106 



INTRODUCTION. 



enamel, are soldered together : such teeth are compound ; the grinders 

 of the Elephant are an example. Ivory, enamel, and crusta petrosa, 

 combine, according to F. Cuvier, in four different modes of arrange- 

 ment. Some teeth are composed of ivory, enamel, and crusta petrosa ; 

 others of ivory and enamel ; others (as in the Cachalot), of ivory and 

 crusta petrosa ; and others, again, of ivory only. Ivory consists of 

 gelatine and phosphate of lime, and, when cut, has a silky, fibrous 

 appearance ; the fibres, which are, in fact, minute tubes, being nearly 

 parallel to the external surface of the tooth. The enamel is much harder 

 than the ivory, which it merely envelopes, and from which it can be 

 separated : it consists essentially of fluate of lime, and, a section being 

 made, it presents the appearance of brilliant needles, perpendicular to 

 the surface of the ivory. Crusta petrosa, or cortical substance, is of 

 the same composition as ivory, consisting of gelatine and phosphate of 

 lime : it is, in fact, an outer layer of ivory, deposited after the formation 

 of the internal ivory and enamel (in compound teeth), from a peculiar 

 membrane. In teeth, consisting only of ivory and cortical substance, 

 the latter assumes the place of enamel, and is deposited over the ivory, 

 varying in thickness according to circumstances. On the teeth of the 

 Cachalot, it is of great thickness, and its structure is dense and compact. 

 In the compound molars of the Elephant, it forms a thick solid bed, in 

 which the folds of enamel, investing the ivory (fig. 109), appear to be im- 

 pacted.* According to the experiments of F. 

 Cuvier, the anterior part of the incisors of the 

 Rodentia is covered with a delicate layer of cor- 

 tical substance, which, as in the Beaver, the 

 Porcupine, and many other species, is of a bright 

 orange colour this colour is said to be owing to 

 the presence of iron in the form of a simple 



Griudiug surface of Elephant's tooth. -j T j n -i -r-i-i i 



oxide. In the dentition of the Elephant, instances 



are presented, at the same time, of the simplest and of the most com- 

 pound structure of teeth : the grinders are compound, as already stated 

 the tusks are solely ivory. 



The following valuable observations are from a paper by Professor 

 Owen, " On the structure of teeth, and the resemblance of bone to ivory, 

 as illustrated by microscopical examination of the teeth of Man, and va- 

 rious existing and extinct animals." In this paper Professor Owen 

 alludes to the recent investigations of Purkinje, Miiller, and Retzius, with 

 respect to the intimate structure of teeth, and follows up the results of 

 their experiments by a detail of the conclusions to which his own re- 



* In the Horse, the cortical substance, forming the external part of the molars, is of great 

 thickness ; as it is, also, in the molars of the Hippopotamus. On the summits of the projecting surfaces, 

 in the molars of the Ruminantia, it is thin, but becomes thicker in the depressions between them. 



