TUBES WITH NOURISHING, COLORLESS FLUID. JX 



of compartments or cells, and extremely minute tubes. 

 The earthy particles have a twofold arrangement, be- 

 ing either blended with the animal matter of the in- 

 terspaces and parietes of the tubes, or contained in a 

 minute granular state in their cavities. The density 

 of the dentine arises principally from the proportion 

 of earth in the first of these states of combination. 

 The tubes contain, near the formative pulp, filament- 

 ary processes of that part, and convey a colorless fluid, 

 probably transuded ' plasma.' They thus relate not 

 only to the mechanical conditions of the tooth, but to 

 the vitality and nutrition of the dentine. This tissue 

 has few or no canals large enough to admit capillary 

 vessels with the red particles of blood, and it has been 

 therefore called ( unvascular dentine.' 



" Cement always closely corresponds in texture with 

 the osseous tissue of the same animal ; and whenever 

 it occurs of different thickness, as upon the teeth of 

 the horse, sloth, or ruminant, it is also traversed, like 

 bone, by vascular canals. When the osseous tissue is 

 excavated, as in dentigerous vertebrates above fishes, 

 by minute radiated cells, forming, with their contents, 

 the ' corpuscles of Purkinje/ these are likewise present, 

 of similar size and form, in the cement, and are its 

 chief characteristic as a constituent of the tooth. The 

 hardening material of the cement is partly segregated 

 and combined with the parietes of the radiated cells 

 and canals, and is partly contained in disgregated 

 granules in the cells, which are thus rendered white 

 and opaque, viewed by reflected light. The relative 

 density of the dentine and cement varies according to 

 the proportion of the earthy material, and chiefly of 

 that part which is combined with the animal matter 

 in the walls of the cavities, as compared with the size 



