TEETH. 



865 



The third tissue, when present, is situated 

 between the dentine and cement, and is called 

 "enamel" (encaustum, adamas, Lat.). 



"Dentine" consists of an organised animal 

 basis disposed in the form of extremely minute 

 tubes and cells, and of earthy particles : these 

 particles have a twofold arrangement, being 

 either blended with the animal matter of the 

 interspaces and parietes of the tubes and cells, 

 or contained in a minutely 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 and cells contain, besides the granular 

 earth, a colourless fluid, probably transuded 

 " plasma" or "liquor sanguinis," and thus re- 

 late not only to the mechanical conditions of 

 the tooth, but to the vitality and nutrition of 

 the dentine. 



This typical structure of dentine is well 

 illustrated in the article TOOTH : such " true 

 dentine " has no canals large enough to ad- 

 mit capillary vessels with the red particles of 

 blood, and it has been therefore called " un- 

 vascular dentine." 



The simplest modification of dentine is that 

 in which capillary tracts of the primitive vas- 

 cular pulp remain uncalcified, and permanently 

 carry red blood into the substance of the 

 tissue. These so-called " medullary canals " 

 or " vascular canals " present various disposi- 

 tions in the dentine which they modify, and 

 which I have proposed to call " vaso-dentine." 

 It is often combined with true dentine in the 

 same tooth ; e.g. in the scalpriform incisors 

 of certain Rodents*, the tusks of the Ele- 

 phant f, the molars of the extinct Iguanodon.J 



A third modification of the fundamental 

 tissue of the tooth is where the cellular basis 

 of the dentine is arranged in concentric layers 

 around the vascular canals, and contains " ra- 

 diated cells " like those of the osseous tissue : 

 it is called " osteo-dentine." The transition 

 from dentine to vaso-dentine, and from this to 

 osteo-dentine, is gradual, and the resemblance 

 of osteo-dentine to true bone is very close. 



" Cement " always closely corresponds in 

 texture with the osseous tissue of the same 

 animal ; and wherever it occurs of sufficient 

 thickness, as upon the teeth of the horse, 

 sloth, or ruminant, it is also traversed, like 

 bone, by vascular canals. In reptiles and 

 mammals, in which the animal basis of the 

 bones of the skeleton is excavated 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 ma- 

 terial 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 



O 



* Odontosraphy, 4to, p. 405. 

 t Ib. p. 643. 

 J Ib. p. 251. 



VOL. IV. 



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 and number of the 

 cavities themselves. In the complex grinders 

 of the elephant, the masked boar, and the 

 capybara, the cement, which forms nearly half 

 the mass of the tooth, wears down sooner 

 than the dentine. 



The "enamel" is the hardest constituent 

 of a tooth, and, consequently, the hardest of 

 animal tissues ; but it consists, like the other 

 dental substances, of earthy matter arranged 

 by organic forces in an animal matrix. Here, 

 however, the earth is mainly contained in the 

 canals of the animal membrane ; and, in 

 mammals and reptiles, completely fills those 

 canals, which are comparatively wide, whilst 

 their parietes are of extreme tenuity. The 

 hardening salts of the enamel are not only 

 present in far greater proportion than in the 

 other dental tissues ; but, in some animals, 

 are peculiarly distinguished by the presence 

 of fluate of lime. 



The following are characteristic examples 

 of the above-defined tissues, and their different 

 combinations, in different teeth. 



The examples are extremely few, and, as 

 far as I know, are peculiar to the class Pisces, 

 of calcified teeth which consist of a single 

 tissue, and this is always a modification of 

 dentine. The large pharyngeal teeth of the 

 Wrasse (Labrim} consist of a very hard kind 

 of unvascular dentine. Fig. 544. shows a ver- 



Fig. 544. 



Section of pharyngeal tooth of Labrus, magnified. 

 ^Owen's Odantography.}\ 



tical section of one of these teeth, supported 

 upon the very vascular osseous tissue of the 

 pharyngeal bone : p is the pulp cavity. 



The next stage of complexity is where a 

 portion of the dentine is modified by vascular 

 canals. Teeth, thus composed of dentine 

 and vaso-dentine, are very common in fishes. 

 The hard dentine is always external, and 

 holds the place, and performs the office, of 

 enamel in the teeth of higher animals ; but it 

 is only analogous to enamel, not the same 



3 K 



