MAMMALIA. 39 
This extremity is continued by an arm, a fore-arm, 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 tubereles, ‘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 pons Varolit, 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 simply cellular. 
The ear always contains a cavity called the tympanum, or 
~ drum, which communicates with the mouth by the L'ustachian 
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 ineus 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; and, finally, a 
