246 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



a tooth is constituted of what is known as dentine, a section of which 

 reveals it to be formed of a densely packed mass of curving tubes with 

 distinct walls, imbedded in a dense, bone-like matrix, which run from the 

 pulp-cavity to the outer surface of the dentine near which they ramify. 

 The material between the tubules or the matrix of the dentine is a per- 

 fectly homogeneous substance, containing nearly the whole of the earthy 

 matter contained in the tooth, arranged in all animals in superimposed 

 layers. The tubules are the one four-thousandth of an inch in diameter, 

 and when fresh contain nerve and vascular processes from the pulp. 

 The third substance found in teeth is known as the cement, or crusta 

 petrosa, and in the simple tooth merely covers the fang, whereas it dips 

 in between the layers of enamel in com- 

 pound teeth on the crown, and when the 

 tooth is wholly inclosed within its cavity 

 also covers the crown (Fig. 94). In 

 carnivora the teeth, as already remarked, 

 are simple ; in other words, their crowns 

 are permanently covered with enamel, and 

 when in extremely old subjects the incisor tsSShhiai^fe!a'S3^i!i3&S. — s 



Fig. 93.— Enamel Prisms, after Kolliker. 



(Klein.) 

 A, in longitudinal view ; B, in cross-Beetion. 



AS--- c 



Fig. 94. —Section through a 

 Canine Tooth of Man, after 

 Waxdeyer. (Klein.) 



A, crusta petrosa, with large bone-cor- 

 puscles : B. interglobular substance ; C, den- 

 tinal tubules. 



teeth and canine teeth wear down, — then only does the dentine of the 

 teeth become exposed. In herbivora compound teeth are invariably met 

 with. In other words, on the free surface of the teeth of herbivora 

 different substances of varying degrees of density and hardness are 

 always met with, the function of which is to insure a constantly rough 

 surface for the purposes of grinding; for a good mill-stone is composed of 

 materials which wear with different degrees of rapidity, and thus, alwa3's 

 remaining rough, most effectually grinds the substances over which it 

 passes. In the compound tooth, as found in the herbivora, the cement has 

 originally covered the entire crown, but as the tooth is erupted simply 

 remains on the biting or grinding surface of the tooth. Thus, in the 



