FORMATION OF THE DEXTAL SUBSTANCES. 



797 



matrix containing a few very fine molecules, thinly disseminated in it. At the 

 exterior of the pulp, the cells become elongated, and arranged perpendicularly to the 



Fig. 555. VERTICAL TRANSVERSE SEC- Pig 555^ 



TION OF THE DENTAL SAC, PULP, &C. , 



OF A KITTEN (from Kolliker after a 

 preparation by Tiersch). \* 



a, dental papilla or pulp, the outer 

 darker part consisting of the dentine 

 cells ; 6, the cap of dentine formed upon 

 the summit ; c, its covering of enamel ; 

 d, inner layer of epithelium of the 

 enamel organ ; e, gelatinous tissue ; /, 

 outer epithelial layer of the enamel 

 organ ; g, inner layer, and h, outer 

 layer of the dental sac. 



surface, so as to form a tolerably regular 

 layer, resembling a columnar epi- 

 thelium. The pulp contains white 

 areolar fibres, without any elastic or 

 yellow tissue, and it is highly vascular. 

 The capillary vessels are most abun- 

 dant at the points where ossification 

 is to commence ; they form a series of 

 loops between rows of cells arranged 

 in a radiate manner, but they do not 

 reach the surface. Besides this, the 

 entire pulp is covered by a fine pellucid 

 homogeneous membrane, named the 

 preformative membrane (Purkinje, 

 Raschkow), or basement membrane. 

 The space between the pulp and the 



wall of the sac is occupied by a delicate substance accurately applied to its surface. 

 This is the outer pulp of Hunter, termed also the enamel-organ (organon adamantines, 

 Purkinje), being generally considered to be connected with the formation of the enamel. 

 It presents three layers ; viz., externally, an epithelial layer with prominences which 

 fit in between vascular processes of the surrounding mucous membrane ; internally, a 

 layer of cylindrical nucleated cells, named the enamel-membrane, resting on the pre- 

 formative membrane ; and between these, a bulky substance, consisting of small stellate 

 cells anastomosing by long processes, and having the large meshes between them filled 

 with clear fluid. This structure was formerly supposed to be similar to the primitive 

 pulp ; but, as was first stated by Huxley and since confirmed by Kolliker, the whole 

 enamel-organ is epithelial in nature, being derived by invagination from the cuticle. 



The dentine is formed at the surface of the pulp, beneath the preformative mem- 

 brane, but the precise manner in which it is derived from the soft tissues is still 

 a matter for investigation, According to Purkinje, Retzius, and Raschkow, the 

 preformative membrane is the part which first undergoes calcification, and after- 

 wards the tissue of the pulp immediately beneath it. On gently separating the 

 newly formed cap of dentine from the formative pulp, in the growing teeth of the 

 human subject or of animals, and examining it under the microscope, the elongated 

 cells of the pulp are found adhering in numbers to the inner surface of the newly- 

 formed dentine. Owen states that the nuclei of the elongated cells, having them- 

 selves become lengthened, divide both longitudinally and transversely to develop 

 secondary cells which continue included within the primary cells. The secondary 

 cells then elongate, and together with their nuclei join end to end. Calcification 

 proceeds in all parts, except in the nuclei of the secondary cells which remain as 

 the cavities or lumina of the tubes ; the walls of the secondary cells are supposed 

 to form the parietes of the tubes, and the material between the secondary cells 

 together with the walls of the primary cells to be converted into the intertubular sub- 

 stance. The bifurcation of the tubuli is said to result from the junction of two 



