DENTAL SYSTEM 19 



one is a higher form than the other, since they both serve important 

 and different purposes in the animal economy. 



As is almost always the case in nature, intermediate conditions 

 between these two forms of teeth are met with. Thus some teeth, 

 as the molars of the Horse, and of many Rodents, are for a time 

 rootless, and have growing pulps producing very long crowns with 

 parallel sides, the summits of which may be in use and beginning 

 to wear away while the bases are still growing ; but ultimately the 

 pulp contracts, forms a neck and distinct roots, and ceases to grow. 

 The canine tusks of the Musk Deer and of the Walrus have 

 persistent pulps, and are open at their base until the animal is of 

 advanced age, when they close, and the pulp ceases to be renewed. 

 The same sometimes happens in the tusks of very old Boars. 



The simplest form of the crown of a tooth is that of a cone ; 

 but this may be variously modified. Thus it may be flattened, with its 

 edges sharp and cutting, and pointed at the apex, as in the laterally 

 compressed premolars of most Carnivora ; or it may be chisel- or 

 awl-shaped, with a straight truncated edge, as in the human incisors ; 

 or it may be broad, Avith a flat or rounded upper surface. Very 

 often there is a more or less prominent ridge encircling the whole or 

 part of the base of the crown just above the neck, called the cingu- 

 lum, which serves as a protection to the edge of the gum in masti- 

 cating, and is most developed in flesh -eating and insectivorous 

 animals, in which the gums are liable to be injured by splinters of 

 bone or other hard fragments of their food. The form of the 

 crown is frequently rendered complex by the development upon its 

 surface of elevations or tubercules called cusps or cones, or by 

 ridges usually transverse, but sometimes variously curved or folded. 

 When the crown is broad and the ridges are greatly developed, as 

 in the molars of the Elephant, Horse, and Ox (Fig. 1, V.), the inter- 

 spaces between them are filled with cement, which supports them 

 and makes a solid compact mass of the whole tooth. When such a 

 tooth wears away at the surface by friction against the opposed 

 tooth of the other jaw, the different density of the layers of 

 the substances of which it is composed — enamel, dentine, and 

 cement — arranged in characteristic patterns, causes them to wear 

 unequally, the hard enamel ridges projecting beyond the others, 

 and thus giving rise to a grinding surface of great mechanical 

 advantage. 



Succession. — The dentition of all mammals consists of a definite 

 set of teeth, almost always of constant and determinate number, 

 form, and situation, and, with few exceptions, persisting in a 

 functional condition throughout the natural term of the animal's 

 life. In many species these are the only teeth which the animal 

 ever possesses, — the set which is first formed being permanent, or, if 

 accidentally lost, or decaying in extreme old age, not being replaced 



