CHAP. XXIII.] 



THE ENAMEL. 



171 



Fig. 151. 



The following facts illustrate the foregoing account of the structure 

 of dentine. The granularity of the ultimate tissue may be best seen 

 in specimens in course of development. On the surface of the pulp- 

 cavity the orifices of the tubuli can be seen ; and in transverse sec- 

 tions of the tubuli * (fig. 151), their proper 

 walls, the width of their walls and calibre, 

 and their distance apart, are all discernible. 

 In broken fragments, especially if torn after 

 the tooth has been softened in acid, the 

 tubuli may be observed to stand out from 

 the surface, being broken off at different 

 lengths, as if their structure was distinct 

 from the intertubular tissue. Their hollow- 

 ness is proved by the chasing of bubbles 

 along them, visible under the microscope, 

 when turpentine is added to a dry section, 

 and also by the gas which may be seen to 

 be disengaged in bubbles, chiefly from their 

 interior, when a section is similarly treated 

 with acid. The latter experiment seems 

 also to shew that the parietes of the tubuli 

 contain a denser deposit of earthy matter, 

 and are consequently harder and more re- 

 sisting than the intertubular tissue. We 



do not regard the tubuli as filled Up by Human molar.-Magn. 400 diam 



solid contents, but as possessing a truly hollow bore, designed to 

 give passage to fluids. 



The enamel, investing the crown of the tooth, and forming that 

 part which is exposed in the mouth to the contact of external sub- 

 stances, is harder and compacter than the dentine, and of peculiar 

 structure, although formed, as will be afterwards shewn, on the 

 same general plan as the other portions of organized bodies. As 

 its earthy constituents are in much larger proportion than in the 

 dentine (for they make up 98 instead of 72 out of 100 parts, 

 according to Berzelius), the enamel requires much less nutrient 

 change, and its interstitial passages are very minute. 



The enamel (fig. 150, A. and fig. 152) consists of a congeries 

 of hexagonal rods, placed endwise side by side, so as to form a 

 layer, of which the surfaces are formed by the ends of the rods, 

 and the thickness is determined by their length. The deep surface 



* Mr. Topping, of York Place, New Road, mounts these and other objects 

 very skilfully. 



, Transverse sections of tubules of 



dentine, shewing their cavities, their 



waiis, and the intertubular tissue. 



a - Ordinary distance apart. 



* More crowded. 



c. Another view. 



