INTRODUCTION. XXI 



the radiated cells are present in the bone of the skeleton and in the 

 dental cement there is a close conformity as to their size and shape 

 in both tissues. 



The most remarkable modification of mammalian cement is 

 presented by the thick layer of that substance which invests the 

 molars of the extinct megatherium ;(1) besides abounding in calcige- 

 rous cells it is here traversed by straight, parallel and occasionally 

 bifurcated medullary canals, arranged with regular intervals, and 

 directed from the exterior of the tooth somewhat obliquely to the 

 surface of the unvascular dentine, close to which they anastomose 

 by loops, corresponding with, and opposite to those formed by the 

 medullary canals of the vascular dentine of the same tooth (2). 



Under every modification the cement is the most highly organ- 

 ized and most vascular of the dental tissues, and its chief use is to 

 form the bond of vital union between the denser and commonly un- 

 vascular constituents of the tooth and the bone in which the tooth is 

 implanted. In a few reptiles (now extinct) and in the herbivorous 

 mammalia the cement not only invests the exterior of the teeth, but 

 penetrates their substance in vertical folds, varying in number, form, 

 extent, thickness and degree of complexity, and contributing to main- 

 tain that inequality of the grinding surface of the tooth which is 

 essential to its function as an instrument for the comminution of vege- 

 table substances. 



The higher an animal is placed in the scale of organization, the 

 more distinct and characteristic are not only the various organs of 

 the body, but the different tissues which enter into their composition. 



(I) Plate 84, 6. (2) PI. 84, «. 



