DEVELOPMENT OF THE TEETH. 



379 



Fig. 240. 



destitute of them; and is bounded by a simple membrane the mem- 

 Irana prceformativa. (Raschkow.) (Fig. 239.) Beneath this is a layer 

 of elongated cells ( ? 5 to -gfo of an inch long, by g^ to 2 g ! gg of 

 an inch wide), with vesicular nuclei and distinct single or double 

 nucleoli, arranged like an epithelium, though not so sharply defined 

 internally; and next is the paren- 

 chyma of the pulp, consisting of 

 a sort of rudimentary collagenous 

 tissue, with many rounded or elon- 

 gated nuclei, and the vessels. (Fig. 

 240.) The latter become very nu- 

 merous at the period when the den- 

 tine begins to be formed, the most 

 numerous perpendicular loops of 

 capillaries, ^^ of an inch in di- 

 ameter, being in contiguity with 

 the surface of the dentine. The 

 nerves are developed later. Their 

 distribution, as well as that of the 

 vessels, in the pulp of the perfect 

 tooth, has already been described 

 (p. 375). 



It is from the epithelium-like 

 layer of cells that the dentine is formed; and the former seems to 

 maintain a constant thickness by the elongation of the original cells 

 internally (while the dentine is formed externally), accompanied by 

 a continual multiplication of their nuclei. The parenchyma of the 

 pulp, therefore, progressively diminishes as the dentine increases ; 

 the latter being formed in concentric layers from without inwards, 

 like the lamellae of the Haversian rods of bone (p. 356). 



It appears that the cells just described become the solid dentine 

 by the gradual reception of calcareous salts. The largest tubuli 

 are probably the remaining unossified portions of the cavity of the 

 cells, and are hence analogous to the Iacuna3 of bone. The divi- 

 sions of the tubuli may result from a longitudinal division of the 

 cells, or from a single cell coalescing with two of its predecessors. 

 The finest lateral branches appear to be of secondary origin, and 

 probably result from resorption of already formed dentine, like the 

 pore? of bone (p. 354). 



During the ossification of the dentine, and while recently formed 



Surface of the dentinal pulp of a new-born 

 infant, a. Dentinal cells, b. Their appendages. 

 c. Vascular part of the pulp. Magnified 300 

 diameters. (Kottiker.) 



