DENTITION. 643 



ducts of the glands are dilated into large reservoirs, from 

 which the contents are injected into the mouth of the young 

 animal by the action of a compressor muscle." In all other 

 Mammals the young suck the milk from the mammae. 



Dentition. 



The teeth of Mammals are developed in the gum or soft 

 tissue which covers the borders of the premaxillae, maxillae, 

 and mandibles. As in other animals, they are in part of 

 epidermic, in part of dermic origin. In the course of their 

 development their bases are enclosed in sockets formed in 

 the subjacent bones. 



In most teeth there are three or four different kinds of 

 tissue. The greater part consists of dentine or ivory ; out- 

 side of this there is a layer of very hard glistening enamel ; 

 in the interior there is a cavity which in growing teeth con- 

 tains a gelatinous tissue or pulp, supplied by blood vessels 

 and by branches of the fifth nerve, and contributing to the 

 increase of the dentine ; lastly, around the narrowed bases 

 or roots of the tooth, or between the folds of the enamel 

 if these have been developed, there is a bone-like tissue 

 called the crusta petrosa or cement. 



The development of teeth begins with the formation of 

 a dental ridge, an invagination of the ectodermic epithelium. 

 From this ridge a number of " enamel germs " are next 

 differentiated. Beneath each germ a papilla of the vas- 

 cular mesodermic dermis is defined off as the " dentine 

 germ." The crown of this papilla becomes hard, and 

 the ossification proceeds downwards and inwards, while 

 above the dentine crown the enamel begins to form a hard 

 cap. Meantime the tissue around the base of the tooth- 

 papilla becomes differentiated into an enclosing follicle or 

 sac, from the inner layer of which the cement is developed. 

 The pulp is but the uncalcified core of the papilla. 



The base of a tooth may remain unconstricted, and the core of pulp 

 may persist. Such a tooth goes on growing, its growth usually keeping 

 pace with the rate at which the apex is worn away with use, and it is 

 described as " rootless " and " with persistent pulp." The incisors of 

 Rodents and of Elephants illustrate this condition. 



In the development of most teeth, however, the base is narrowed 

 and prolonged into a root or several roots which become firmly fixed 

 in the socket. Through a minute aperture at the end of the root, 



