78 Vertebrata. 



numerator of a fraction, while those of the lower are 

 placed as the denominator; thus, in an adult man 



the dental formula is /- , C^=, P^^, M^^ ; 

 22 i i 22 33' 



that is, on each side of each jaw there are two in- 

 cisors, on each side of each jaw one canine, two pre- 

 molars, and three molars. The jaws are almost 

 always protected by fleshy lips, except in the first 

 order. 



52. Viscera. Mammals have well developed 

 brains, and usually acute sense-organs. The lungs 

 and heart are separated from the intestine and other 

 digestive organs by a muscular partition, called the 

 diaphragm, which is an important agent in breathing. 

 The heart consists of four cavities, and the opening 

 between the right auricle and right ventricle is guarded 

 by a membranous valve consisting of three flaps. 

 There is but one aortic arch in the adult, and it 

 arches to the left side ; there are two carotid arteries 

 for conveying blood to the brain. The blood con- 

 tains round, non-nucleated corpuscles, and therein 

 differs from that of any of the foregoing classes. 



There are seventeen orders of mammals at pre- 

 sent living, but representatives of several additional 

 and most remarkable intermediate orders have been 

 found in a fossil state, especially in the tertiary beds 

 in America. 



