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MAMMALIA. 39 



This extremity is continued by an arm, a fore-arraj and a 

 hand, the latter being composed of two ranges of small bones 

 called the carpus, of another range called the metacarpus, and 

 of the fingers, each of which consists of two or three bones, 

 termed phalanges. 



With the exception of the cetacea, the first part of the pos- 

 terior extremity, in all animals of this class, is fixed to the 

 spine, forming 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 terminated by parts 

 similar to those of the hand, i. e. by a tarsus, metatarsus and 

 toes. 



The head of the mammalia is always articulated by two con- 

 dyles, 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 

 ventricles, and enveloping four pairs of tubercles, named the 

 corpora striata, or striated bodies, the thalami nervorum op- 

 ticorum, or beds of the optic nerves, and the nates and testes. 

 Between the optic beds is a third ventricle, which communi- 

 cates with a fourth under the cerebellum, the crura of which 

 always form a transverse prominence under the medulla ob- 

 longata, called the yjons Varolii, or bridge ofVarolius. 



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 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 call- 

 ed the membrana tympani, and contains a chain of four little 

 bones, named the iyicus or anvil, malleus or hammer, the os 

 orbiculare or circular bone, and the stapes or stirrup ; a ves- 

 tibule, on the entrance of which rests the stapes, and which 

 communicates with three semicircular canals j and, finally, a 



