INTRODUCTION. XXXI 



These authors describe the parenchyme of the dentinal pulp as 

 being composed of minute uniform spherical granules, without any 

 of the characteristic filaments of cellular tissue, and, in this respect, 

 differing from the enamel-pulp. The free surface of the granular 

 tissue is covered by a peculiarly dense, structureless pellucid mem- 

 brane, which they term the ' preform ative membrane ' because the 

 formation of the dentine commences therein. Blood-vessels soon 

 penetrate the granular pulp, form several anastomoses in their course, 

 through its substance, and terminate in a rich and delicate net-work 

 of capillaries on that part of the surface of the pulp where the 

 dentine has begun to be formed ; the rest of the pulp's surface is 

 covered by the preformative membrane and does not display any 

 capillary reticulation. True nervous filaments cannot be distinguished 

 in the pulp until after its vascularity has been established. The 

 granules of the pulp immediately beneath the preformative membrane 

 have a more elongated form than the rest, and are placed either 

 vertically, or at an acute angle with the membrane. 



The formation of the dentine is preceded by the development 

 of numerous minute elevations on the surface of the pulp, at and 

 near its apex ; these are conjectured to be subsequently transformed 

 into the undulating ridges in which the enamel-fibres are firmly 

 inserted. The preformative membrane becomes of a stony hardness, 

 except at the margin of the recently formed dentine, where it is 

 soft and easily rent. The dentine begins to be formed at the apex 

 of the pulp immediately beneath the preformative membrane. 



Of the exactness of the preceding observations by Purkinje and 

 Raschkow I have had repeated evidence. The more obscure parts 

 of their description of the development of the dentine are quoted 

 and commented on by Dr. Schwann, whose observations on this 



