the lienal (splenic) swings around on the greater curvature of 

 the stomach and joins the gastroduodenal branch of the 

 hepatic. A second branch of the gastro-duodenal curves 

 along the underside of the duodenum and connects with the 

 superior mesenteric artery. 



The second main division of the dorsal aorta is the supe- 

 rior mesenteric. This artery extends out through the mesen- 

 tery where it fans out into a large number of branches serv- 

 ing the small and much of the large intestine. 



The third stem from the dorsal aorta is the inferior mes- 

 enteric; this is interconnected with the superior mesenteric, 

 a loop supplying the transverse and descending parts of the 

 colon. The inferior mesenteric also supplies the sigmoid 

 colon and rectum. Feeding into the inferior mesenteric stem 

 are the middle and inferior hemorrhoidals which arise 

 from the hypogastric artery. The kidneys and gonads also 

 receive branches from the dorsal aorta, and there are verte- 

 bral branches extending into the body-wall musculature. 



Posteriorly the dorsal aorta divides to form two large, 

 common iliac arteries, between which a small middle sacral 

 artery continues posteriorly below the vertebral column. 

 The common iliac artery passes laterally and downward, 

 giving off the hypogastric artery medially and continuing 

 as the external iliac artery into the lower limb. The external 

 iliac artery becomes the femoral artery in the base of the 

 limb. The femoral sends branches to the hip area and to the 

 various parts of the lower limb. Among the mammals there 

 is much variation in the exact arrangement of the various 

 branches but a general pattern is evident. 



EMBRYOLOGiCAL DEVELOPMENT The dorsal aorta is formed 

 by the fusion of bilateral channels. From it, segmentals extend 

 upward between the myotomes and outward over the yolk 

 sac as the vitelline arteries. Of the several vitellines, a single 



channel finally develops and later becomes the superior 

 mesenteric. The coeliac and inferior mesenteric condense 

 from a number of segmental mesenteric vessels. The um- 

 bilical arteries follow the allantois outward and supply the 

 placenta. The intraembryonic umbilical becomes the com- 

 mon iliac and the hypogastric stem of the adult which con- 

 nects with the body wall through the lateral umbilical 

 ligament. 



The functional nature of the origin of the definitive vessels 

 is well shown by the development of a limb stem (Figure 

 11-35). At first there are several segmental vessels involved. 

 The number decreases until one remains. This vessel is lost 

 distally in a reticulum of channels from which a final chan- 

 nel or channels is selected. 



Reptiles The reptile pattern is comparable to the mammal. 

 The dorsal aorta may give rise to a small gastric branch, 

 passing to the dorsal wall of the stomach, and then a large 

 coeliac stem which serves the spleen (lien), a part of the 

 greater and the lesser curvature of the stomach as well as 

 the anterior part of the duodenum. The coeliac stem may 

 include a part of the superior mesenteric, which serves the 

 small intestine and the anterior part of the large intestine. 

 In Sphenodon, the superior mesenteric is separate from the 

 coeliac stem but is continuous with the inferior mesenteric 

 which springs from the dorsal aorta posterior to the renal 

 arteries. In Lacerta there are several small mesenteric stems 

 arising from the dorsal aorta. Among lizards there is a large 

 number of patterns representing the basic stem as well as 

 some of the large branches of these stems. 



Amphibians In the Amphibia the pattern of main arteries 

 is like that already described, and the nature of the devel- 

 opment of this system is apparent. In a series of forms, the 



brochial vein and arter/ 



marginal vein 



B 



digital vessels' 



Figure 1 1-35. Three stages in the formation of the arteries and veins of the forelimb of a tetra- 

 pod. (After Grodzinski, 1933) 



CIRCULATION IN THE BODY • 367 



