42 MAMMALIA. 



a girdle or pelvis, which, in youth, consists of three pairs of bones, 

 the ilium which is attached to the spine, the pubis which forms the 

 anterior part of the girdle, and the ischium, the posterior. At the 

 point of union of these three bones is situated the cavity with which 

 the thigh is articulated, to which, in its turn, is attached the leg, 

 formed of two bones, the tibia and fibula; this extremity is termi- 

 nated by parts similar to those of the hand, i. e. by a tarsus, meta- 

 tarsus and toes. 



The head of the mammalia is always articulated by two condyles, 

 with the atlas, the first vertebra of the neck. 



The brain is always composed of two hemispheres, united by a 

 medullary layer, called the corpus callosum^ containing the ventri- 

 cles, and enveloping four pairs of tubercles, named the corpora 

 striata or striated bodies, the thalami nervorum opticorum or beds 

 of the optic nerves, and the nates^ and testes. Between the optic 

 beds is a third ventricle, which communicates with a fourth under 

 the cerebellum, 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 protected by two lids 

 and a vestige of a third, and has its crystalline fixed by the ciliary 

 processes — its sclerotic is sim.ply 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 membrana 

 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 semicircular canals; and 

 finally, a cochlea, which terminates 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 ethmoidal bones, the middle by the 

 two ossa parietalia and the os ethmoides, and the posterior by the 

 OS occipitis. Between the ossa parietalia, the sphenoidalis and the 

 OS occipitis, are interposed the two temporal bones, part of which 

 belong properly to the face. 



In the foetus, the occipital bone is divided into four parts: the sphe- 

 noidal into two halves, which are again subdivided into three pairs 

 of lateral wings; the temporal into three, one of which serves to 



