8l4 PHYLUM CHORDATA : CLASS MAMMALIA 



mouths of the young, which do not seem to be able to suck. 

 An anterior prolongation of the larynx to meet the posterior 

 nares establishes a complete air passage, and enables the 

 young to continue breathing while they are being fed. In 

 Cetacea the milk ducts are dilated into large reservoirs, the 

 contents of which can be rapidly injected into the mouth of 

 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 criista 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 diiferentiated. Beneath each germ a papilla of the 

 vascular mesodermic dermis is defined ofi^ 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 diflferentiated 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 unconstricte'd, and the core of pulp 

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



