TEETH. 



869 



tnin distance from the base of the tooth ; and 

 this column sends radiating outwards, from 

 its circumference, a series of vertical plates, 

 which divide into two once or twice before 

 they terminate at the periphery of the tooth. 



Each of these diverging and dichotomising 

 plates gives off throughout its course smaller 

 processes, which stand at right angles, or 

 nearly so, to the main plate ; they are gene- 

 rally opposite, but sometimes alternate ; many 

 of the secondary plates or processes, which 

 are given off near the centre of the tooth, also 

 divide into two before they terminate; and 

 their contour is seen, in the transverse section, 

 to partake of all the undulations of the folds 

 of cement which invest and divide the den- 

 tinal plates and processes from each other. 



The dental pulp-cavity is reduced to a 

 mere line about the upper third of the tooth, 

 but throughout its whole extent fissures radiate 

 from it, corresponding in number with the 

 radiating plates of dentine. Each fissure is 

 continued along the middle of each plate, 

 dividing where this divides, and extending 

 along the middle of each bifurcation and pro- 

 cess to within a short distance of the line of 

 cement. The pulp-fissure commonly dilates 

 into a canal at the origin of the lateral pro- 

 cesses of the radiating plates, before it divides 

 to accompany and penetrate those processes. 



The main fissures or radiations of the pulp- 

 cavity extend to within a line or half a line 

 of the periphery of the tooth, and suddenly 

 dilate at their terminations into spaces, which, 

 in transverse section, are subcircular, oval, or 



rally smaller spaces. All these spaces, or 

 canals, in the living tooth, must have been 

 occupied by corresponding processes of the 

 vascular pulp : they constitute so many cen- 

 tres of radiation of the fine calcigerous tubes, 

 which, with their uniting clear substance, 

 constitute the dentine.* 



An analogous complexity is produced by 

 numerous fissures radiating from a central 

 mass of vaso-dentine, which more or less fills 

 up the pulp-cavity of the seemingly simple 

 conical teeth of the extinct family of fishes 

 which I have called " Dendrodonts."f Fig. 

 553. is one of these fossil teeth, of the natu- 



Fig. 553. 



Tooth of a Dendrodus, natural size. 



ral size ; a a transverse section ; and^g. 554- a 

 reduced view of a portion of the same section, 

 enlarged twenty diameters. 



Thus magnified, a central pulp-cavity, of 



Fig. 554. 



Transverse section of tooth of Dendrodus. A, natural size ; B, the portion c, of A, magnified 20 diameters. 



pyriform, p : the branches of the radiating relatively small size, and of an irregular lobu- 

 lines, which are continued into the lateral lated form, is discerned, a portion of which 



secondary plates or processes of the dentinal 

 UunelUe, likewise dilate into similar, and gene- 



* Odout<M?raphy, pp. 195217, pi. G4A, 64 B. 

 f Ib. p. 171. 



3 K 3 



