DENTAL TISSUES. 



363 



240 



Section of pharjngeal tooth of Lahrus, 

 magnified, v. 



The examples are extremely few, and peculiar to the class 

 Pisces, of calcified teeth which consist of a single tissue, and this is 

 always a modification of dentine. The large pharyngeal teeth of 

 the Wrasse (Labrus} consist of 

 a very hard kind of unvascular 

 dentine. Fig. 240 shows a ver- 

 tical section of one of these teeth, 

 supported upon the vascular 

 osseous tissue of the pharyngeal 

 bone : p is the pulp cavity. 



The next stage of complexity 

 is where a portion of the dentine 

 is modified by vascular canals. 

 Teeth, thus composed of dentine 

 and vaso-dentine, are very com- 

 mon in fishes. The hard dentine 

 is always external, and holds the 

 place, and performs the office, of 

 enamel in the teeth of higher 

 animals ; but it is only analogous 

 to enamel, not the same tissue. 

 Fig. 241 exemplifies this struc- 

 ture in a longitudinal section of the tooth of a Shark (Lamna). 



The molars of the Dugong (Halicore) are composed of dentine 

 and cement . the latter substance forming a thick outer layer, 

 fig. 242, c. 



In the teeth of the Cachalot (Physeter) the pulp-cavity of the 

 growing tooth becomes filled up by osteodentine, the result of a 

 modified calcification of the dentinal pulp ; when the tooth presents 

 three tissues, as shown in fig. 239, in which c is the thick external 

 cement, d the hard dentine, and o the osteodentine ; sometimes 

 developed in loose stalactitic-shaped nodules. 



In the teeth of the Sloth, and its great extinct congener, the 

 Megatherium, the hard dentine is reduced to a thin laver. fig-. 238, 



v O 



t, and the chief bulk of the tooth is made up of a central body of 

 vaso-dentine, ib. v., and a thick external crust of cement, ib. c. 



Besides the number of constituent tissues teeth become 'complex' 

 in structure by the proportion and disposition, chiefly inflection, 

 of more or fewer of those tissues. 



Certain fishes and the extinct ( Labyrinthodont ' reptiles ex- 

 hibit this complexity in a remarkable degree. In fig. 243, the 

 tooth of the Labyrinthodon salamandro'ides feebly indicates its 

 singular structure by the longitudinal striae. But every streak is 



