142 THE VITAL FUNCTIONS. 



The formation of the teeth of herbivorous qua- 

 drupeds, which have three kinds of substance, is 

 conducted in a still more artificial and complicated 

 manner. Thus in the Elephant, the pulp which 

 constructs the ivory is extended in the form of a 

 number of parallel plates ; while the capsule which 

 invests it, accompanies it in all its parts, sending 

 down duplicatures of membrane in the intervals 

 between the plates. Hence the ivory constructed 

 by the pulp, and the enamel formed over it, are 

 variously intermixed ; but besides this, the crusta 

 petrosa is formed by a similar process on the out- 

 side of the enamel. Cuvier asserts that it is con- 

 structed by the same capsule which has formed the 

 enamel, and which, previously to this change of 

 function, has become more spongy and vascular 

 than before. But his brother, M. Frederic Cuvier, 

 represents the deposit of crusta petrosa, as per- 

 formed by a third membrane, wholly distinct from 

 the two others, and exterior to them all, although it 

 follows them in all their folds. In the Horse and 

 the Ox, the projecting processes of the pulp have 

 more of a conical form, with undulating sides ; and 

 hence the waved appearance, due to the undulating 

 course of the fibres, which is presented by the 

 enamel, on making sections of the teeth of these 

 animals. 



The tusks of the elephant are composed of ivory, 

 and are formed precisely in the same manner as the 

 simple conical teeth already described, excepting 

 that there is no outer capsule, and therefore no outer 

 crust of enamel. The whole of the substance of 

 the tusk is constructed by the successive formation 

 of layers, having a conical shape, from deposits of 



