142 PHYSIOLOGY. 



the grinding surfaces of the tooth. The external membrane of the 

 tooth [matrix] does not adhere to it at first, but forms the enamel on 

 the outside of the bony parts covering the jellies [divisions of dentinal 

 pulp]. As there are more jellies than one to each grinder, there are 

 inequalities on the external surface, or narrow notches, into which 

 these pass in folds of the external membrane, like the pia mater, so as 

 to fill or lie close to all the external bony surfaces. 



These bony processes, which we might term sheaths, unite at their 

 edges to one another ; so that there are cavities between them through 

 their whole length, running parallel to the others which contain the 

 jellies. 



These cavities or canals are filled up with gelatinous substances, which 

 pass into them from the general covering at the base [of the matrix] 

 and terminate at the bottom of the tooth in a ragged white end between 

 it and the basis of the other jellies touching them ; for, at first, these 

 interstitial cavities are pervious at both ends [top and bottom], and 

 continue so till the tooth is almost completely formed. 



These interstitial jellies form the enamel which runs the whole length 

 of the tooth, along its centre in a very irregular manner, answering to 

 the form of the irregular cavities formed by the union of the bony parts. 

 The doublings or folds of the internal part of the external covering 

 which we mentioned as pressed in laterally, have the same white ter- 

 mination at bottom which we took notice of in the above described. 

 When the tooth is pretty far advanced, the bottom or apices of these 

 interstitial cavities close up, and then may be said to form but one 

 cavity, all the jellies uniting into one : afterwards, two or three fangs 

 arise, as in the human grinders. 



The external membrane, with its folds, forms the external enamel ; 

 and, when that is formed, it then forms a thin bony covering [cement] 

 to the whole, which fills up the niches and makes the external surface 

 more equal at this time ; at this time, too, it unites with these bony 

 coverings, beginning first at the bases and proceeding to the root or 

 fang, as the tooth advances through the gum. The interstitial jellies, 

 also, form the interstitial enamel ; and, when that is completed, they 

 line the whole with a bony matter [csementum], or, in other words, fill 

 up the whole interstices with a bony matter. When the interstitial 

 jellies begin to form this bony matter, they also adhere to it as the 

 external membrane did first, beginning at the bases ; which adhesion 

 goes to the bottom as the external adhesion did. The interstitial jelly 

 becomes more and more membranous as it fills up this cavity. By the 

 time the tooth is just ready to cut the gum, these interstitial cavities 



