28 MAMMALIA. 



tremity is terminated by the foot, which is composed of parts 

 analogous to the hand> viz. a tarsus, metatarsus, and toes. 



The head in the Mammalia is always articulated by two 

 condyles upon the atlas or first vertebra. The brain is com- 

 posed of two hemispheres, united by a medullary lamina cal- 

 led the corpus callosum, and contains two ventricles, enclosing 

 four pairs of tubercles, called corpora striata, the thalami op- 

 tici, nates, and testes. Between the thalami optici is a third 

 ventricle communicating with the fourth situated beneath the 

 cerebellum. The crura of the cerebellum form always under 

 the medulla oblongata a transverse prominence called pom 

 Varolii. 



The eye, always lodged in its orbit, is protected by two eyelids 

 and a vestige of a third. Its crystalline lens is fixed by the 

 ciliary process and its cellular sclerotic coat. 



In the ear there is always found a cavity shut up by a mem- 

 brane called the tympanum, with four little bones ; a vestibule 

 at the entrance of which one of these bones is placed, and which 

 communicates with three semicircular canals ; finally, a spiral 

 canal termed the cochlea, which terminates by one of its canals 

 in the tympanal cavity and by the other in the vestibule. 



The cranium is divided into three compartments. The an- 

 terior part is formed of the two frontal bones and the ethmoid, 

 the intermediate by the parietal and the sphenoid bones ; and 

 the posterior by the occipital bone. Between the occipital, the 

 parietal, and the sphenoid, are inserted the temporal bones, 

 which, to a certain extent, belong to the face. 



In the foetus the occiput is divided into four parts, the body 

 of the sphenoid into two, and three of its pairs of alae are se- 

 parate; the temporal bone into three, of which one serves to com- 

 plete the cranium, another to enclose the labyrinth of the ear, 

 the third to form the walls of its cavity, &c. These portions of 

 the cranium unite more or less quickly, according to the species, 

 and end by perfect union in the adult. 



The face is formed by the two maxillary bones, between which 

 the nasal canal passes. Before these are two intermaxillary, 

 behind two palate bones, and between them descends the 

 single plate of the ethmoid bone, named the vomer. At the 

 entrance of the nasal canal are the bones which form the nose. 



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