50 DIGESTION AND FOOD. 



structure in the body. In the compound tooth the enamel 

 dips into the surface of the crown, and in some animals, as 

 the elephant, we may regard a compound tooth as a series 

 of flattened teeth arranged in a row, and connected by a 

 structure called cement, or crusta petrosa. This cement only 

 covers the fang of a simple tooth, whereas it dips in between 

 the layers of enamel in compound teeth, and when the tooth 

 is still wholly enclosed within its cavity, a layer of cement 

 covers the cxown also. The pointed fang or fangs of the 

 teeth are pierced by an opening which enters a cavity shown 

 at Figs. 12 or 18. This is the pulp cavity, containing blood- 

 vessels and nerves, whieh ramify in a delicate fibre-cellular 

 structure, constituting the pulp. The latter is prolonged all 

 over its surface into an infinite number of small funnel-shaped 

 apertures, which are continuous with tubes of the dentine or 

 inner structure of the tooth. The dentine constitutes the 

 bulk of both crown and fang, and a section of the dentine 

 proves it to be formed of a densely packed mass of tubes 

 with distinct walls, and which run from the pulp cavity to 

 the outer surface of the dentine, near which they ramify. In 

 this course the dentinal tubes bend, and have a beautiful 

 wavy appearance. The material between the tubes, or matrix 

 of the dentine, is a perfectly homogenous substance, arranged 

 probably in all animals in superimposed layers. 



The enamel is composed of pentagonal or hexagonal long 

 prisms or enamel fibres, closely packed together and arranged 

 in a radiating manner from the dentinal or attached surface. 

 The enamel prisms take a course outwards, similar to that 

 of the dentinal canals, and decussate, so that entire band- 

 shaped layers of them extend in very various directions from 

 the dentine as far as the outer surface of the enamel The 

 enamel covers the crown, and passes over the neck of the 

 tooth, getting thin and only partially covering the fang. 



