472 SPECIAL HISTOLOGY. 



the tooth retains the exact form of the dentine and its external struc- 

 ture also ; the tubules, however, are seen with difficulty. If it be 

 macerated in acids or alkalies until quite soft, the matrix undergoes 

 incipient solution, but the dentinal tubules, with their walls, offer greater 

 resistance, and may be readily and abundantly isolated (see " Mikr. 

 Anat." ii. 2, p. 61, fig. 189). By still longer maceration, all is dis- 

 solved. If teeth be heated to redness, or treated with caustic alkalies, 

 the inorganic portions likewise retain the form of the tooth. It follows, 

 then, that the same intimate mixture of inorganic and organic parts 

 occurs in the teeth, as in the bones, with which they so closely agree 

 in their chemical composition. 



The apparent walls of the dentinal tubuli, which are commonly visible 

 in transverse sections (Fig. 184), are not the actual walls of the canals, 

 but rings, which result from our invariably viewing a certain length of 

 the canals in the always more or less thick sections, their undulated 

 course giving the walls a greater apparent thickness than they really 

 possess. If in any transverse section the apertures of the canals be 

 exactly brought into focus, we perceive, instead of the dark ring, only 

 a very narrow yellowish border, which is what I consider to be the 

 actual wall. That such is the case, appears from the examination of 

 transverse and oblique sections of canals filled with fluid, in which short, 

 yellow tubules and small rings of almost the same diameter as that of 

 the cavities of the canals, may be clearly recognized. 



The dentine occasionally presents indications of lamination in the 

 form of arched lines running more or less parallel and at different dis- 

 tances, often quite close together (Fig. 187) ; which in transverse sec- 

 tions appear as rings, and are especially distinct in the crown. These, 

 the contour lines of Owen,* are not the same with the glistening, 

 indistinctly defined striae observed by Schreger, which run exactly 



* [This is not exactly correct. The term " contour lines," as used by Professor Owen 

 ("Report of British Association" for 1838, p. 135, and " Odontography," pp. 460, 464, 611), 

 includes both descriptions of markings mentioned in the text, but is more especially em- 

 ployed for Schreger's. The ordinary contour lines, in fact, are stated by Professor Owen to 

 proceed from " a short bend" of the tubuli " parallel with the outer contour of the crown ;" 

 from these, the Professor distinguishes the " strong contour lines," in the ivory of the ele- 

 phant's tusk, as being produced by " strata of extremely minute opaque cellules." It should 

 be observed, however, that Retzius had long before drawn attention to these peculiar striae. 

 In his admirable memoir, published in Muller's Archiv, for 1837, he says, p. 507, "In the 

 incisor teeth of the horse, also, many less transparent striae running parallel with the cavitas 

 pulpcB may be seen, like the annual rings in the trunk of a tree. They proceeded in this 

 case, however, not merely from certain parallel flexures of the tubes, but especially from 

 similar calcareous cells, which had accumulated in one zone for the greatest part of the length 

 cf the tooth. Tab. xxii Fig. 3." See also his explanation of the zones in the Elephant's 

 tooth, at pp. 510-11. TRS.] 



