786 



MAMMALIA. 



ting more deeply into the socket, the fang is gradually formed, and the 

 central pulp shrinks until, in the fully-formed tooth, it becomes reduced 

 to a thin membrane richly supplied with vessels and nerves, which lines 

 the small central cavity that remains. 



(2268.) 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. 



(2269.) The most complex condition of the dental organs is that 

 found in the molar teeth of herbivorous quadrupeds, which, being de- 

 stined to act the part of millstones in grinding down and comminuting 

 vegetable substances, must necessarily, like the millstones of human con- 

 trivance, 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 attain this end, the ivory and enamel 

 interdigitate, 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 dif- 

 ferent degrees of hardness, the softer substances are most easily worn 

 away, and thus these compound teeth always offer an efficient grinding 

 surface. 



(2270.) By inspecting the accompanying figure (fig. 400) represent- 



Fig. 400. 



Structure of the molar teeth of the Elephant. 



ing a section of the tooth of an Elephant, the disposition referred to will 

 be better understood : the layers of enamel are seen to alternate with 



