MICROSCOPIC STRUCTURE. 469 



secondary curves of the tubes were equally present, but the direction 

 of the former somewhat different, the principal bend in the upper 

 and lower tubes being turned rather from, than towards, the 

 centre of the tooth ; but the general principle of arrangement, 

 perpendicular to the tooth's surface equally prevailed. The lower 

 end of the pulp-cavity was quite closed by dentine, the tubes of 

 which descended vertically ; the pulp-cavity forming a centre from 

 which the dentinal tubes radiated on all sides. The normal part 

 of the dentine was developed, as in a premolar, into two cusps, 

 and the pulp-cavity v was single as in a bicuspid of the lower 

 jaw. The enamel was unusually thick, and the cusps were divided 

 by a cleft extending nearly to the dentine. The enamel fibres had 

 the usual form, structure, and direction. The cement became un- 

 usually thick at the closed end of the fang and there formed the 

 medium of adhesion between the dentine and a small portion of 

 true bone. Three or four vascular canals (6) were detected per- 

 forating the cement and dentine which closed the pulp-cavity, 

 and communicating with similar canals in a mass of osteo- 

 dentine (o) which almost blocked up the cavity of the tooth. The 

 thin layer of non-cellular coronal cement was conspicuous over 

 the whole outer surface of the enamel, and presented no increase 

 in thickness in the abnormal median cleft. 



The presence of all the beautiful prospective arrangements of 

 the minutest particles of an organ, by which it is adapted to 

 its office, in an example of the same organ developed abnormally 

 under conditions unfitting it for the exercise of its proper office, 

 affiards no argument against the contrivance, but, rightly consi- 

 dered, enlarges our conceptions of the liberal provision of the 

 contriver. The matrix of a tooth, wherever it may chance to be 

 formed, must so operate in the disposition of the hard building 

 materials of the tooth as to ensure the utmost strength and power 

 of resistance in all those directions in which such tooth would 

 have been acted on, if it had been developed in its proper po- 

 sition. 



Of the four substances composing the human tooth that 

 which is most exposed to outward influences, and which is first 



