74) CLASS MAMMALIA. 



which are analogous to those of the hand, and are 

 called the tarsus, metatarsus, and toes. 



The head of the mammalia is always articulated 

 by two condyles upon the atlas or first vertebra. 



Their brain is composed of two hemispheres, 

 united by a medullary lamina called the corpus cal- 

 losum, containing two ventricles and enclosing four 

 pair of tubercular, or eminences, named the corpora 

 striata or striated bodies, thalami optici or optic beds, 

 nates and testes. Between the optic beds is a third 

 ventricle which communicates with a fourth situated 

 beneath the cerebellum. The crura of their cere- 

 bellum always form under the medulla oblongata a 

 transverse prominence denominated the tuber annu- 

 lare or bridge of Varolius. 



Their eye, lodged invariably in its orbit, and pro- 

 tected by two eyelids and a vestige of a third, has 

 its crystalline humour fixed by the ciliary process. 

 Its sclerotic coat, or investing tunic, is simply cellular. 



In their ear there is always found a cavity called 

 the tympanum or drum, closed from without by a 

 membrane named the membrana tympani; there are 

 also four small bones, called the incus or anvil, mal- 

 leus or hammer, stapes or stirrup, and q$0 orbiculare, 

 or spheroid bone; a vestibule, at the entrance of 

 which is placed the stapes, and which communi- 

 cates with three semicircular canals ; finally, a 

 volute, or spiral canal termed the cochlea, which 

 terminates, by one of its seala? or canals, in the 

 tympanum or cavity, by the other, in the ves- 

 tibule. 



