90 



GENERAL ZOOLOGY 



a colorless membrane, highly resistant to the action of chemical 

 reagents. The pulp cavity is a space in the dentine, open at the 

 base of the root, and rilled with blood vessels and odontoblasts 

 (cells that produce dentine). Human teeth have the same 

 general structure except that a cuticula dentis is absent, and the 

 enamel is more evenly distributed over the entire crown (Fig. 55). 

 The pulp also contains nerve endings. Each human tooth is 

 inserted in a socket or alveolus, the dentine of the root of the tooth 



Fig. 56. 



Upper teeth of the dog. c, canine; i, incisor; p, premolar; m, molar. 

 The carnasial or flesh-cutting tooth is the fourth premolar. 



being attached to the bone of the jaw by a cement substance. 

 The dentine of the vertebrate tooth is formed of mesoderm, the 

 same embryonic germ layer that gives rise to bones, while the 

 enamel is a secretion of ectodermal cells of the oral cavity. 



The vertebrate tooth evolved apparently from what is known 

 as a placoid scale, such as is found in the skin and in a modified 

 form, in margins of the jaws of the Elasmobranchii (sharks). 

 The placoid scale consists of a curved spine projecting from the 

 outer surface of a flattened base of dentine embedded in the skin 

 (Fig. 25). The spine, which extends beyond the surface of the 

 skin is covered with an enamel-like substance of ectodermal origin 

 and is also provided with a pulplike cavity, open below. Near 



