24 THE FROG 
it unites in the median line to form the dorsal aorta. The dorsal 
aorta passes posteriorly, giving off branches to the viscera and 
body wall, and bifurcates near the posterior end of the body cavity. 
(a) Branches of the systemic arch. 
The small laryngeal artery arises from the dorsal suntie: of 
the systemic arch near its origin. It is distributed to the larynx 
and pharynx. 
The occipito-vertebral artery arises from the systemic arch 
at about the level of the third vertebra; it passes anteriorly 
and turns dorsalward at the level of the posterior end of the 
cranium, sending one branch anteriorly to the cranium and another 
posteriorly along the vertebral column. To trace the course of 
these branches turn the frog dorsal surface up, slit up the skin a 
little to one side of the dorsal median line from the tip of the nose 
to the “hump” of the frog’s back, and reflect the more lateral 
flap. Dissect away the suprascapula (expanded dorsal portion 
of the shoulder skeleton) and the muscles covering the dorsal 
surface of the vertebre. The occipito-vertebral artery will be 
seen emerging from the body cavity just posterior to the skull; it 
divides into an anterior branch, the occipital artery, and a pos- 
terior branch, the vertebral artery. Trace the course of each. 
The small esophageal artery arises near the angle between the 
systemic arch and the occipito-vertebral or sometimes from the 
latter. It passes to the dorsal surface of the cesophagus. 
The subclavian artery arises just posterior to the occipito- 
vertebral and passes laterally to supply the muscles of the shoulder 
and arm. On its course toward the arm it sends off several small 
branches to neighboring muscles and the body wall. 
(b) Branches of the dorsal aorta. 
The ceeliaco-mesenteric artery is a large artery arising near the 
junction of the systemic arches; it divides into the celiac and the 
anterior mesenteric arteries. The cceliac artery gives off the left 
gastric artery and finally divides into the hepatic and right 
gastric arteries; trace their distribution. The anterior mesenteric 
artery sends branches to the spleen (splenic artery ) and small and 
large intestines. 
The urogenital arteries are four or five (or even more) small 
arteries which arise from the ventral surface of the dorsal aorta 
