274 HISTORY OF THE HUMAN BODY 



merely diminishes the size of the pulp cavity without increas- 

 ing the length of the tooth as a whole. Both methods are in 

 a way a provision against the loss of substance due to constant 

 use ; in the first type the outward growth and the loss through 

 wear usually balance one another so that the tooth remains 

 of about the same length throughout life; in the second 

 the tooth grows gradually shorter while the addition of new 

 layers beneath merely protects the sensitive pulp by keeping 

 a constant thickness of dentine between it and the free surface. 

 Thus, in man, whose teeth are of the second type, the 

 chewing surface in old age is at a level which in youth would 

 lay bare the pulp. Illustrations of the first type are seen in 

 the teeth of the hippopotamus, the tusks of swine and ele- 

 phants, and the chisel-like incisors of rodents; in the last in- 

 stance the front side only is covered with enamel, and as this 

 substance is harder than dentine, it wears away more slowly, 

 and constantly presents a projecting edge, thus keeping the 

 teeth sharp. Such teeth depend upon constant use in order to 

 be kept at the proper length, and in abnormal cases in which 

 some irregularity of the jaw prevents the meeting of opposite 

 teeth, they grow past one another and become eventually 

 disposed in coils and other eccentric forms which may prove 

 the death of their possessor through an inability to feed prop- 

 erly. Similar, although here perfectly normal, instances are 

 those of the ornamental tusks of elephants and wild swine, 

 and in one of these latter, the babyroussa of the East Indies, 

 an upper tooth upon each side bores upward through the lip 

 and erects a curved point high above the snout. 

 - The primitive form for a tooth is that of an elongated 

 cone, a type which occurs with slight variation in proportions 

 among all the lower vertebrates. In mammals, however, the 

 teeth are characterized by the introduction of numerous 

 modifications, sometimes very complex in nature, resulting in 

 a large number of variations, not merely in different species, 

 but in different regions of the same jaw, which correspond 

 to a differentiation in use between the front teeth, that are 

 in a position for grasping the food, and the back teeth, which 



