170 



DIGESTION. 



[CHAP, xxiii. 



to occupy the spaces which would be left by the radiation of un- 

 branched tubes from a common centre. The tubuli are in some 

 parts about their own width asunder, in others they run in mutual 



Fig. 150. 



Sections of a human incisor, shewing : 



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 enamel. 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 '.he surface 

 of the pulp-cavity. 



c. Branching of the tubuli of the fang, and their termination in the small irregular lacunee of 

 the " granular layer." 



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

 Magnified 300 diameters. 



contact ; and their compact proper wall is about as thick as their 

 cavity is wide. Except near the pulp-cavity, they are, as it were, 

 hairy with filamentary canaliculi, which diverge on all sides, and form 

 innumerable junctions with one another ; so that the tubuli, through- 

 out most of the dentine, may be said to communicate with each other 

 independently of the pulp-cavity, into which they all open. To- 

 wards the outer surface of the dentine, where it is encrusted on the 

 crown with enamel and on the root with bone, the tubuli gradually 

 taper, and finally terminate in diminutive canals, which open, some 

 into small, irregular lacunae (forming a layer on the root, termed by 

 Mr. Tomes the granular layer), some into the lacunae of the osseous 

 investment of the fang, and others upon the surface on which the 

 enamel rests. Occasionally the tubuli are dilated into true lacunae, 

 or form free arches of communication. 



