STRUCTURE OF THE ELEMENTARY TISSUES. 61 



This is due to the growth of the permanent teeth, which push their way 

 up from beneath, absorbing in their progress the whole of the fang of each, 

 milk-tooth and leaving at length only the crown as a mere shell, which, 

 is shed to make way for the eruption of the permanent teeth (Fig. 66). 



The temporary teeth are ten in each jaw, namely, four incisors, two 

 canine,*, and four molars, and are replaced by ten permanent teeth, each 

 of which is developed in a way almost exactly similar to the manner of 

 development already described, from a small process or sac set by, so to 

 speak, from the enamel germ of the temporary tooth which pfecedes it, 

 and called the cavity of reserve. 



The number of permanent teeth in each jaw is, however, increased to six- 

 teen, by the development of three others on each side of the jaw after much 

 the same fashion as that by which the milk-teeth were themselves formed. 



FIG. 66. Part of the lower jaw of a child of three or four years old, showing the relations of the 

 temporary* and permanent teeth. The specimen contains all the milk teeth of the right side, to- 

 gether with the incisors of the left; the inner plate of the jaw has been removed, so as to expose the 

 sacs of all the permanent teeth of the right side, except the eighth or wisdom tooth, which is not yet 

 formed. The large sac near the ascending ramus of the jaw is that of the first permanent molar, and 

 above and behind it is the commencing rudiment of the second molar. (Quain.) 



The beginning of the development of the permanent teeth of course 

 takes place long before the cutting of those which they are to succeed. 

 One of the first steps in the development of a milk-tooth is the out- 

 growth of a lateral process of epithelial cells from its primitive enamel 

 organ (Fig. 63, c, f p}. This epithelial outgrowth ultimately becomes 

 the enamel organ of the permanent tooth, and is indented from below by 

 a primitive dental papilla, precisely as described above. 



The following formula shows, at a glance, the comparative arrange- 

 ment and number of the temporary and permanent teeth: 



Mo. Ca. In. Ca. Mo. 



( Upper 2 



Temporary Teeth 4 -=20 



(Lower 21412 =10 



Mo. Bi. Ca. In. Ca. Bi. Mo. 



( Upper 3 2 



Permanent Teeth 1 =32 



( Lower 321412 3 = 16 



