SKELETON AND TEETH 



95 



Fig. 44a. — First upper 

 molar, right side of 

 Deer (Odocoileus). On 

 the left, the masti- 

 cating surface ; heavy 

 black line, enamel. 

 On the right, external 

 side, showing crown 

 and roots. Brachyo- 

 dont. 



materials of which the teeth are constructed and the manner 



in which those materials are combined. In all primitive 



mammals and in many of the higher and 



more advanced ones (including Man) a tooth 



is composed of the crown, or portion which 



is exposed above the gum, and the roots * one 



or more in number, by means of which the 



tooth is firmly inserted in the jaw-bone ; the 



roots are at least partly formed before the 



tooth comes into use. Such a tooth is said 



to be short or low-crowned, or brachyodont. 



In many plant-feeders, such as horses, 



cattle, elephants, beavers, etc., the teeth 



continue to grow in height for a long time 



and do not form roots until late in life, or perhaps not at all. 



Such teeth are said to be long- or high-crowned, or hypsodont, and 



in very many instances the development of brachyodont into 



hypsodont teeth may be fol- 

 lowed through every step of 

 the change. The advan- 

 tage of the change is ob- 

 vious in lengthening the 

 animal's life, especially in 

 those which feed upon abra- 

 sive substances, like grass, 

 for the growth of the teeth 

 long continues to make up 

 for the loss through wear. 

 Serious trouble has often 

 been caused for captive el- 

 ephants by giving them too 

 soft food, so that the growth 

 of the teeth is not properly 

 balanced by abrasion. Still 

 another category of teeth is 



Fig. 45. — First upper molar, left side, of a fos- 

 sil horse (Equus sp.). On the right, external 

 side. On the left, the grinding surface, 

 showing two stages of wear. Heavy black 

 line, enamel ; white, dentine ; shaded, cement. 

 Hypsodont, roots not yet formed. 



