784 



ORGANS OF DIGESTION. 



behind. While there is a slight diminution in the height of the exposed 

 parts of the teeth from the incisors backwards to the wisdom-teeth, there is 

 in man a general uniformity in the amount of projection of the crowns 

 throughout the whole series. In consequence of the large proportionate 

 breadth of the upper central incisors, the other teeth of the upper jaw are 

 thrown somewhat outwards, so that in closure of the jaws the canine and 

 bicuspid teeth come into contact partly with the corresponding lower teeth 

 and partly with those next following ; and in the case of the molar teeth, 

 each cusp of the upper lies behind the corresponding cusp of the lower 

 teeth. Since, however, the upper wisdom-teeth are smaller than those 

 below, the dental ranges terminate behind nearly at the same point in both 

 jaws. 



The Milk-teeth. The temporary incisor and canine teeth resemble those of 

 the permanent set in their general form ; but they are of smaller dimen- 

 sions. The temporary molar teeth present some peculiarities. The hinder 

 of the two is much the larger ; it is the largest of all the milk-teeth, and 

 is larger even than the second permanent bicuspid, by which it is afterwards 

 replaced. The crown of the first upper milk molar has only three cusps, 



Fig. 542. 



Fig. 542. MILK TEETH OP THE RIGHT SIDE OF THE UPPER AND LOWER JAWS. 

 a, the incisors ; 6, the canines ; c, the molar teeth. 



two external and one internal ; that of the second has four distinct cusps. 

 The first lower temporary molar has four cusps, and the second five, of which 

 in the latter case three are external. The fangs of the temporary molars 

 resemble those of the permanent set, but they are smaller, and are more 

 divergent from the neck of the tooth. 



Structure. On making a section of a tooth, the hard substance of 

 which it is composed is found to be hollow in the centre. The form of the 

 cavity bears a general resemblance to that of the tooth itself : it occupies the 

 interior of the crown, is widest opposite to or a little above the neck, and ex- 

 tends down each fang, at the point of which it opens by a small orifice. In 

 the crown of the incisor teeth the cavity is prolonged into two fine linear 

 canals, which proceed one to each corner of the crown ; in the bicuspid and 

 molar teeth it advances a short distance into each cusp. In the case of a root 

 formed by the blending of two or more fangs, as occurs occasionally in the 

 wisdom-teeth, each division has a separate canal prolonged down to its apex. 



