46 MAMMALIA. 



valves, agrees for the most part with that of Man, though there 

 sometimes occurs, as in the aorta, a sacciform expansion of its com- 

 mencement, e. g. in the Narwhal, and in a less degree in many of 

 the Dolphins also. The number of pulmonary veins varies consider- 

 ably, and there frequently occur upon one side a greater number than 

 upon the other (3+2), a circumstance chiefly occasioned by the 

 number of the lobes of the lung. 



Valves occur in the Veins of the body, and frequently, even as in 

 the Ox, in the portal veins, where they are wanting in Man. The 

 trunk of the superior vena cava is very frequently double, in indi- 

 vidual animals from all the orders, as in the common Bat, Hedge- 

 hog, Squirrel, Ornithorynchus, Elephant ; as a rule, however, it is 

 single, as in the Apes, Ruminantia, most Carnivora, &c. The 

 inferior vena cava is commonly dilated in diving animals, previous 

 to entering the heart, and while yet within the liver, as in the Seals ; 

 in a less degree also in the Dolphin and Otter, still less so in the 

 Beaver and Ornithorynchus ; in these it forms a true sinus, like 

 that of Fishes. This large size of the veins, in relation to that of 

 the arteries, exerts unquestionably an important influence upon the 

 circulation and the process of diving ; and the discovery is a re- 

 markable one, of a peculiar annular muscle, about an inch in 

 breadth, which is met with in the Seals on the trunk of the inferior 

 vena cava, above the diaphragm and venous sac, and which can 

 cut off the return of blood to the heart. In the Cetacea remarkably 

 developed venous plexuses occur ; one of these lies, e. g. in the canal 

 formed by the inferior spinous processes of the tail ; another much 

 more conspicuous (plexus iliacus) lies between the psoas muscle and 

 the peritoneum. 



The absorbent vessels exhibit in general the same conditions as 

 in Man, in reference to the chyliferous ducts. The lymphatic 

 glands of the mesentery are usually less numerous, and more 

 blended together, than in Man. They sometimes form only a 

 single mass lying at the root of the mesentery, called the Pancreas 

 Asellii (as in the Dog and the Carnivora generally), near to which, 

 however, some smaller lymphatic glands usually occur. This me- 

 senteric gland is most conspicuous in the Cetacea, where the lym- 

 phatic vessels are very much developed. 



The Blood of Mammalia very uniformly presents small, round, 

 disc-shaped corpuscles, very similar to, but n^stly somewhat 

 smaller than in Man ; this is especially the case in the Ruminantia. 

 The largest animals, as the Elephant, have still very small cor- 



