MASTICATION. 51 



The cement is the true bone of tooth, or substantia os- 

 toidea. 



I have said that in a compound tooth the wearing surface 

 is composed of materials of different degrees of hardness. 

 The substances are the enamel, dentine, and crusta petrosa or 

 cement, the chemical composition of which explains this cir- 

 cumstance, as seen by the annexed table. 



Dentine. Enamel. Cement. 



Organic substances, . . . 28'01 3'59 32-24 



Inorganic substances, . . 71 '99 96*41 67'76 



Thus the sharp angles and prominences of the compound 

 teeth (see Fig. 23), are formed by enamel, the deeper hollows 

 by wearing of cement, and the material worn between the 

 two is the dentine. 



I have referred to the pulp, which is a vascular structure 

 endowed with exquisite sensibility,, and lodged in the central 

 cavity of the tooth (see Fig. 21.) The pulp is popularly 

 termed the ' quick ' of the tooth, aud when exposed to the 

 contact of air OP foreign substances, great pain is felt 

 by man or animal. But, in addition to the pulp, we have 

 connected with the teeth, the membrane or periosteum lining 

 the tooth socket, which is applied over the fangs, is soft, and 

 contains vessels and many delicate nerves. The last of the 

 soft tissues compose- the gum, whieh is the lining membrane 

 of the mouth reflected over the jaw, and embracing the necks 

 of the teeth. 



Authors are not all agreed as to the periods of eruption 

 and change in the teeth occurring in the domestic animals, 

 and this may be seen by comparing Kreutzer's table on the 

 annexed page with the tables which follow it, especially as 

 regards the dentition in ruminants and in the pig. 



