148 THE VOYAGE OF H.M.S. CHALLENGER. 



supplies branches to both walls of the stomach, but chiefly to the anterior or ventral wall. 



From this trunk are derived the hepatic arteries. 



This branch is absent in Spheniscus mendiculus, its place being supplied by branches 



from the posterior gastric artery. 



T/ie Hepatic Arteries are two in number, and arise from the anterior gastric branch 

 close to its origin. They pass forwards together, and reaching the posterior surface of the 

 liver, are distributed, the one to the right and the other to the left hepatic lobe. 



In Spheniscus mendiculus the hepatic arteries are derived from the posterior gastric 

 artery. In Spheniscus minor and Spheniscus demersus there is only one hepatic artery. 



(3) The Posterior Gastric Artery. — A branch of the coeliac axis, this artery passes 

 backwards, to reach the right margin of the stomach close to its anterior extremity. 

 Here it applies itself to the stomach, and travels backwards parallel to the anterior gastric 

 artery, but on the dorsal (posterior) surface of the stomach, at the posterior extremity of 

 which it terminates. This artery gives off numerous branches to both surfaces of the 

 stomach, but principally to the dorsal wall of that viscus. 



In Pygosceles, as well as in Ap)tenodytes, the posterior gastric artery comes off from 

 the anterior gastric artery, instead of from the coeliac axis as in the other species which 

 I have examined. 



(4) Tlie Intestinal Artery. — This, the largest branch of the coeliac axis, travels hori- 

 zontally backwards, parallel wath, but at some distance from, the right margin of the 

 stomach. Passing between the folds of the mesentery, it breaks up into numerous 

 branches, which supply the upper third or fourth of the small intestine as well as the 

 pancreas. The upper branches of this artery inosculate with the anterior gastric, the 

 lower with the superior mesenteric artery. 



V. — The Superior Mesenteric Artery- 

 Comes off from the abdominal aorta, half an inch behind the coeliac axis. It travels 

 backwards, and, Ipng between the folds of the mesentery, gives off numerous branches 

 from either side. These secondary branches divide dichotomously, but do not anastomose 

 with one another. Hence there is an absence of the arterial arcades met with in this 

 situation in the mammal. The smallest branches of this trunk only anastomose when in 

 contact with the wall of the intestine, and from this anastomosis the minute vessels which 

 supply the gut are derived. The superior mesenteric artery supplies the lower half or 

 three-fourths of the small intestine, in other words, so much of the gut as is not supplied 

 by the intestinal branch of the coehac axis. 



In Spheiiiscus minor the coehac axis gives off no intestinal branch. In this species, 

 therefore, the entire length of the small intestine is supplied by the superior mesenteric 

 artery, additional branches of which take the place of the intestinal branch of the coeliac 

 axis of other species. 



