DEVELOPMENT AND MOEPHOLOGY OF THE TEETH. 67 



the lingual aspect of the roots of the milk teeth (Fig. 49), and if 

 the milk teeth be roughly extracted the permanent bud may also 

 be torn out. 



Dentigerous and other Cysts of the Jaw. — Cysts with 

 epithelial walls, containing fluid, teeth or other dermal contents, 

 occasionally develop in the jaw. They are formed from epithelial 

 remnants of the dental shelf, which normally breaks up and dis- 

 appear completely, or from detached parts of the enamel buds 

 (see Figs. 48 and 49). 



Nature of Teeth. — A tooth must be regarded as an ossified 

 dermal papilla which has received a coating of enamel from the 

 epidermis covering it. In nature they correspond to the placoid 

 scales of the shark's skin. The placoid scales and teeth of the 

 shark are similar in structure, the one series becoming continuous 

 with the other at the margin of the mouth. The dental papilla 

 and enamel bud represent an invaginated or depressed part of 

 dermis and epidermis. 



Number of Dentitions. — In many lower vertebrate forms, such 

 as sharks, the dental shelf gives off constantly a series of buds, 

 so that as soon as one tooth is lost another springs up from 

 behind in its place. In mammals generally, as in man, the dental 

 shelf gives off only two series of buds — one for the milk set and 

 another for the permanent set. In marsupials it gives off only 

 one series, so that the first set of teeth is never replaced by a 

 second. 



Morphology of Human Teeth. — The crowns of all the human 

 teeth seem to be modifications of the same type, the tritubercular 

 a form undoubtedly evolved from the simple conical tooth found 

 in fishes and reptiles (see Fig. 50). The conical peg-like tooth is 

 to be regarded as the most primitive type, and in man vestigial 

 teeth of this type occasionally occur. In the incisor teeth the 

 two outer or labial cusps are represented by the cutting edge of 

 the crown ; the inner remains as the heel at the base of the 

 crown. Secondary divisions of the two outer cusps into two or 

 three cuspules may be seen in newly-erupted incisors. In the 

 canine the outer two cusps of the tritubercular type are fused 

 into one while the inner remains slightly marked as a rule, 

 but it may rise up and form a prominent cusp as in the 

 premolars (Farmer). In the premolars or bicuspids the outer 



