COMPLEX AND COMPOUND TEETH. 



235 



Fiff. 55. 



ner, the megatliermm, the hard dentine is reduced to a 

 thin layer, and the chief bulk of the tooth is made up of 

 a central body of vaso-dentine, and a thick external crust 

 of cement. The hard dentine is, of course, the firmest 

 tissue of a tooth so composed, and forms the crest of the 

 transverse ridges of the grinding surface, like the enamel 

 plates in the elephant's grinder. 



The human teeth, and those of the carnivorous mam- 

 mals, appear at first sight to be composed 

 of dentine and enamel only; but their 

 crowns are originally, and their fangs 

 are always, covered by a thin coat of 

 cement. There is also commonly a small 

 central tract of osteo-dentine in old 

 teeth. 



The teeth, called compound or com- 

 plex, in mammalia^ differ as regards their 

 composition from the preceding only by 

 the different proportion and disposition 

 of the constituent tissues. Fig. 55 is a 

 longitudinal section of the incisor of a 

 horse ; d is the dentine, e the enamel, and 

 c the cement, a layer of which is reflected 

 into the deep central depression of the 

 crown ; s indicates the colored mass of 

 tartar and particles of food which fills up 

 the cavity, forming the " mark" of the horse-dealer. 



A very complex tooth may be formed out of two tissues 

 by the way in which these may be interblended, as the 

 result of an original complex disposition of the constitu- 

 ents of the dental matrix. 



Certain fishes, and a singular family of gigantic extinct 



SECTION OF HORSE S 

 INCISOR. 



