426 ELEMENTARY ANATOMY. [LESS. 



as in the Monotremes. Instead of a single superior cava 

 there may be two, the blood from the right and left sides of 

 the head and pectoral limbs being gathered by two entirely 

 distinct sets of vessels, each set ending respectively in a right 

 and in a left vena cava, the two venae cavas each opening by a 

 distinct aperture into the right auricle. This is the case in 

 very many Mammals, e.g. in the Rabbit. The middle sacral 

 vein of course increases in importance with the increase of 

 the coccygeal region being thus very large in the Cetacea. 



Below man's class, and that of Birds, the blood from the 

 caudal region and the pelvic limbs may enter the kidneys and 

 be therein re-distributed by ramifying branches, similar to 

 the re-distribution of the blood by the portal circulation in 

 the liver. This is the case, e.g., in Batrachians. Here, how- 

 ever, part of the blood from these sources is carried on, by 

 abdominal veins, directly to the liver. But these veins may 

 go directly to the vena cava inferior, as in Birds. 



The great veins, before entering the heart, may dilate into a 

 rhythmically contractile sinus venosus, as in most Batrachians 

 and Reptiles. 



The blood of the body may be brought back by cardinal 

 veins and hepatic veins exclusively, as is the case in Fishes. 

 These cardinal veins consist, on each side of the body, of an 

 anterior one receding from the head and a posterior one 

 advancing fom the tail the anterior and posterior cardinal 

 vein of each side uniting to form a venous trunk (the ductus 

 Cuvieri) which descends from the point at which these join, 

 to the sinus venosus of the heart ; thus exactly reproducing 

 the primitive condition of man's venous system. 



The portal vein itself may be rhythmically contractile, as 

 in Myxinevcs\di A mphioxus ; or the caudal vein may possess 

 a pair of small contractile vesicles, as in the Eel. The root 

 veins of the limbs may be contractile, as in many Batra- 

 chians, or veins so remote from the centre of the circula- 

 tion as those which traverse the wing membranes of Bats 

 may be similarly contractile. 



Finally, as in Amphioxus, we may find many veins to be 

 possessed of this property. 



The veins may be destitute of valves, even in man's own 

 class, as e.g. in the Cetacea. 



n. We see, then, that not only structurally, but also 

 physiologically, the circulating system, even in man's own 

 sub-kingdom, may present very important divergences from 

 the conditions we find in him. 



