DIGESTION OF FOOD: THE MOUTH 



75 



so that they can be pushed back toward the throat more 

 easily. This living mucous membrane and the outer skin 

 of the body come together at the lips. They are much alike 

 in structure save that 

 the blood vessels 

 come nearer the sur- 

 face in the mucous 

 membrane and make 

 it look redder. 



The Teeth. Back of 

 the lips, which aid 

 slightly in holding and 

 directing the food, are 

 the teeth. These, by 

 catting, tearing and 

 grinding the food, pre- 

 pare it for digestion. 

 Their shapes are ad- 

 mirably adapted for 

 this work, which is 

 called mastication. 



The teeth of each 

 side of each jaw, begin- 

 ning at the middle in front, comprise two incisors, one canine, 

 two bicuspids and three molars, or grinders. The incisors (Fig. 

 36 /) with chisel edges are used almost exclusively for cutting 

 pieces from large morsels; in chewing they come into action 

 very little. The canines (Fig. 36 C), named from their similarity 

 to the tearing, tusk-like teeth of dogs, are of no great service 

 to civilized man, who, though he may eat fruits without first 

 cutting them, usually cuts his other foods with knife and 

 fork. The bicuspids (Fig. 36 B), so called from the two prom- 

 inences, or cusps, on their free ends, are of use partly for tear- 

 ing, and partly for grinding. The molars (Fig. 36 M), or heavy, 

 many-cusped " back teeth," are solely for grinding. Their 



.. Artery 

 Nerve 



FIG. 36. SHOWING THE UPPER JAW OF A 

 CHILD AND THE LOWER JAW OF AN ADULT 



In the upper figure Bu indicates the buds of 

 the permanent teeth nearly ready to push out 

 the first set, or milk teeth. 



