be followed; (2) a somewhat larger posteriorly directed artery, oc, going to the muscles at the occipital region of the skull; (3) a short laterally directed stem, z¹. The last-named branch, z¹, in turn, leads in three directions: (a) to the collateralis colli artery through the connective x¹; (b) a short anteriorly directed vessel, e, that passes into the skull, possibly to the ear, through the large foramen that lies between the exoccipital and quadrate bones; it gives off a small twig, q, to the muscles in the region of the jaw articulation (quadrate); (c) the main stem of the branch z continues laterad and cephalad as one of the chief arteries, z², to the anterior region of the skull, giving off a fairly wide branch, jm¹, to the large jaw muscle, and then two branches, o¹ and o², to the lateral surface of the eyeball and socket; it then anastomoses, just cephalad and laterad to the eye, with the forward continuation, cm¹, of the corresponding main stem, cm, of the common carotid, already mentioned. The vessel cm¹, after almost meeting its fellow in the middle line, passes cephalad and laterad across the ventral surface of the eye to the union, above mentioned, with the lateral branch, z²; at the posterior-mesial border of the eye it gives off a branch that divides into two twigs, one, o³, for the posterior eye muscles, and one, e¹ to the region of the ear and the top of the skull.
At the point of union of the branches cm¹ and z² a sort of simple plexus may be formed from which two vessels, n, pass to the posterior nasal region, and two vessels pass forward along the side of the upper jaw. Of the latter two vessels one, which may be called the inferior dental of the maxilla, dm, is very small and extends along the maxilla to its very tip, at the base of the teeth and ventral to the palatine bone; the other, which is larger and may be called the superior dental of the maxilla, dm¹, extends cephalad along the mesial side of the maxilla, dorsal to the palatine bone; it sends numerous twigs into the maxillary bone among the roots of the teeth. After passing nearly to the end of the snout, the superior dental, dm¹ suddenly forms a loop towards the median line and passes as a straight branch, n¹, directly caudad, near and parallel to the median plane. The branch n¹ extends along the floor of the nasal cavity and, after giving off twigs to this chamber, ends in a network of vessels, o⁴, on the anterior surface of the eyeball and socket.
A pair of very small arteries, n², may be seen in the nasal chamber between and parallel to the branches, n¹; they lie close to each side of the nasal septum and supply the anterior nasal region. They apparently arise, as shown by the broken lines, from the loop of the superior dental artery, dm¹, though this could not be definitely determined.