196 ANIMAL PHYSIOLOGY. [LESSON 55. 



837. In a tusk of the Hog, as seen in longitudinal section (Fig, 

 318), the enamel covers the outer surface that portion which comes 

 in contact with resisting substances, and within it the ivory ; the 

 pulp cavity is seen at the lower part. 



838. This is probably still better seen in a transverse section of 

 the same tooth (Fig. 319) ; in this view, the tooth appears to be tri- 

 angular in form. The large pulp cavity occupies the centre, which 



FIG. 318. 



IB 



FIG. 319. 



Longitudinal section, tusk of Hog. Transverse section, Hog's tusk. 



is succeeded by the tubes of the ivory ; the enamel entirely surrounds 

 the section. 



839. It is reserved for the Herbivorous quadruped to show the 

 most interesting and beautiful forms of teeth known to us among the 

 higher animals. The same three tissues are present here, but ar- 

 ranged in a different manner to meet a special want. 



840. These animals having uniformly to masticate grass, either 

 in a dried or green state, have their teeth constantly brought in 

 contact with large quantities of pure silica, incorporated in the 

 cuticle of the grass. If the surface of the teeth were smooth, 

 they would be inadequate to perform the grinding motion assigned 

 to them. We know that the miller requires to have the surface of 

 his mill-stones hacked into cavities to form a grinding surface, and 

 this is precisely the want of the herbivorous quadruped in relation to 

 the surface of its teeth they must not be flat, but hollowed out into 

 unequal cavities. To meet this imperative want, the tissues enamel, 

 ivory, and bone are all arranged on the same plane. 



841. Examine the crown of the molar (grinding) tooth of a Cow 

 or a Sheep ; its surface will present a series of ridges standing up 

 above the other tissues ; next to these are inclined planes of a differ- 

 ent tissue, descending from the ridges to large, hollow, unsightly 

 cavities lying at their base evidently another tissue. 



842. Now it must happen that if flint be brought constantly in 



