468 HUMAN DENTITION. 



and from true bone, and as easily recognizable."(l) Of these is 

 the tissue, there first defined and which I have since called 

 * osteo-dentine,' from its combining the vascular concentric-coated 

 canals of the osseous tissue, with a development of the fine tubes 

 resembling those of true dentine, but with stronger and less re- 

 gular curvatures. This substance is found lining the pulp-cavity 

 of old teeth ; and sometimes forms the middle part of the grinding 

 surface of much worn molars : but the conversion of the remains 

 of the pulp into the osteo-dentine is constant in human teeth after 

 the age of twenty years. (2) 



To ascertain what modifications of structure the dental tissues 

 might present in teeth which are sometimes developed, with hair 

 and portions of bone, in the human ovarium, I examined micro- 

 scopically one of those ovarian teeth, of which a longitudinal slice, 

 taken through the middle, is figured in PI. 124. The intimate 

 structure of the dentine d, enamel e, and cement c, was essentially 

 identical with those of the ordinary maxillary teeth. The dentinal 

 cells and tubes presented the same dimensions : the primary and 



(1) "One of these substances is characterised by being traversed throughout by numerous 

 coarse canals, filled %vith a highly vascular medulla or pulp, sometimes anastomosing re- 

 ticularly — sometimes diverging and frequently branching, — sometimes disposed nearly parallel 

 with one another, and presenting more or fewer dichotomous divisions. The canals in many 

 cases are surrounded by concentric lamellae, and thus resemble very closely the Haversian 

 canals of true bone ; but the calcigerous tubes which every where radiate from them are 

 relatively much larger. ITie highly organized tooth-substance just described differs from true 

 osseous substance, and from coementum, in the absence of the Purkingian corpuscles or cells." 

 Trans. Brit. Assoc. 1838. p. 137. 



(2) Lintott 'On the structure of the Human teeth.' 1843. John Hunter appears first 

 to have called attention to this process which prevents the exposure of the pulp-cavity 

 when the crown is worn low down ; he calls the substance ' new matter' and says " it may 

 be easily known from the old, for when a tooth has been worn down almost to the neck 

 a spot may always be seen in the middle, which is more transparent and at the same 

 time of a darker colour, and generally softer than the other." Hunter's success in in- 

 jecting vessels in the cavities of the teeth in very old people is explained by the existence 

 of vascular canals in the osteo-dentine and their communication with similar canals per- 

 forating the thickened cement. I have already noticed the abundance of this substance 

 in the teeth of the Cetacea, and its centrifugal development from detached centres in the 

 substance of the pulp of the Cachalot, forming there sometimes detached masses (p. 360). 

 A modification of osteo-dentine forms, as we have seen, the main body of the teeth of 

 the Sloths and Megatheroid quadrupeds ; but the transition from the normal hard dentine 

 to its vascular modification is more gradual in the Human teeth. 



