314 COMPARATIVE ANATOMY 



tion with one another, and give rise respectively to the calcined 

 enamel with its " cuticula dentis," and to the dentine, of which the 

 teeth are composed : the dentine consists of calcined hard dentine 

 and vascular vaso-dentine. The enamel is the harder and contains 

 little organic matter, and the dentine (ivory) is permeated by a 

 system of fine canals into which delicate processes of the odonto- 

 blasts extend. A third, bone-like substance, the cement, is also 

 formed from the mesoderm round the bases of the teeth and 

 between the folds of enamel when these are present; it may 

 unite with the bones of the jaw. 



The root of the tooth, embedded in the gums, is provided at 

 its lower end with an opening leading into the central pulp-cavity 

 (Fig. 229), containing the pulp of the tooth, which consists of cells, 

 blood-vessels, and nerves, and which renders the tooth sensitive. 



The form of the teeth is largely dependent on that of the 

 jaws and on the mode of articulation of the latter, as well as on 

 the nature of the food. Functional adaptation may result in com- 

 plications in the relations of the enamel, dentine, and cement, so 

 as to produce a cutting or grinding surface of different degrees of 

 hardness, and these modifications are generally more marked in the 

 upper than in the lower jaw. 



In most Vertebrates below Mammals all the teeth are essenti- 

 ally similar in form (homodont dentition] : in Mammals, on the 

 other hand, they become differentiated into distinct groups (hetero- 

 dont dentition), viz. into incisors, canines, and cheek-teeth or grinders 

 (premolars and molars). 



A succession of teeth takes place throughout life in almost all 

 Vertebrates except Mammals, in which, owing to specialisation, a 

 reduction in the number of tooth-generations has taken place, so 

 that there are practically only two functional sets, the so-called 

 deciduous or milk-teeth and the snccessional teeth. This difference 

 is expressed by the terms polyphyodont and diphyodont. 



Fishes and Amphibians. The homology of the teeth and 

 their similarity with the dermal denticles of Elasmobranchs have 

 already been referred to (p. 39). The most primitive form of tooth is 

 that of a simple cone, but even amongst Plagiostomes, in which 

 the teeth are arranged in numerous parallel rows upon the carti- 

 laginous jaws, this form has already become modified in various 

 ways for seizing or crushing the food. 



Of those Anamnia which possess a bony skull, four groups of 

 tooth-bearing bones may in general be distinguished, viz. (1) the 

 maxillary arch (premaxilla and maxilla} ; (2) the palatal arch 

 (vomer, palatine, pterygoid)', (3) the unpaired parasphenoid ; and 

 (4) the mandibular arch (dentary and splenial). 1 



True teeth, with enamel, enamel-epithelium, and odontoblasts, 



1 The teeth of Elasmobranchs are comparable to those of the palatal arch 

 and splenial. (For those of Holocephali, cf. p. 88.) 



