MAMMALIANS. 39 
They have all the upper jaw fixed to the skull, and the lower composed of two 
pieces only, articulated by a projecting condyle to a fixed temporal bone ; the neck 
consists of seven vertebrae, one single species excepted, 
which has nine*; the anterior ribs are attached in 
front, by cartilage, to a sternum formed of a certain 
number of pieces placed in a row ; their fore-limb 
commences in a blade-bone, which is not articulated, 
but merely suspended in the flesh, often resting on 
the sternum by means of an intermediate bone, called 
a clavicle. Tkis extremity is continued by an arm, a 
fore-arm, and a hand, the last composed of two ranges 
of small bones, called a wrist or carpus, of another 
range of bones termed metacarpus, and of digits or 
fingers, each of which consists of two or three bones, 
named phalanges. 
Excepting the Cetacea, they have all the first part of 
the hinder extremity fixed to the spine, and 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 fore part of the girdle, and the ischium, which constitutes 
the hind part. At the point of union of these three bones is situate 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 the foot, which is composed of 
parts analogous to those of the hand, namely, a tarsus, metatarsus, and digits or toes. 
The head of mammalians is always articulated by two condyles upon the atlas, or 
first vertebra. 
Their brain is composed of two hemispheres, united by a medullary layer termed 
the corpus callosum, containing two ventricles, and enveloping the four pairs of tuber- 
cles named the corpora striata, the thalami nervorum opticorum, or beds of the optic 
I nerves, and the nates and testes. Between the optic beds is a third ventricle, which 
I communicates with a fourth situated under the cerebellum, the crura of which always 
I form a transverse prominence under the medulla oblongata, called pons Varolii. 
I Their eye, invariably lodged in its orbit, is protected by two lids and a vestige of a 
j third, and has its crystalline fixed by the ciliary process and its simply cellular sclero- 
tica [or white] . 
In their ear, there is always found a cavity named the drum, or tympanum, which 
communicates with the back part of the mouth, by a canal termed the trumpet, or 
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 hammer, anvil, 
orbicular, and stirrup bones ; a vestibule, on the entrance of which rests the stirrup- 
bone, and which communicates with three semicircular canals ; and, finally, a cochlea, 
which terminates by one passage in the drum, and by another in the vestibule. 
Their cranium subdivides into three portions : the anterior is formed by the two 
frontal and the ethmoidal bones ; the middle, by the parietal bones and the sphenoidal ; 
* The sloth is alluded to, in which, however, distinct rudiments of ribs are attached to the eighth and ninth, as shown in the above figure 
(«, V) ; so that, in reality, this constitutes no exception to the universal rule. — Kd. 
Fiff. 2. 
