466 ■ HUMAN DENTITION. 





apart ; they are more plainly seen in transverse sections of the crown 

 than in longitudinal sections, and they have the same relation to the 

 fibres of the enamel which the contour-lines of the dentine bear to 

 the calcigerous tubes. Without doubt they indicate, in like manner, 

 strata of segments of the fibres and stages in the formation of 

 the substance. Where these strata, which are arranged very ob- 

 liquely to the vertical surface of the dentine, crop out upon that 

 surface they occasion those wavy transverse annular delicate 

 markings which Leeuwenhoek(l) noticed upon the exterior of the 

 enamel, and which he supposed to indicate successive stages in 

 the protrusion of the tooth through the gum, in taking its place 

 in the dental series. The various conditions of the enamel at 

 different periods of its formation have been mentioned in the 

 description of the development of the tooth. (2) 



The cement in the human tooth,(3) as in other simple teeth, 

 is confined to the exterior surface, with the exception of a small 

 portion which, in old teeth, is usually reflected into the entry to 

 the pulp-cavity and sometimes closes it up. This third substance 

 is thinnest upon the crown and very gradually increases in thick- 

 ness as it approaches the end of the fang : it is only on the im- 

 planted part of the tooth that the radiated cells, which demon- 

 strate the close analogy between cement and bone, exist ; elsewhere 

 the clear basis of the cement alone is present, and this is soon 

 worn away from the enamel of the crown. There are no vascular 

 canals in human cement, except when it happens to be abnormally 

 thickened ; in which case, as in the naturally thickened masses of 

 that substance in the teeth of Herbivora, it acquires also this addi- 

 tional feature of resemblance to true bone. The chemical consti- 

 tution of both the animal basis and earthy salts of this dental 

 tissue and of bone is identical ; the hardening particles are chiefly 

 blended in the finest state of sub-division with the clear basis, a 

 portion being contained in a coarser state in the cells ; it is this 

 which occasions their milk-white colour when viewed by reflected 

 light. (4) The cemental cells are generally oblong, sometimes cir- 



(1) ' Select Works' by Hoole, 4to. 1800, p. 115. 



(2) Introduction, p. xlvi. (3) PL 122, fig. 1—7, c. 



(4) The calcareous matter does not occupy the whole of the cavity of the cell ; space 

 is left as in the dentinal tubes, for the transit of fluids. Mr. Smee, (Med. Gazette, 



