470 MAMMALIA. 



a muscular partition, called the diaphragm, the 

 movements of which, by enlarging the chest, are the 

 principal cause of the inspiration of air. Tlie heart 

 is double — that is, consists of two ventricles for the 

 l^ropulsion of blood through the arteries, and two 

 auricles for its reception from the veins. The mouth 

 is closed by fleshy lips, and the skin, with but few 

 exceptions, is covered with hair. The teeth in 

 Mammalia are organs of great importance to the 

 Zoologist. They are generally placed in single 

 series, and vary much in their form, according to the 

 nature of the food, as well as according to their 

 position in the mouth. In man, there are in each 

 half of e^ich jaw two front teeth, having the name of 

 incisors or cutting teeth ; one more pointed, called 

 the canine or dog-tooth, or sometimes the eye-tooth ; 

 two somewhat flattened at the top, with single fangs, 

 called false molars, and three, situated behind all 

 the rest, called true molars or grinders. For sim- 

 plicity, naturalists have invented the following tabu- 

 lar method of expressing the number and arrange- 

 ment of the teeth, which is called the dental 

 formula : — 



I^^ C-^-^ FM^-^ M^-^^32 

 2-2 1-1 2-2 3-3 ' 



meaning— incisors, two on each side in the upper and 

 two on each side in the lower jaw — canines, one on 

 each side in the upper and one on each side in the 

 lower jaw — false molars, two on each side in both 

 upper and lower jaws — molars, three on each side in 

 botli jaws — making in all thirty-two teeth. 



In those races which feed exclusively on flesh, the 

 molar teeth partake of a cutting character, while in 

 those that subsist on grain and herbage, the molar 

 or grinding structure prevails. Sometimes the 

 incisors are curiously developed ; in the Squirrels, 

 liats, and other similar animals, they project for- 

 wards, and are continually growing; in the Ele- 

 phant they stand out in the form of huge curved 



