MAMMALIA 1515 



the crura of which always form a transverse promi- 

 nence under the medulla oblongata, called the pons 

 Varolii, or bridge of Varolius. 



The eye, invariably lodged in its orbit, is pro- 

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

 its crystalline fixed by the ciliary processes its 

 sclerotic is simply cellular. 



The ear always contains a cavity called the 

 tympanum, or drum, which communicates with the 

 mouth by the Eustachian tube; the cavity itself is 

 closed externally by a membrane called the mem- 

 brana tympani, and contains a chain of four little 

 bones, named the incus or anvil, malleus or hammer, 

 the os orbiculare or circular bone, and the stapes 

 or stirrup; a vestibule, on the entrance of which rests 

 the stapes, and which communicates with three semi- 

 circular canals; and finally a cochlea, which termi- 

 nates by one canal in the vestibule and by the other 

 in the tympanum. 



Their cranium is subdivided into three portions; 

 the anterior is formed by the two frontal and ethmoi- 

 dal bones, the middle by the two ossa parietalia and 

 the os ethmoides, and the posterior by the os occi- 

 pitis. Between the ossa parietalia, the sphenoidalis, 

 and the os occipitis are interposed the two temporal 

 bones, part of which belong properly to the face. 



Their face consists of two maxillary bones, be- 

 tween which pass the nostrils; the two intermaxil- 

 laries are situated before and the two ossa palati 

 behind them; between these descends the vomer, a 

 bony process of the os ethmoides; at the entrance of 

 the nasal canal are placed the ossa nasi; to its ex- 



