Class 1. 



J.IAMMAXIA. 



27 



Fiff. a. 



lliey have all the upper jaw fixed to the skuU, and the lower composed of two 

 pieces only, articulated by a projecting condyle to a fixed temporal hone ; the neck 



consists of seven vertebrse, one smgle species excepted, 

 v/hich 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. This extremity is continued by an arm, a 

 fore-arm, and a hand, the last composed of two ranges 

 of small bones, called a MTist 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 m.ammalians 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 corjnis callosum, containing two ventricles, and envelopmg the four pairs of tuber- 

 cles named the corpora striata, the thalami nervorum opticorum, or beds of the optic 

 nerves, and the iiates and testes. Between the optic beds is a third ventricle, which 

 communicates v.'ith a fourth situated under the cerebellum, the crura of which always 

 form a transverse prominence under the medulla oblongata, called the ^;ohs Varolii. 



Their 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 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 stimip bones ; a vestibule, on the entrance of which rests the stirrup- 

 bone, and which communicates Avith three semicircular canals ; and, finally, a cochlea, 

 v.'hich 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 alUuled to, in which, however, distinct rudiments of ribs arc attaciicd to the ci^jhth nnd ninth, as shown in the above fi^nire 

 '(/, h) ; so that, in rcalitv, this coDstUutcs no excejition to the universal rule. — Kd. 



