DIGESTIVE ORGANS. 



211 



Nerves (branches of the trigeminal) and blood-vessels enter the cavity 

 through the base of the tooth. Usually, when the tooth is fully formed, 

 the odontoblasts cease to act, but exceptionally, even in mammals 

 (tusks of elephants, incisors of rodents) they function through life and 

 the tooth continues to grow. In the mammals an additional layer of 

 modified bone, the cement, is formed around the root of the tooth 

 and may extend on to the crown. 



Just as the scales are arranged in quincunx on the surface of the 

 body, so are the teeth in the mouths of skates and some other elasmo- 



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 ; ^;-vi(>:--' . $b 



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FIG. 212. FIG. 213. 



FIG. 212. Diagram of germs of milk and permanent dentitions in a mammal, based on 

 Rose, b, basal layer of e, ectoderm; dr, dental ridge; ds, dental shelf; eo, enamel organ of 

 milk tooth; m, mesenchyme; p, pulp of milk tooth; pg, germ of permanent tooth. 



FIG. 213.- Diagrammatic section of incisor tooth, c, cement; d, dentine; e, enamel; 

 p, pulp cavity. 



branchs, where they form a tessellated pavement above and below, the 

 teeth being flattened and used for crushing the molluscs on which these 

 animals feed. More commonly the teeth are flattened in the antero- 

 posterior direction and have sharp cutting edges. In such cases, as 

 a rule, only the anterior row of teeth is functional, the others lying folded 

 down behind, ready to come into use when one of the first row is lost. 

 Most vertebrates have a succession of teeth (polyphyodont dentition) 

 and the elasmobranchs show how this has come about. The second 

 arises on the (morphologically) posterior side of the first and so on. 

 In the non-mammalian classes the number of such dentitions is in- 

 definite (polyphyodont), but in the great majority of mammals there 

 are two, the first or milk dentition and the second or permanent 

 dentition (diphyodont condition). 



