76 THE AMERICAN BEAVER. 



In the beaver the inferior cava begins to enlarge 

 opposite the kidney. The largest part of the sinus is 

 where it receives the hepatic veins. After passing 

 through the diaphragm it contracts to its original size. 

 The four hepatic veins are also capable of containing 

 a large quantity of blood, the largest readily admit- 

 ting the adult fore finger. On opening the vena cava 

 in its length, its linear width, opposite the kidney, is 

 two inches; in the hepatic fissure it is three inches; 

 and before reaching the right auricle it is two inches. 

 The capacity of the venous sinus is not fully indi- 

 cated by these measurements, as the vein probably 

 yields to distention. The " sigmoid valves," described 

 by M. Sarrasin, are merely the openings of the three 

 hepatic veins seen from above. The blood corpuscles 

 of the beaver measure 37,00" in diameter. The mean 

 of 24 rodents, as given in Gerber's Anatomy, is 375?". 

 The eye of the beaver is small. The optic nerve is 

 but 5'" in diameter. In decussating within the skull 

 the nerve of the right side passes under the left. The 

 reputed sagacity of the beaver is not accounted for 

 by the size or development of the brain. The impla- 

 cental mammals (marsupials and monotremes) are 

 the lowest of the mammiferous class, according to 

 Prof. R. Owen; their brains resembling those of birds, 

 in the absence of the great commissure, or corpus 

 callosum. The brains of rodents are a step in ad- 

 vance. The beaver brain is entirely smooth on the 

 surface, and, although the cerebellum is uncovered, 

 the posterior development is greater than in the mar- 

 supials. The olfactory lobe is large. The optic lobes 

 are covered. Width of cerebral hemisphere, 83'"; of 

 corpus callosum, 60'"; length of brain before removal, 



