EXOSTOSIS OF THE ROOT. xvii 



the deciduous teetli, but is relatively thinner, and the 

 Purkinjean cells are more irregular. 



"*In growing teeth, with roots not fully formed, 

 the cement is so thin that the Purkinjean cells are 

 not visible. It looks like a fine membrane, and has 

 been described as the periosteum of the roots, which 

 are wholly composed of it ; but it increases in thick- 

 ness with the age of the tooth, and is the seat and ori- 

 gin of what are called exostoses of the roots.' These 

 growths are subject to the formation of abscesses, and 

 all the morbid actions of true bone. 



"It is the presence of this osseous snbstance which 

 renders intelhgible many well-known experiments of 

 which human teeth have been the subjects, such as 

 their transplantation and adhesion into the combs of 

 cocks, and the establishment of a vascular connection 

 between the tooth and the comb. 



"Under every modification the cement is the most 

 ■highly organized and most vascular of the dental tis- 

 sues, and its chief use is to form the bond of vital 

 union between the denser and commonly unvascular 

 constituents of the tooth and the bone in w'hich 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 con- 

 tributing to maintain that inequality of the grinding 

 surface of the tooth which is essential to its function 

 as an instrument for the comminution of vegetable 

 substances."* 



* CEivrENT MisTAKET^r FOR Taetak (Odontot/tthosX — Sur- 

 geon E. May hew says ("The Horse's Mouth," &c.) : "Within 

 the alveolar cavityj the crusta petrosa^ which becomes of con- 



