60 THE ORGANS OF SPECIAL SENSE. 



Its base or root is directed backward and downward and 

 is attached by muscles to the hyoid bone and the lower 

 jaw, the hyoid bone being a horseshoe-shaped bone lying 

 just below and as it were within the inferior maxillary. 

 The base of the tongue is attached also to the epiglottis 

 and at the sides to the soft palate by the anterior pillars. 

 Except at its base and the posterior part of its under 

 surface the tongue is free, but a fold of mucous membrane, 

 thefrenum, holds it somewhat in front. Thus it possesses 

 great versatility of motion and serves as an auxiliary in 

 articulation, mastication, and deglutition. 



The Teeth. Securely embedded in either jaw are the 

 teeth, nature's instrument for the first preparation of the 

 food for digestion through tearing and grinding. The 

 incisors, which are in front, have wide sharp edges for 

 cutting the food. Next come the canine teeth with a 

 sharp point for tearing it, while at the back are the molars 

 with a broad flat top for grinding. 



There are two sets of teeth: 1. the temporary or milk 

 teeth, twenty in number four incisors, two canines, and 

 four molars in each jaw which appear at from six 

 months to two years, and 2. the permanent teeth, thirty- 

 two in number four incisors, two canines, known as eye 

 teeth in the upper jaw and as stomach teeth in the lower 

 jaw, four bicuspids, so-called because they have two 

 cusps where the molars have four or five, and six molars 

 in each jaw which come from the sixth to the twenty- 

 first years. The first to appear are the two lower middle 

 incisors, which come at the age of six months. The last 

 to appear are the wisdom teeth, the farthest back of the 

 molars, which come at the age of twenty-one years or 

 thereabouts. 



Each tooth consists of a crown or body above the 

 gum, a neck, and a fang or root within the gum. The 

 body is of dentine or ivory with a thin crust of enamel 

 and contains the pulp, a vascular connective tissue 

 containing many nerves. Beginning at the neck and 

 covering the fang is a layer of cement or true bone. 



