16 



Basic Structure of Vertebrates 



the bone of the jaw. The "cheek-teeth" of elephants, horses, and cud- 

 chewing hoofed mammals ( sheep, cattle) have not only their roots but 

 also their crowns covered by a layer of cement. The working surfaces of 

 these teeth are strongly ridged. The ridges are caused primarily by an 

 up-and-down folding of the dentine-and-enamel wall of the tooth (Fig. 

 44). Then, before the tooth is erupted, the cement-forming tissue 

 around the root grows up over the crown 

 and deposits cement external to the 

 enamel. As the tooth is used, the rela- 

 tively soft cement is worn away, expos- 

 ing the enamel on the tops of the folds. 

 Eventually this enamel is worn away, 

 exposing the dentine of the folds. From 

 then on, all three substances are worn 

 down, but each upstanding layer of the 

 more resistant enamel produces a ridge 

 (Fig. 44). Thus the efficient grinding 

 surface of the tooth is produced and 

 maintained by wear. The "ivory" of the 

 elephant's tusk is a modified dentine 

 covered by a thin layer of cement. 



A variety of dentine results from its 

 invasion by fine blood-vessels. Ordinary 

 dentine, containing no blood, is white or 

 yellowish. Vascular dentine, or vasoden- 

 tine, found in the teeth of many fishes 

 and occasionally elsewhere, is more or 

 less reddened by the blood in it and is 

 softer than ordinary dentine. 

 In some cases the root, instead of tapering down to a point pierced 

 by only a narrow foramen (Figs. 31, 45A), remains widely open at the 

 base (Fig. 45B). This condition of having an "open root" or persistent 

 pulp is characteristic of teeth which attain great length or grow con- 

 tinuously at the base to compensate for wear at the apex of the crown, 

 e.g., teeth of some hoofed mammals, tusks of hog and elephant, chisel- 

 like front teeth (incisors) of rodents (Fig. 45C). Such teeth, in which 

 there is no sharp distinction between crown and root, are sometimes 

 described as being "rootless." 



The relation of teeth to the oral skeleton varies. In sharks and some 

 other fishes, the teeth are carried by the oral membrane (equivalent 

 to the external skin) and are not joined to the skeleton of the jaws or 

 to other skeletal parts. If teeth are firmly joined to the jawbone, they 

 may be fused (ankylosed) to its extreme edge (lower edge of upper 



Fig. 41. Section of elephant 

 molar, (c) Cement; (d) dentine; 

 (e) enamel; (p) pnlp-cavity; (r) 

 root. (Courtesy, Owen: "Com- 

 parative Anatomy and Physi- 

 ology of Vertebrates," London, 

 Longmans, Green & Co., Ltd.) 



