THE DENTAL TUBULES. 787 



far correspond, that the tubes are on the whole nearly parallel, being only 

 slightly divergent as they pass towards the surface ; and as they divide 

 several times dichotomously, and at first without being much diminished in 

 size, they continue to occupy the substance of the dentine with nearly equi- 

 distant tubes, and thus produce, when seen in fine sections of the tooth 

 made parallel to their course, a striated appearance, as if the dentine were 

 made up of fine parallel fibres. The concurrence of many of these parallel 

 curvatures of the dental tubuli produces, by the manner in which they 

 reflect the light, an appearance of concentric undulations in the dentine, 

 which may be well seen with a low magnifying power. This, however, is 

 not to be confounded with another set of curved marks called contour lines, 

 which depend on conditions of the matrix, and will be afterwards described. 

 The average diameter of each tubule near its inner and larger end is -j^^th 

 of an inch, and the distance between adjacent tubules is about two or three 

 times their width. (Retzius.) From their sides numerous immeasurably 

 fine branches are given off, which penetrate the hard iutertubular substance, 

 where they either anastomose or terminate blindly. These lateral ramuscles 

 are said to be more abundant in the fang. Near the periphery of the ivory 

 they are very numerous, and, together with the main tubules themselves, 

 which there, by rapid division and subdivision, also become very fine, ter- 

 minate by joining together in loops, or end in little dilatations, or in the 

 cells of the granular layer to be described. 



The dental tubules, when highly magnified, appear like dark lines against 

 transmitted light, but are white when seen upon a black ground. Their 

 tubular character is proved by the fact that ink, or other coloured fluids, 

 together with minute bells of air, can be made to pass along them, in sec- 

 tions of dry teeth. Their walls, in transverse sections, may often appear 

 thicker than they are in reality, owing to a certain length of the tubes being 

 seen in the section : but if the orifice of the canal be brought exactly into 

 focus, the wall appears as only a very thin, yellowish border ; and, indeed, 

 Kolliker denies the existence of any wall distinct from the matrix. From 

 the researches of Nasmyth, Tomes, and Kolliker, it appears that in the 

 recent state the tubules are filled with substance (dental fibres), continuous 

 with the pulp of the tooth : and it is suggested by Tomes that this is not 

 only subservient to the nutrition of the dentine, but probably also confers 

 on it a certain degree of sensibility. It has been noticed, indeed, that the 

 dentine is more sensitive near the surface than deeper in its substance, a 

 fact not easily intelligible on the supposition that the sentient tissue is 

 confined to the pulp-cavity. 



In the temporary, and sometimes even in the permanent teeth, the tubules are 

 constricted at short intervals, so as to present a moniliform character. The terminal 

 branches of tubules are occasionally seen to pass on into the cement which covers the 

 fang, and to communicate with the small ramified canals of the characteristic lacunas 

 found in that osseous layer. Tubules have likewise been observed by Tomes passing 

 on into the enamel, more especially in the teeth of marsupial animals, but in a less 

 marked degree in human teeth. 



The intertubular substance is translucent. The animal matter which 

 remains in it, after the earthy matter has been removed by an acid, 

 exhibits a tendency to tear in the direction of the tubules, but is in reality 

 a homogeneous substance, deposited in a laminated manner. This was 

 shown by Sharpey, who observed that in the softened teeth of the cachalot 

 or sperm-whale the animal substance was readily torn into fine lamellae, 

 disposed parallelly with the internal surface of the pulp-cavity, and there- 



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