460 HUMAN DENTITION. 



straight ; but, as soon as ihey begin to incline from the perpendicular, 

 they become bent into two or, more commonly, three gentle wavy 

 curves ; at the sides of the crown and along the upper half of 

 the fang the tubes form a short bend concave towards the crown, 

 then a longer bend in the opposite, and finally a third bend in the 

 first direction ; the general curve being concave downwards : in 

 the lower half of the fang the first curvature with the concavity up- 

 wards is the principal one. These * primary curves' are on the same 

 plane or nearly so. The * secondary curves, '(1) of which two hun- 

 dred may be counted in an extent of Ath of an inch of the length of 

 a tube, are not on the same plane, but form slight gyrations through 

 the whole course of the tube. Both the primary and secondary curves 

 of adjacent tubes are parallel ; and occasionally the tubes make a 

 short bend along a line parallel with the outer contour of the crown, 

 giving rise to the appearance which may be called ' contour lines ;'(2) 

 the parallelism of the entire tubes being affected only by the 

 amount of their divergence in radiating from the pulp-cavity. As a 

 general rule the tubes are nearly perpendicular to the surface of 

 the dentine, and take an almost direct course from the pulp-cavity 

 to that surface. Thin sections of dentine taken across the tubes, 

 viewed by transmitted light with a five hundred times linear mag- 

 nifying power, show the clear area of the tube more or less occupied 

 by a dark subgranular opake calcareous substance, (3) and a clear 

 border(4) to the area, neatly defined from the intertubular sub- 

 stance, (5) and indicating the proper parietes of the tube. The addi- 

 tion of dilute muriatic acid dissolves the opake contents of the tube 

 and clears the whole circular area : it dissolves, at the same time, 

 the finer particles of lime in combination with the animal basis of 

 the intertubular substance and parietes of the tubes, which latter 

 then cease to be distinguishable. This shows that either the lime- 

 particles have a peculiar arrangement in the parietes of the tubes, 

 or are more abundant there than in the intermediate substance, since 

 the parietes can be distinguishable only whilst those particles are 

 present: the appearance can scarcely be an optical deception because 



(I) PI. 122 a, fig. 5. PI. 122 a, fig. 1. (2) PL 122, fig. 7, I- 



(3) PI. 124 a, fig. 4. (4) lb. h. (5) lb. i. 



