364 



ANATOMY OF VERTEBRATES. 



241 



a fissure, into which a thin outer layer of cement, fig. 244, c, is 

 reflected into the body of the tooth, following the sinuous wavings 



of the lobes of dentine, d, which 

 diverge from the central pulp-cavity, a. 

 The inflected fold of cement, c, runs 

 straight for about half a line, and then 

 becomes wavy, the waves rapidly in- 

 creasing in breadth as they recede from 

 the periphery of the tooth ; the first two, 

 three, or four undulations are simple ; 

 then their contour itself becomes 

 broken by smaller or secondary waves 

 these become stronger as the fold ap- 

 proaches the centre of the tooth, when 

 it increases in thickness, and finally 

 terminates by a slight dilatation or 

 loop close to the pulp-cavity, from 

 which the free margin of the inflected 

 fold of cement is separated by an 

 extremely thin laj^er of dentine. The 

 number of the inflected converrrino; 

 folds of dentine is about fifty at the 

 middle of the crown of the tooth 

 figured, but is greater at the base. 

 All the inflected folds of cemeut at 

 the base of the tooth have the same 

 complicated disposition with increased 

 extent ; but, as they approach their ter- 

 mination towards the upper part of the tooth, they also gradually 

 diminish in breadth, and consequently penetrate to a less distance 

 into the substance of the tooth. Hence, in such a section as is 

 delineated, fig. 244, it will be observed that some of the convo- 

 luted folds, as those marked c, extend near to the centre of the 

 tooth ; others, as those marked c', reach only about half way to 

 the centre ; and those folds, c" , which, to use a geological ex- 

 pression, are ' cropping out,' penetrate to a very short distance 

 into the dentine, and resemble, in their extent and simplicit}^ the 

 converginc; folds of cement in the fanirs of the tooth of tlie 

 Ichthyosaurus and Lepidostcns. 



The disposition of the dentine is still more complicated than 

 that of the cement. It consists of a slender, central, conical 

 column, excavated by a conical pulp-ca^'ity for a certain distance 

 from the base of the tooth ; and this column sends radlatinn; out- 



section of tooth of a Shark (Lamna), 



magn. ; v vaso-4entiiie, d gano-dentine. 



