Arterial System of Hyla Aurea.
The truncus arteriosus (fig. 4, t.) comes from the ventricle of the heart and divides almost immediately into two main branches. After running for a short distance each half divides into three branches, known as the “aortic arches.” (For the sake of simplification, one side only—namely, the left side—will be described.)
A. Carotid Arch (fig. 4, i).—This is the upper of the three arches, and after running for a short distance it divides into two branches—viz., the lingual artery (fig. 4, l.), which comes off just in front of a swelling in the carotid, known as the “carotid gland,” and runs forward along the muscles of the lower jaw towards the snout, breaking up into smaller branches as it proceeds; and the carotid artery (fig. 4, c.), which runs round the œsophagus, then, after bending backwards so as to overlap the systemic arch, runs forwards and downwards and enters the skull a little to the left of the median line. I have not been able to make out very clearly the course of the carotid artery after it enters the head, but it appears to divide up into internal and external carotids.
B. Systemic Arch (fig. 4, ii).—This is the middle of the three arches, and runs round and down over the œsophagus to the dorsal body-wall. It then continues on as a large artery and joins with its fellow on the other side, just anterior to the kidneys, to form the dorsal aorta. When opposite the arm this arch gives off two arteries, viz.:—
(a.) The occipito-vertebral (fig. 4, oc.): This is seen as a very short, thick artery running downwards and forwards, and disappears into the muscles of the dorsal body-wall. In Rana this divides into two branches—namely, the occipital and the vertebral—but in Hyla I have only been able to make out the occipital (fig. 4, oc.), which runs up to the muscles on the side of the head and also to the orbit.
(b.) The subclavian artery (fig. 4, s.) branches off from the systemic arch near the origin of the occipital vertebral. It runs out as a large artery to supply the arm.
C. Pulmo-cutaneous Arch (fig. 4, iii).—This is the third of the aortic arches, and just before it reaches the lung it divides into two, viz.:—
(a.) The cutaneous artery (fig. 4, cu.) runs outwards and downwards, and disappears in the muscles at the angle of the jaws. It can be traced to the dorsal surface, where it runs as a large vein along the skin from the pectoral down to the pelvic girdle. It has a number of small branches which supply the skin.
(b.) The pulmonary artery (fig. 4, pl.) bends round and runs down the outer side of the lung, through which it ramifies.
The dorsal aorta (fig. 4, d.a.), as stated before, is made up by the union of the right and left systemic arches. It runs down close to the vertebral column, and just below the kidneys it divides into the two iliac arteries (fig. 4, il.), which supply the legs.
Just where the two systemic arches unite, a large median artery, the cœliaco-mesenteric (fig. 4, c.m.), is given off to the viscera. This divides into two smaller arteries—viz. (a) Cœliac artery (fig. 4, cœ.), which breaks up again into (1) the hepatic artery, running to the gall-bladder and liver, and (2) the gastric artery, supplying the stomach; (b) mesenteric artery (fig. 4, m.), which breaks up again into (1) the anterior mesenteric, supplying the duodenum and the proximal end of the intestine, and (2) the posterior mesenteric, supplying the distal end.
Just as the iliac artery passes the pelvic girdle it gives off two or three branches that supply the body-wall and some of the muscles of the thigh; these seem to be what Marshall* calls the “lumbar” (fig. 4, lm.), and Ecker† the “external iliae arteries.”
[Footnote] * “The Frog,” A. Milnes Marshall, 7th ed, p. 31 (b, 3).