MAMMALIA. 



669 



Before the progressively advancing tooth issues from the nidus 

 wherein it is produced, the enamel is deposited upon the surface of 

 the ivory by the lining membrane of the capsule (c), and becomes 

 arranged in crystalline fibres placed perpendicularly to the surface 

 of the ivory, until the whole crown of the tooth is adequately 

 coated with this important additional substance. Meanwhile the 

 growth of the tooth still proceeds by the lengthening of its root, 

 until at last the crown issues from the jaw, and the enamel-secreting 

 membrane (c) becomes obliterated. 



(747.) The most complex condition of the dental organs is 

 that found in the molar teeth of herbivorous quadrupeds, which, 

 being destined to act the part of mill-stones in grinding down 

 and comminuting vegetable substances, must necessarily, like 

 the mill-stones of human contrivance, have a grinding surface, 

 presenting prominent edges and deep sulci, not liable to become 

 worn even by the continual abrasion to which they are subjected. 

 In order to obtain this end, the ivory and enamel indigitate, as it 

 'were, in the substance of the tooth ; and are, moreover, imbedded 

 in a third material, not met with in the simpler forms, called the 

 cementum or crusta petrosa. In consequence of this arrangement, 

 seeing that the plates of ivory, of enamel, and of cement, are all 

 of different degrees of hardness, the softer substances are most 

 easily worn away; and thus these compound teeth always offer an 

 efficient grinding surface. 



By inspecting the accompanying figure (fig. 314), representing 

 a section of the tooth 



of an Elephant, the Fig.3\4. 



disposition referred 

 to will be better un- 

 derstood : the layers 

 of enamel are seen 

 to alternate with 

 plates of ivory, while 

 all the interstices are 

 filled up by the cir- 

 cum fused cementum. 



During the growth 

 of a compound 

 tooth of this descrip- 

 tion, the enamel- 

 secreting membranes derived from the capsule of the tooth, of 



