30 ZOOLOGY. 



innominate artery. Posteriorly tho arch is continued into 

 a dorsal aorta, which soon comes to lie in the middle line 

 on the dorsal side of the ccelom. From this two kinds 

 of arteries come off median and paired. 



The most important median ones are those which run 

 into the mesentery to supply the abdominal portion of the 

 alimentary canal. These are three in number, two very 

 large ones, not far behind the diaphragm, and a third much 

 smaller and farther behind. The first is the coeliae artery, 

 which divides very soon into a gastro-hepatic and a lieno- 

 gastric : these (as their names imply) convey blood to the 

 capillaries of the stomach, liver, and spleen. The second 

 is the superior or anterior mesenteric, supplying the greater 

 portion of the intestines. The third is the inferior or 

 posterior mesenteric, supplying the rectum. 



Smaller median arteries go off dorsalwards, to the 

 vertebral column and spinal cord, at intervals corresponding 

 to the several vertebrae. 



The paired arteries given off from the dorsal aorta corre- 

 spond very closely with the veins entering the postcaval : 

 thus there are phrenic, renal (with dorso-lumbar), genital, 

 ilio-lumbar, and external and internal iliacs. The two last- 

 named, however, start off together as common iliacs. 



14. Comparison of Arteries and Veins. A comparison 

 of the course of the veins and arteries in the rabbit gives 

 some interesting results. Almost all the smaller arteries 

 run side by side with the corresponding veins, just as the 

 up and down lines of a railway run side by side : obviously 

 what is the best route for the one is also the best for the 

 other. A certain number of the larger vessels show the 

 same arrangement this is the case with the subclavians, 

 renals and iliacs ; it is also so with the vessels of the mesentery 

 and intestines, except for the very largest, where the 

 existence of a portal system makes it impossible. But when 

 we come to the very largest vessels of all, the attempt to 

 compare particular veins and particular arteries breaks down 

 altogether. The portal and hepatic veins cannot be compared 

 with the co3liac and mesenteric arteries ; nor the jugulars 

 with the carotids. There are no arteries answering to the 



