I 



THE TEETH. 621 



more distinct, and the lacunas, with their contents (the cement-corpuscles), 

 the more numerous, the broader the diameter of the layer of the cementum. 



The outer surface of the cementum is covered on its upper part with a 

 narrow layer of connective tissue and with epithelial elements, in close 

 resemblance with those of Nasmyth's layer of the enamel. This layer turns 

 over into the epithelial coat of the gum. 



I have met once with striking formations on the neck of a tooth. The 

 ordinary cement of the neck is interrupted by grooves or pits containing the 

 elements of pericementum. The inner per- 

 iphery of the pit is covered with a well- 

 developed, evidently isolated, formation 

 of cementum. The island of the cemen- 

 tum is broadest above the bottom of the 



pit, and slopes down along the walls of the |T "j * F 



pit until it is lost within the layer of the 

 cementum of the neck. (See Fig. 266.) 



Enamel. Up to this time the impression 



of most examiners has been that the FlG ' 2 6 7. -LONGITUDINAL SEC- 

 enamel is built up by bundles of rods or TION OF ENAMEL - 



prisms, crossing each other, and traversed ER, enamel-rods, traversed by pre- 

 by faint vertical lines, which give each of Bailing vertical spaces ; EF, enamel- 

 them the appearance of a column sub- 

 divided into small squares. The enamel- diameters, 

 rods doubtless exist, and are wavy close 



to the dentine, and straight on the periphery and the main mass of the enamel. 

 They may be considered as columns of a calcined substance, between which 

 minute spaces are left, analogous to the cement-substance of epithelial for- 

 mations. 



In longitudinal sections we see delicate beaded fibers, which occupy the 

 central portion of the interstices between the enamel-rods. These fibers I 

 propose to term the " enamel-fibers." From such a fiber arise very minute 

 conical fibrillse, which traverse the rims between the fiber and the neighbor- 

 ing outlines of the rods, and fade away from the moment they enter the 

 latter. The columns of the basis-substance themselves are pierced by deli- 

 cate canaliculi, running in an almost vertical direction through the enamel- 

 rods, regularly enough to give the appearance of squares, although these are 

 much smaller than usually represented. In the middle of a minute square 

 light canals are seen, not infrequently running parallel with the outlines of 

 the enamel-rod. The square fields thus produced by the rectangular crossing 

 of light channels look, under the power of 1200 diameters, finely granular. 

 In specimens not fully decalcified it is impossible to decide whether there is a 

 light net-work within the enamel prisms analogous to that in the basis- 

 substance of the dentine and cementum, or whether the granular appearance 

 is merely due to the depositions of lime-salts. (See Fig. 267.) 



Cross-sections of the enamel, which we obtain also in longitudinal sections 

 of the tooth, on account of the different directions of the bundles of the 

 enamel-rods, plainly exhibit the irregular polyhedral fields of the enamel- 

 rods. The light interstices between the polyhedral fields contain in many 

 instances delicate beaded fibers surrounding the polyhedral fields of the 

 enamel-rods. The fibers, if cut transversely, have the appearance of dots, 

 and connect with each other directly or by means of intervening delicate 



