MAMMALIA. 29 



The molar or cheek bone of each side unites the maxillary to 

 the temporal, and often to the frontal bone; and finally, the la- 

 chrymal cavity occupies the internal angle of the orbit, and some- 

 times part of the cheek. 



The tongue in the Mammalia is always fleshy, and attached 

 to the hyoid bone, which bone is suspended by ligaments to the 

 cranium. 



Their lungs, two in number, are composed of a mass of small 

 cells, inclosed without adhesion in a cavity formed by the sides 

 of the diaphragm, and lined by the pleura. Their organ of voice 

 is at the upper extremity of the trachea or windpipe ; and a 

 fleshy continuation, named velum palati, establishes a direct 

 communication between their larynx and the back part of their 

 nostrils. 



Living on the earth's surface, as do the greater part of the 

 Mammalia, they are exposed to alternations of heat and cold, 

 and their bodies have in consequence a covering of hair, which 

 is thicker in the colder, and more scanty in the warmer 

 regions. The Cetacea, which inhabit the sea, are, however, to- 

 tally destitute of this covering. 



The intestinal canal of the mammiferous animals is suspend- 

 ed by a fold of the peritonaeum, called the mesentery, which 

 contains numerous conglobate glands for the lacteal vessels. 

 Another production of the peritonaeum, named the epiploon, 

 hangs before and beneath the intestines. 



The generation of the Mammalia is essentially viviparous. 

 The foetus after conception descends into the uterus, to the 

 inner surface of which it is attached by means of an arrange- 

 ment of vessels termed the placenta, through the medium of 

 which nourishment is derived. The young for some time after 

 birth are nourished by a particular secretion of the mother (milk,) 

 produced in the mammiferous animals after parturition, and 

 drawn by the young from mammce or teats. It is from this last 

 character that the term Mammalia has been applied to this Class 

 a character exclusively proper to them, and by which they are 

 more easily recognized than by another external distinction. 



The total number of mammiferous animals described, accord- 

 ing to Desmarest, is about 850, including, however, many species 



