XXV] TEETH 643 



connected by only a narrow neck with the adjacent connective 

 tissue, this appearing in the dried tooth as a small hole through 

 which a blood-vessel passes. The term root is applied to the 

 dentine surrounding the narrow neck. When it is formed, growth 

 of the tooth ceases. 



The teeth of Mammalia are amongst their most characteristic 

 organs ; they are more differentiated than those of other Craniata, 

 and their peculiar structure enables us to identify many fossil 

 remains as mammalian. 



They are typically differentiated into four kinds, viz. incisors or 

 cutting teeth, canines or stabbing teeth, premolars and molars, 

 which taken together are termed cheek-teeth or back-teeth (Fig. 

 317). The incisors are borne by the premaxilla and have sharp, 

 straight edges adapted for cutting morsels of convenient size from 

 the food. The canines and hinder teeth are borne by the maxilla. 

 The canines, popularly known as the eye-teeth, are pointed teeth 

 used for the purpose of killing prey or for defence against enemies, 

 or in the fights which occur among males for the possession of 

 females. The premolars have at least one cutting edge, often two 

 or more parallel to one another; they are used to cut up the 

 morsels which have been taken into the mouth. Finally the molars 

 have broad surfaces with which the food is sufficiently broken up 

 to permit of its being swallowed. The teeth of the lower jaw are 

 of course all borne by the dentary, and they are divided into the 

 same varieties as those in the upper jaw. In Elasmobranchii, as 

 we have seen, the teeth are enlarged placoid denticles developed 013. 

 a fold of skin which is invaginated within the lip, and as one row 

 of teeth becomes worn out another takes its place, the skin bearing 

 the old teeth slipping forward over the lip. In the higher Craniata 

 this fold is represented by a solid wedge of ectoderm, called the 

 enamel organ, and in Amphibia and Reptilia it produces succes- 

 sive rows of teeth throughout life as they are needed. In Mam- 

 malia it normally produces two, the first of which lasts only for a 

 short time during the youth of the animal, and is known as the 

 milk dentition ; the teeth belonging to this row are pushed out of 

 the gum by those of the second row, or permanent dentition, 

 which last throughout the life of the animal. In the milk dentition 

 there are only incisors, canines, and molars ; the milk molars are 

 succeeded by the premolars of the permanent dentition, while the 

 permanent molars have no predecessors and are regarded as belated 

 members of the first dentition. The teeth of Mammalia have 



412 



