22 ANLMAL DICXTISTRY. 



it soon hc'conics lliicl<l_\- encrusted witli crnsta pctrosa on 

 l)otli the liuccal and lingual surfaces. The molar teeth have 

 no outer enamel co\'erin^" after they ha\-e been in wear for 

 a short time. The enamel surrounding^ the infundibula of 

 the incisors is arranged in the form of an oblong or elliptical 

 ring at the table surface, ^^■hilc that of the molars is folded 

 into a very irregular boundary. The external enamel con- 

 sists of an undulated i)late following the course of the longi- 

 tudinal ridges and groo\-es of the molar teeth. At the table 

 end of the longitudinal ridges it terminates in a sharp point, 

 constituting the "sharp teeth" of herbivora. Chemically, it 

 consists of : 



Carbonate of lime 4-37 



Phosphate of lime 89.82 



Fluoride of lime a trace 



Phosphate of magnesia 1.34 



Soluble salts 88 



Organic matter 3.59 



Microscopically it presents a thin cutic'e or skin called 

 the cuticle of the enamel, which is separable when a section 

 of enaniel is treated with hydrochloric acid, and hexagonal 

 prisms projecting at a right angle from the surface of the 

 dentine, giving the appearance of a mosiac floor. 



CRUSTA PETROSA OR CEMENT. 



The crusta petrosa is the softest of the three hard bodies 

 composing the teeth. It is an external osseous incrustation 

 of the fang by which the tooth is cemented to the alveolar 

 cavity. Tn the \-irgin footh it is but a thin layer, but as the 

 tooth ages it becomes thick from the deposition of osseous 

 tissue. Crusta peti'osa covers the entire outer surfaces of 

 the mature molars, the fangs of the incisors, the grooves in 

 the crowns of the incisors of old animals and parllv fills the 



