TISSUES OF THE BODY. 



261 



in the alveolus. It is hollow internally, traversed by a canal which, 

 commencing above in the crown, terminates below at the point of the 

 root by a free opening. In the incisor and canine teeth this cavity is 

 single, and is divided in the others according to the number of their roots. 

 It is filled with a peculiar connective-tissue, very vascular, and largely 

 supplied with nerves, which is called the pulp. The nutrition of the whole 

 organ takes place from this as from the Haversian canals of bone. 



From a histologies! point of view the tooth may be regarded as made 

 up of three kinds of tissue (fig. 250), of a coating on the root, called the 

 cement, i.e., a bony substance ; then of a layer covering the crown, 

 known as the enamel (see next section) ; and 

 finally, of a mass situated internally, the proper 

 tissue of the tooth surrounding the cavity just 

 mentioned. This has received the names of the 

 " ivory" "dentine" 



The latter possesses a hardness exceeding that of 

 bone, and must be looked upon as a species of the 

 latter without bone-corpuscles, and with a more regular 

 course in its canaliculi. It appears white, in thin 

 sections, with a satiny lustre frequently, as long as 

 the system of canals is filled with air and is not 

 occupied by a fluid. 



These passages or dental canaliculi appear, in dried 

 sections containing air, as extremely numerous and fine 

 dark tubes of from O'OOH-0'0023 mm. and upwards. 

 They maintain a tolerably parallel course, side by 

 side, perpendicular to the surface of the cavity of 

 the tooth. This is consequently vertical in the 

 middle of the crown (fig. 250), oblique at the 

 sides of the latter, becoming horizontal below towards 

 the root (2). In transverse section the middle and 

 under portions of a tooth display a radiating arrange- 

 ment of the canaliculi. f~ 



If the latter become filled with fluid, they blend 

 into the ground-substance and are rendered partly or 

 altogether invisible, reminding us of what takes place 

 in bone under similar circumstances. 



They correspond farther with the canaliculi of bone in having a 

 special lining layer, which is, however, thicker than in the latter. In 

 macerated dentine this layer appears on sections in 

 the form of tubes projecting beyond the surface. 

 The latter may be easily isolated by the softening 

 action of acids, as well as boiling of the tooth-cartilage 

 or treatment with alkalies, on which they are pre- 

 sented to us as intercommunicating structures (Koelliker, 

 Hoppe, Neumann, Frey, Waldeyer). 



In suitable sections of dentine we may likewise see 

 the canals transversely opened (fig. 251). 



If we now examine into the more minute arrangement 

 of the canaliculi in thin leaves of dentine containing 

 air, we find their number to be greater in that portion of 

 tissue surrounding the central cavity, and in the crown, 

 than in the root. We remark also in the whole course of one of these tuben, 



Fig. 250. A human in- 

 cisor, with the cavity 

 in the axis surrounded 

 by dentine, which lat- 

 ter is covered ahove by 

 enamel, below by ce- 

 ment. 



251. Softened 

 dentine, with trans- 

 versely cut canali- 

 culi. 



