Chap. II.] EXTERNAL CHARACTERS. 13 



being lost and not replaced, and one incisor on each side in the 

 lower jaw. The grinding teeth of the young have been shed 

 and replaced by permanent teeth, the premolars, and in addition 

 to these, three more grinding teeth on each side, and in each jaw, 

 are formed behind the premolars. They are the molars. The 

 arrangement of teeth in the young is called the milk dentition, 

 which consists of eighteen teeth ; that in the adult is called the 

 permanent dentition, which comprises twenty-eight teeth. 



The teeth are implanted in sockets, are devoid of fangs, and 

 grow continuously during life, constant waste of substance being 

 constantly made good. Each is composed of three substances of 

 different hardness : cement (softest and bony), dentine (harder), 

 and enamel (hardest). The cement forms a thin incrusting 

 external layer. * The enamel encases the dentine except where 

 it has been infolded. In the large incisors it ^forms, for 

 example, a casing to the dentine ; but the layer of enamel is 

 much thicker along the anterior face, especially in the mid-line, 

 where it is, so to speak, tucked in along a median groove. On 

 the posterior face it is very thin or absent. In consequence of 

 this arrangement of hard and soft substance, the anterior edge 

 of the tooth wears away less rapidly than the posterior, so that, 

 by the friction against each other of the upper and lower 

 incisors, a chisel-like cutting edge is maintained. In the little 

 incisors the front edge wears away slightly more rapidly than 

 the hinder edge, so that the larger and smaller incisors together 

 form a sort of notch into which the incisors of the lower jaw fit. 

 The premolars and molars vary from each other somewhat in 

 form and arrangement of their substance ; but the four median 

 grinding teeth have, from constant use, crowns which present two 

 furrows running across the axis of the jaw. These are separated 

 by a central ridge. Enamel forms the outer layer of the tooth 

 (coated with a little cement), and is infolded along the central 

 ridge, which owes its existence to this harder infolded layer. 

 Dentine occupies the troughs of the furrows. 



Two pairs of passages communicate from the mouth to the 

 nasal chambers, (1) the large posterior nares, which lie far back 

 behind the roof of the mouth, (2) the minute naso-palatine 



