82 MAMMALIA. 
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 condyles, with 
the atlas, the first vertebra of the neck. 
The brain is always composed of two hemispheres, united by a medul- 
lary layer, called the corpus callosum, containing the ventricles, and en- 
veloping 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 al- 
ways form a transverse prominence 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 simply cellular. 
The ear always contains a cavity called the tympanum, or drum, which 
communicates with the mouth by the Kustachian tube; the cavity itself is 
closed externally by a membrane called the membrana tympani, and con- 
tains a chain of four little bones, named the zneus 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 communi- 
cates with three semicircular canals; and, finally, a cochlea, which ter- 
minates 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 parie- 
talia 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 sphenoi- 
dal into two halves, which are again subdivided into three pairs of lateral 
wings; the temporal into three, one of which serves to complete the cra- 
nium, the second to close the labyrinth of the ear, the third to form the 
parietes of the tympanum, &c. These bony portions, still more numerous 
in the earliest period of the foetal existence, are united more or less 
promptly, according to the species, and the bones themselves finally be- 
come consolidated in the adult. 
Their face consists of the two maxillary bones, between which pass the 
nostrils; the two intermaxillaries 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 
