28 MAMMALIA. 
Some of the Mammalia, however, can raise themselves in the air, by 
means of elongated limbs connected by extensible membranes; others have 
their imbs so much shortened, that they can move with facility only in 
water; but these cireumsvances by no means exclude them from the cluss 
to which they are allied, by other essential characters. 
All the Mammalia have the upper jaw fixed to the craarum; the lower 
is composed of two pieces, articulated by a projecting condyle to a fixed 
temporal bone. The neck is composed of seven, and, in one species, of nine 
vertebre. The anterior ribs are attached to a sternum, formed of a number 
of pieces, placed vertically. Their anterior extremity commences at the 
scapula, which is not articulated to any other bone, but simply suspended in 
the muscular attachments, and often resting on the sternum, by an interme- 
diate bone, denominated the clavicle. This extremity is ccntinued by an 
arm, a fore-arm, and a hand, which last is formed of two rows of little 
bones, called the carpus, of another row named the metacarpus, and of 
fingers, each composed of two or three bones, called phalanges. 
With the exception of the Cetacea, all this class have the first part of the 
posterior extremity fixed to the spine. This part, in the form of a girdle, 
or basin, is named the pelvis. In youth, it is divided into three pairs of 
bones,—the os iliwm, which is attached to the vertebral column; the os 
pubis, which forms the anterior part; and the ‘ischium, which forms the 
posterior portion. At the junction of these three bones, is the cavity where 
the bone of the thigh is articulated, to which again is joined the leg, com- 
posed of two bones, the ¢idia and the fibula. This extremity is terminated 
by the foot, which is composed of parts analogous to the hand, viz; a tarsus, 
metatarsus, and toes. 
The head, in the Mammalia, is always articulated by two condyles. spon 
the atlas, or first vertebra. The brain is composed of two hemispheres, 
united by a medullary lamina, called the corpus callosum, and contains two 
ventricles, inclosing four pairs of tubercles, called corpora striata, the thalami 
optici, nates, and testes. Between the thalami optici is a third ventricle, com- 
municating with the fourth, situated beneath the cerebellum. The crura 
of the cerebellum form always under the medulla oblongata, a transverse 
prominence, called pons Varoli. 
The eye, always lodged in its orbit, is protected by two eyelids, and a 
vestige of a third. Its crystalline lens is fixed by the ciliary processes, and 
its cellular sclerotic coat. 
In the ear there is always found a cavity, shut up by a membrane, called 
the tympanum, with four little bones; a vestibule, at the entrance of which 
one of these bones is placed, and which communicates with three semicir- 
cular canals; finally, a spiral canal, termed the cochlea, which terminates 
by one of its canals in the tympanal cavity, and by the other into the vesti- 
bule. The cranium is divided into three compartments. The anterior part 
is formed of the two frontal bones and the ethmnoid; the intermediate, by the 
parietal and the sphenoid bones; and the posterior, by the occipital bone. 
