88 ELEMENTS OF COMPARATIVE ZOOLOGY. 



unite in a large vein (mesenterial vein) which follows along 

 the colon, thence forward into an anterior fold, where it is 

 joined by other veins (gastric) from the stomach and 

 (splenic) from the spleen. From the union of these is 

 formed the portal vein which enters the liver from behind. 

 The small arterial branches arise from a mesenterial artery 

 which accompanies the mesenterial vein for some distance 

 and then can be traced back to the median line of the 

 dorsal surface of the body-cavity, where it joins the great 

 arterial trunk, the aorta. From the aorta, just in front of 

 the origin of the mesenterial artery, arises the cceliac artery, 

 which gives off. a branch to the liver (hepatic artery), and 

 then divides into splenic and gastric arteries, going to the 

 spleen and stomach respectively. Trace these arteries. 

 Where does the hepatic enter the liver ? 



The single vessels in the mesenteries are the lymphatics. 

 Their purpose is to carry the products of digestion forward, 

 and eventually empty them into the blood-vessels. These 

 lymphatics unite in a lymphatic duct, which runs closely 

 parallel to the mesenterial artery and empties into a thoracic 

 duct running parallel with the aorta. 



Sketch the blood-vessels (X 2) so far made out, on a 

 sheet large enough to accommodate the whole circulatory 

 apparatus of the rat. 



In the mesentery supporting the duodenum find the 

 fatty-looking, irregular pancreas. Where does its duct 

 enter the intestine ? 



How many lobes are there in the liver ? Are they sym- 

 metrically placed ? Beside the portal vein and the hepatic 

 artery is the bile-duct. Trace it forward and see how its 

 branches arise from the liver-lobes. Trace it backwards and 

 see where it enters the intestine. Look on the posterior 

 surface of liver for the gall- bladder. Tip the liver back- 



