714 



ORGANS OF DIGESTION. 



Fig. 388. Vertical Sec- 

 tion of a Bicuspid Tooth. 

 (.Magnified.) 



tch 



Fang 



open at their inner ends into the pulp cavity. They pursue a wavy and undu- 

 lating 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 5^5^ 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 ex- 

 treme minuteness are given off, which join together in 

 loops in the intertubular substance, or terminate in small 

 dilatations, from which branches are given off. Near 

 the periphery of the dentine, the finer ramifications of 

 the tubuli terminate 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, slen- 

 der cylindrical prolongations of the pulp-tissue. 



The intertubular substance is translucent, finely granu- 

 lar, 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 re- 

 maining 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 lamina 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 boiling into gelatin. The earthy matter consists of phosphate of lime, car- 

 bonate of lime, a trace of fluoride of calcium, phosphate of magnesia and other 

 salts. 



The Enamel is the hardest and most compact part of a tooth, and forms a thin 

 crust over the exposed part of the crown, as far as the commencement of the 

 fang. It is thickest on the grinding surface of the crown, until worn away by 

 attrition, and becomes thinner towards the neck. It consists of a congeries of 

 minute hexagonal rods. They lie parallel with one another, resting by one 

 extremity upon the dentine, which presents a number of minute depressions for 

 their reception; and forming the free surface of the crown by the other extre- 

 mity. These fibres are directed vertically on the summit of the crown, horizon- 

 tally at the sides; they are about the S^B of an inch in diameter, and pursue 

 a more or less wavy course, which gives to the cut surface of the enamel the 

 appearance of a series of concentric lines. 



Numerous minute interstices intervene between the enamel-fibres near their 

 dentinal surface, a provision calculated to allow of the permeation of fluids from 

 ihe dentinal tubuli into the substance of the enamel. The enamel-rods consist 

 of solid hexagonal or four-sided prisms, connected by their surfaces and ends, 

 and filled with calcareous matter. If the latter is removed, by weak acid, from 

 newly-formed or growing enamel, it will be found to present a network of deli- 

 cate prismatic cells of animal matter. 



Chemical Composition. According to Bibra, enamel consists of 96.5 per cent, 

 of earthy matter, and 3.5 per cent of animal matter. The earthy matter consists 

 of phosphate of lime, with traces of fluoride of calcium, carbonate of lime, phos- 

 phate of muiiiH'sia, and other salts. 



The Cortical Substance, or Cement (crusta petrosa), is disposed as a thin layer 



