648 



ORGANS OF DIGESTION. 



Fig. 322. Vertical 

 Section of a Molar 

 Tooth. 



Fig. 323. Vertical Section 

 of a Bicuspid Tooth. 

 Magnified. 



STRUCTUEE OF THE TEETH. 



Structure. On making a vertical section of a tooth (fig. 322), a hollow cavity 

 will be found in the interior. This cavity is situated at the base of the crown, and 

 is continuous with a canal which traverses the centre of 

 each fang, and opens by a minute orifice at its extremity. 

 The shape of the cavity corresponds somewhat with that of 

 the tooth : it forms what is called the pulp cavity, and con- 

 tains a soft, highly vascular, and sensitive substance, the 

 dental pulp. The pulp is richly supplied with vessels and 

 nerves, which enter the cavity through the small aperture at 

 the point of each fang. 



The solid portion of the tooth consists of three distinct 

 structures, viz., ivory (tooth-bone or dentine), which forms 

 the larger portion of the tooth ; enamel, which covers the 

 exposed part or crown ; and the cortical substance or cement 

 (crusta petrosa), which is disposed as a thin layer on the surface of the fang. 



The IVOKY or dentine (fig. 323) forms the principal mass of a tooth ; in its 

 central part is the cavity inclosing the pulp. It is a modification of the osseous 

 tisue, from which it differs, however, in structure and chemical composition. On 



examination with the microscope, it is seen to consist 

 of a number of minute wavy and branching tubes, 

 having distinct parietes. They are called the dental 

 tubuli, and are imbedded in a dense homogeneous sub- 

 stance, the intertubular tissue. 



The dental tubuli are placed parallel with one ano- 

 ther, and open at their inner ends into the pulp cavity. 

 They pursue a wavy and undulating course towards 

 the periphery. The direction of these tubes varies; 

 they are vertical in the upper portion of the crown, 

 oblique in the neck and upper part of the root, and 

 towards the lower part of the root they are inclined 

 downwards. The tubuli, at their commencement, 

 are about ^VTT of an inch in diameter; in their 

 course they divide and subdivide dichotomously, 

 so as to give to the cut surface of the dentine a striated 

 appearance. From the sides of the tubes, especially 

 in the fang, ramifications of extreme minuteness are 

 given off, which join together in loops in the inter- 

 tubular substance, or terminate in small dilatations, from 

 which branches are given off. Near the periphery of 

 the dentine, the finer ramifications of the tubuli ter- 

 minate in a somewhat similar manner. In the fang, 

 these ramifications occasionally pass into the crusta 

 petrosa. The dental tubuli have comparatively thick 

 walls, and contain, according to Mr. Tomes, slender cylindrical prolongations of 

 the pulp-tissue. 



The intertubular substance is translucent, finely granular, and contains the chief 

 part of the earthy matter of the dentine. After the earthy matter has been 

 removed, by steeping a tooth in weak acid, the animal basis remaining is described 

 by Henle as consisting of bundles of pale, granular, flattened fibres, running 

 parallel with the tubes ; but by Mr. Nasmyth as consisting of a mass of brick- 

 shaped cells surrounding the tubules. By Czermak and Mr. Salter it is supposed 

 to consist of laminae which run parallel with the pulp cavity, across the direction 

 of the tubes. 



Chemical Composition. According to Berzelius and Bibra, dentine consists of 

 28 parts of animal, and 72 of earthy matter. The animal matter is resolvable by 



