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 limbs so much sho-tened, that they can move with facility only in 

 water ; but these circumstances by no means excludp them from the class 

 to which they are allied, by other essential characters. 



All the Mammalia have the upper jaw fixed to the cranium ; the lower 

 is composed of two pieces, articulated by a projecting ondyle to a fixed 

 temporal bone. The neck is composed of seven, and, in one species, of nine 

 vertebrae. The anterior ribs are attached to a sternum, foraied 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 continued 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 05 ilium, which is attached to the vertebral column; the os 

 pubis, which forms the anterior part ; and the ischium, which forms the 

 posteiior 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 tibia 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, upon 

 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 Varolii. 



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 ethmoid; the intermediate, by the 

 parietal and the sphenoid bones ; and the posterior, by the occipital bone. 



