230 



SUBSTANCE OF TEETH. 



Fig. 53. 



habits of the animal, and are, therefore, highly interest- 



mg to the physiologist. They form, for the same reason, 



important guides to the naturalist in the classification of 

 animals ; and their value, as zoolo- 

 gical characters, is enhanced by the 

 facility with which, from their posi- 

 tion, they can be examined in liv- 

 ing "or recent animals ; whilst the 

 durability of their tissues renders 

 them not less available to the pal^- 

 onthologist in the determination of 

 the nature and affinities of extinct 

 species, of whose organization they 

 are often the sole remains discove- 

 rable in the deposits of former 

 periods of the earth's history. 



The substance of teeth is not so 

 uniform as in bone, but consists 

 commonly of two or more tissues, 

 characterized by the proportions 

 of their earthy and animal consti- 

 tuents, and by the size, form, and 

 direction of the cavities in the ani- 

 mal basis which contain the earth, 

 the fluid, or the vascular pulp. 

 The tissue which forms the body 



of the tooth is called "dentine" {dentinum, Lat.), Fig. 



63, 1. 



The tissue which forms the outer crust of the tooth is 



called the "cement" {ccemenium^ and amsta petrosa^ Lat.), 



ih. 3. 



The third tissue, when present, is situated between the 



dentine and cement, and is called "enamel" (er^caws/Jt^w, 



or adamas^ Lat.), ih. 4, 



SECTION OF HUMAN INCISOU 



TOOTH (magnified). 



