LEPUS CUNICULUS 313 



line and opens into the postero-dorsal wall of the right atrium. 

 The internal iliac veins drain the back of the thighs, and run in the 

 dorsal wall of the pelvic cavity, at the anterior end of which they 

 unite to form the post-caval. A short distance in front of this 

 the two external iliacs enter the main trunk. They are large 

 vessels coming from the hind limb along the pre-axial side of 

 which they run as the femoral veins. In the abdominal cavity 

 they receive small veins, including a vesicular, from the urinary 

 bladder, and in the female rabbit also branches from the uterus. 

 Just in front of this again the post-caval receives the paired 

 ilio-lumbar veins, which bring blood back from the posterior 

 abdominal waUs. Still more anteriorly are the genital veins, 

 the spermatic veins in the male and ovarian veins in the female. 

 In the female they pass practically straight outwards, but in the 

 male they pass outwards a short way and then backwards in company 

 with the corresponding artery into the scrotal sacs. The renal veins 

 which open more anteriorly, although paired, are asymmetrical, 

 the right being shorter, about three-quarters of an inch long and 

 in front of the left. A certain amount of variation is to be found in 

 the veins of the posterior end of the abdomen, for the left spermatic 

 vein sometimes enters the ilio-lumbar or the renal of the same side 

 instead of entering the post-caval separately. Large hepatic veins 

 return blood from the various lobes of the liver, and enter the post- 

 caval as it passes through this gland. As a rule about four large 

 trunks can be distinguished coming from the liyer. The diaphragm 

 is drained by the small phrenic veins running in its substance, the 

 most ventral of these open into the pre-caval veins, but the others 

 flow into the post-caval as it passes through the central tendon. 



It will be noticed that there is a much closer correspondence 

 between the arteries and veins in Lepus than in Rana, and indeed 

 over the greater part of the system we find that the main vessels 

 of the two systems accompany one another. 



Attention has already been called to the fact that the 

 heart is a very important organ, but as it has only been dealt with 

 in broad outline, we may return to the question here. 



Mammalian Heart. 



The heart of the mammal is an important organ, and as it 

 is so small in the rabbit it is advisable to study its structure in a 

 larger animal such as the pig or the sheep. It should be borne in 

 mind that there are slight differences in the relationship of the main 

 veins to the atria and the main branches of the arteries in the three 

 types. They are only of minor importance, however, and do not 

 affect the general plan, so that they need not be considered here. 



