MAMMALIA. 31 



As their quantity of respiration is moderate, they are designed in 

 general for walking on the earth ; but with vigorous and continued 

 steps. The forms of the articulations of their skeleton are, conse- 

 quently, strictly denned, which determines all their motions with the 

 most rigorous precision. 



Some of them, however, by means of limbs considerably elongated, 

 and extended membranes, raise themselves in the air ; others have them 

 so shortened, that they can move with facility in water only, though 

 this does not deprive them of the general characters of the class. 



The upper jaw, in all these animals, is fixed to the cranium ; the 

 lower is formed 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 before, 

 by cartilage, to a sternum consisting of several vertical pieces; their 

 anterior extremity commences in a shoulder-blade that is not articulated, 

 but simply suspended in the flesh, often resting on the sternum by 

 means of an intermediate bone, called a clavicle. This extremity is 

 continued by an arm, a fore-arm, and a hand, the latter being com- 

 posed 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 posterior 

 extremity, in all animals of this class, is fixed to the spine, forming 

 a girdle or pelvis, which, in youth, consists of three pair of bones ; the 

 ilium which is attached to the spine, the pubis which forms the ante- 

 rior 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 simi- 

 lar 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 

 medullary layer, called the corpus callo&um, containing the ventricles, 

 and enveloping four pair 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 always form a transverse prominence under the medulla 

 oblongata, called the pans Varolii, or bridge of Varolius. 



The eye, invariably lodged in its orbit, is protected by two lids and 



