OP VIVIPAROUS VERTEBRATA. 71 



similarity is greater or less in proportion as the functions for 

 which the limbs are destined, are more or less alike. Besides 

 this, in all the mammalia, with the exception of the cetacea3, 

 the lower limb commences by a bony girdle or pelvis, formed 

 by the bones of the hip fastened to the spine: at first these 

 bones are formed of three distinct parts, the illium, the pubis, 

 and the ischium. The thigh is formed of one single bone, the 

 leg of two principal ones, the tibia and fibula; the foot which 

 terminates this member is composed of a tarsus, metatarsus, 

 and toes. 



The muscles possess a moderate power of contraction; but 

 their irritability is very dependent on the nervous system. 

 The motion is that of walking. Flight can take place in some, 

 by means of elongated limbs and extended membranes; while 

 others again, having their limbs very much shortened, can only 

 swim. The nervous system of the mammalia is chiefly charac- 

 terized by the state of the brain and cerebellum. The latter has 

 lateral lobes or voluminous hemispheres, and there is always a 

 pons varolii under the spinal medulla. In like manner the 

 cerebrum always has the corpora striata, and is formed of 

 two voluminous hemispheres, furnished with circumvolutions, 

 forming two lateral ventricles, connected by the corpus callo- 

 sum. 



The eyes, lodged in the orbits, are defended by two lids, 

 and a vestige of a third; the sclerotica is simply fibrous; the 

 crystalline is fixed by the ciliary processes. In all of them, 

 the ear has a perfect labyrinth, with a cochlea, a. drum and 

 a membrana tympani and small bones. The nasal fossa tra- 

 verse the face, have the ossa turbinata, and extend into the 

 sinuses of the bones. The tongue is fleshy and attached to 

 the os hyoides. The skin of the mammalia, generally is co- 

 vered with hair; the cetaceae alone are wholly deprived of it. 



The intestinal canal is covered with the peritoneum, sus- 

 pended to the mesentery, a fold of that membrane which en- 

 closes the conglobate glands of the chyliferous vessels, and 

 covered with a floating extension of the same membrane, 

 called the epiploon. They have a urinary bladder, that 

 opens, with very few exceptions, into the orifice of the organs 

 11 



