328 THE DIGESTIVE SYSTEM 



becomes much thinner toward the neck. Beneath the enamel it becomes 

 so thin that toward the apex of the tooth it is scarcely demonstrable. At 

 this point also, occasional dentinal tubules are continued for a short 

 distance into the enamel, though this condition is more characteristically 

 developed in some of the lower mammals (e.g., Rodentia) than in man. 

 Enamel. The enamel, which covers the exposed crown of the tooth, 

 is the hardest tissue of the body. About 90 per cent, of its earthy matter 

 is calcium phosphate; about 4 per cent, is calcium carbonate; less than 



5 per cent, of its substance consists of organic matter. 



It contains a slightly larger trace of calcium fluorid 



than dentin. 



Its unit of structure is a calcareous cylinder, the 



enamel prism. These prisms or 'fibers' radiate out- 



ward from the dentin and are disposed after the 

 FIG. 308. ENAMEL manner of a mosaic. They are firmly united to each 

 PRISMS IN THAN- O ^Q T ^ & yer y ^in j ayer of ca i c jfi e( j inter prismatic 



cement substance. They represent calcified colum- 

 From the tooth 



of a calf, x 350. nar ectodermal cells. 

 (After Kolliker.) In transverse section the enamel prisms have a 



polygonal, frequently quite regular hexagonal, out- 

 line. In certain bundles, especially toward the periphery, the prisms have 

 the shape in transverse outline of stout crescents. In longitudinal view 

 the prisms present a slightly beaded appearance, the constricted portions 

 being darker and delicately cross-striped. This peculiar structure and 

 optical condition results from the manner of the formation of the prisms 

 by the deposition of successive globules of preenamel substance, subse- 

 quently becoming calcified to form definitive enamel. The enamel cement 

 exhibits a reciprocal beading. Since the external surface of the enamel is 

 greater than the internal, and since the enamel prisms are of approx- 

 imately uniform diameter throughout, additional shorter prisms, pointed 

 at their inner end, are interpolated toward the surface. The striped and 

 beaded character of the enamel prisms is especially conspicuous in the 

 teeth of rodents. 



The enamel prisms are grouped into bundles within which the con- 

 stituent prisms are parallel. The course of the prism bundles, however, 

 is variable, so that, though following a more or less radial course through 

 the enamel, the prism bundles frequently cross one another at acute 

 angles. In axial longitudinal sections of ground tooth this crossed ar- 

 rangement of the prism bundles produces the appearance of radially 

 disposed alternating dark and light bands. This banding is seen under 



