INTRODUCTION. . xlvU 



primary cells of the pulp is usually one fourth or one half larger, 

 than that of the blood-disc of the species manifesting them. These 

 cells are figured, in the present Work, in the molar of the Mylodon 

 (PI. 79), in the incisive tusk of the Dugong (PI. 95), in the pre- 

 molar (PI. 113) and the canine (PI. 113 a) of the Pteropus, in 

 the incisor of the Chimpanzee (PI. 119 a), and of the Human 

 Subject (PI. 123), and in the molar of a Rhinoceros PI. 139. In 

 the calcification of the dentinal pulp the thin transparent membrane 

 which covers the free surface, or that in contact with the enamel- 

 pulp, is the first to receive or become impregnated with the 

 hardening salts ; and hence has been called the ' preformative 

 membrane.' But at this early stage the calcifying membrane 

 yields to the pressure of the ends of the prismatic cells of the 

 enamel pulp, which are, likewise, beginning to take from the 

 surrounding plasma the hardening salts and impact them in their 

 interior. Thus are formed the pits upon the outer surface of the 

 coronal dentine of enamel-covered teeth, by which the enamel gains 

 a firmer mechanical connection with the dentine. As the process 

 of calcification of the multi-nucleated cells of the dentinal pulp 

 extends in its centripetal course, the pulp, in most teeth, progres- 



refer to any publication of mine from which he derived the idea. I describe the intertubular 

 cells in many of the subjects of my " Memoir" in the ' Transactions of the British Associa- 

 tion/ 1838. In Ptychodus, for example: "The interspaces of the canals are also occupied 

 by the same minute anastomosing reticulate tube-work. Numerous minute calcigerous cells 

 are also present in the interspaces," p. 140. But instead of putting forth this as a discovery, 

 and misrepresenting Retzius as describing those interspaces to be amorphous, I premise by citing 

 Retzius's discovery "of the cells in the clear interspaces of the tubes," p. 136. The true 

 dentinal cells, a figure of which in the tooth of the Mylodon was published in the •2nd Part 

 of this Work, PI. 79, in 1840, are very different from the intertubular cells on which M. Serres 

 reports. In most animals they include many tubes and intertubular spaces, and it is much 

 more exact to say that those cells include a tubular structure, than that the intertubular 

 space is cellular. 



