HUMAN DENTITION. 461 



the thickness of the parietes of the dentinal tubes, in relation to their 

 area, varies in different animals ; it is twice as great, for example, 

 in the incisor of the Dugong as it is in that of Man. In viewing 

 the dentinal tubes lengthwise, a third is usually more faintly seen 

 in the interspace of any two that are in focus (1) ; it is on a different 

 plane, and the tubes on different planes are commonly alternate 

 in position ; viewed in transverse section (2) their arrangement 

 appears quincuncial, yet not regularly so throughout ; so viewed, the 

 distance between each two tubules does not exceed, and often 

 does not equal, the width of one, its parietes and lumen being 

 included. The distance between two tubes viewed longitudinally 

 on the same plane is generally equal to the width of two ; but, if 

 the distance between the opake area of two parallel tubes be 

 estimated, irrespective of the proper tunics of the tubes, it more 

 than equals the width of three such opake arese. Professor Retzius 

 appears to have calculated the relative width of the tubes and 

 their interspaces in the latter mode, and his results have been 

 generally copied by subsequent writers on microscopic anatomy. 

 The method though easy, and applicable to the detection of 

 differential characters in the dental structure of different animals, 

 is inexact in reference to the actual distance between the tunics 

 of the dentinal tubes. The true distance is nearly the same 

 throughout the course of the tubes ; for, as they diverge, they 

 divide and sub-divide dichotomously, maintaining the same diameter 

 to within one fourth or one fifth of their terminations. If the 

 tubes were described according to the order of their formation, 

 the peripheral extremities would be their beginnings, and the 

 dichotomous divisions would rather be the anastomoses of the 

 more numerous and smaller tubes as they gradually converged to 

 terminate, by fewer and wider extremities, in the pulp-cavity. The 

 tubes also send off into the intertubular space very minute branches 

 which divide, and passing into the next interspace, are lost 



(1) PL 123. This is well shown in Retzius' Treatise, PI. 2, (v), fig. 1 b. 



(2) In some parts of the dentine the tubes are occasionally observed to be collected 

 into small groups or bundles. 



