722 MAMMALIA. 



the young. 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 pre- 

 maxillae, 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 usually 

 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 (of 

 which about a third is organic matter); outside of this 

 there is a layer of very hard glistening enamel (practically 

 inorganic) ; in the interior there is a cavity which in grow- 

 ing teeth contains a gelatinous tissue or pulp supplied by 

 blood vessels and by branches of the fifth nerve, and con- 

 tributing 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 bud-like "enamel germs " are 

 next differentiated. Beneath each germ a papilla of the 

 vascular 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 papilla forms the pulp, consisting of connective tissue, 

 with blood vessels and nerves, and an enveloping zone of 

 dentine-forming cells or odontoblasts. 



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 ot 

 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, 

 blood vessels and nerves still enter the pulp-cavity and keep the tooth 



