190 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



through the mesentery to the anterior end of the intestine, and 

 becomes the superior mesenteric artery of the adult. 



Anterior to the superior mesenteric arise arteries supplying 

 the stomach, spleen, liver, pancreas, and duodenum (gastric, 

 splenic, hepatic, and duodenal). In the lower vertebrates these 

 are more or less distinct in their origin from the aorta, while in 

 the adults of the higher groups they unite at their base into a 

 common trunk, the coeliac axis ; and occasionally the superior 

 mesenteric may fuse for a short distance with this, forming a 

 coeliac-mesenteric trunk. 



Between the superior mesenteric (omphalomesaraic) and the 

 hypogastric, other arteries (inferior mesenteries) supply the 

 hinder end of the intestine ; and these again may arise sepa- 

 rately from the aorta, or their roots many fuse into one or 

 more trunks. 



The hypogastric arteries (allantoics of amniotes) are rather 

 more complicated in their relations. In the fishes they are dis- 

 tributed on the ventral wall of the body in front of the vent and 

 to the rectal region of the alimentary tract, while branches, the 

 iliac arteries, are given off from each to the pelvic appendages. 

 In the amphibia, with the development of an allantoic diverticu- 

 lum from this region which later forms the urinary bladder, 

 the vessels going to the rectal region enlarge, and are known as 

 the allantoic arteries. In the embryonic sauropsida and mam- 

 mals the allantois becomes greatly developed, and grows out 

 <^sauropsida) into close connection with the egg shell, or (mam- 

 mals) forms the placenta (see Mammalia) which enters into 

 intimate relations with the uterine walls of the mother. Thus 

 in the amniotes the allantois becomes an important organ of 

 respiration, and in mammals of nutrition, and the arteries which 

 reach this distal portion through the umbilicus become very 

 important. After birth (hatching) respiration and nutrition 

 are accomplished in other ways, and these allantoic vessels con- 

 sequently degenerate. 



With the appearance of legs, the iliac arteries increase in 

 importance. Of these there are two, an anterior external and 

 a more posterior internal, the latter arising from the hypogastric 

 trunk. External and internal iliacs many arise separately from 



