228 



THE TEETH. 



essential element in the classification of the animal king- 

 dom. They occupy the alveolar cavities of the superior 

 and inferior maxillary bones, and are fixed firmly in their 

 respective situations by a species of articulation called 

 gomphosis (70^05, a nail,) from the supposed resemblance 

 to the manner in which a nail is confined when driven into 

 a board, as seen in Fig. 56. 



The teeth in both 

 jaws are arranged in 

 what have been called 

 the dental arches. The 

 size of the arches in the 

 two jaws differs; that 

 of the upper closes over 

 the incisors and canine 

 of the lower, and thus 

 forms the segment of 

 a larger circle this 

 overlapping in the two 

 arches illustrates their 

 adaptation to the cutting action ; and they have been com- 

 pared to the blades of a pair of scissors. The molars, which 

 come in contact by the superior surface of their crowns, 

 have that position which is best adapted for the grinding 

 and pounding motions. Fig. 75, from Nasmyth, illus- 

 trates the superior dental arch of man ; and the same figure 

 shows the superior arch of a chimpanzee. This latter pre- 

 sents a marked difference from the human arch, in having 

 its lateral portions straight, and in presenting a space 

 between the lateral incisor and canine. 

 The teeth have two grand divisions : 



First, the Temporary; Second, the Permanent. 



The first division has 20 teeth; the second 32. Each 

 division is classified into, 



1. Incisors (incisores.) 



2. Canines (cuspidati.) 



3. Bicuspids (bi-cuspidati.) 



4. Molars (molares.) 



