176 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



commence at the pulp-cavity (on whose wall their openings may be seen), 

 and radiate towards the surface. In their course outwards, the tubuli occa- 

 sionally divide dichotomously ; and they frequently give oft' minute branches, 

 which again send off smaller ones. In some animals, these tubuli may be 

 traced at their extremities into cells exactly resembling the lacuna? of bone ; 

 and here the Ivory must be considered as presenting a form of transition into 



[Fig. 78. 



A view of a small portion of a transverse section of 

 the crown of the Tooth seen in fig. 70, viewed under a 

 magnifying power of 350 times ; 1, 2, 3, are the round 

 openings of the tubes, with parietes of a peculiar sub- 

 stance ; 4, 5, 6, are the tubes cut more obljfluely, in 

 consequence of their more external position ] 



A view of the position of the same main 

 tubes, in a transverse section near the root 

 of a bicuspid, magnified 5 diameters. The 

 dark patches in this figure mark the places 

 in which the bone was especially white and 

 less transparent than in the clear interme- 

 diate tracts.] 



[Fig. 79. 



Sections of a human incisor, showing: 



A. Junction of dentine and enamel near the neck of the tooth, a. Tubes of the dentine, dividing and 

 ending on b b, the cupped surface on which the enamel rods vertically rest. c. Free surface of the ena- 

 mel. The enamel rods are crossed by transverse lines and also by oblique dark lines. 



B. Bifurcation of the tubuli of the dentine, soon after their commencement on d the surface of the pulp- 

 cavity. 



c. Branching of the tubuli of the fang, and their termination in the small irregular lacuna? of the " gra- 

 nular layer." 



In these longitudinal views of the tubuli, their cavities only, and not their walls, are visible. 

 Magnified 300 diameters.] 



