VEINS OF MAMMALIA. 553 



caval before it terminates : these two veins meet at a very acute 

 angle, and are separated by a crescentic ridge similar to, but 

 thinner than, that which divides their common orifice from the 

 orifice of the right precaval. In most Lissencephala the left pre- 

 caval descends behind the auricle to terminate with or near to the 

 postcaval : the left vena azygos communicates with the left pre- 

 caval in the Hedgehog and many others, and is larger than the 

 right : opposite proportions prevail in Leporidce and some other 

 Rodents, as in Squirrels, the left azygos being small or wanting. 

 In the Bats the venules of the wino--membrane, owino; to their 



O ' O 



comparatively wide communications with the arterioles, manifest 

 the impulse of the heart's action. 



In Cetacea the left cardinal and precaval veins are reduced to the 

 condition of a coronary sinus which is large ; the blood from the 

 head and fins is returned by the right precaval trunk to the auricle. 

 The definition of any distinct right or left azygos trunks is ob- 

 scured by the characteristic expansion and plexiform multiplication 

 of the veins at the back of the thoracic-abdominal cavity, fig. 427 ; 

 and indeed the precaval system is chiefly brought into communi- 

 cation with the postcaval one by the continuity of the vast venous 

 sinuses surrounding the neural axis and receiving the intercostal 

 and lumbar veins, ultimately opening into the precaval by a short 

 trunk which penetrates the posterior and right part of the chest. 

 In fig. 427, the postcaval vein is cut across at p, where it lies in 

 the interspace of the two masses of depressor muscles of the tail. 

 Veins, m, m, which seem to answer to the iliacs of quadrupeds, 

 return from the side-muscles of the tail ; the caudal vein is repre- 

 sented by a plexus, /, and occupies much of the haemapophysial 

 canal. A plexus from the intestine, G, terminates in the right 

 iliac vein, and thus establishes a communication between it and 

 ths portal system. A hypogastric plexus terminates at i. The 

 peritoneal plexus is shown at /; it becomes more considerable at 

 the season of sexual excitement : but the chief abdominal reser- 

 voir of venous blood is formed by the vast psoadic plexus, k, which 

 extends from behind the hinder end of the kidney, E, to the 

 hinder end of the abdomen. In the Porpoise it forms a mass of 

 reticulate veins upwards of an inch in thickness : it is fed by the 

 caudal veins, a, b, c 3 d, behind, and laterally by from five or seven 

 veins which, returning blood from the dorsal and lateral parietes 

 of the abdomen, pierce the lateral abdominal muscles to join the 

 plexus at e, e. At the mesial margin the psoadic plexus communi- 

 cates by many and wide apertures with the iliac vein ; and anteriorly 

 with veins of the diaphragm or ( phrenic plexuses ' which con- 



