12 THE VOYAGE OF H.M.S. CHALLENGER. 



divided into a superficial and a deep layer by a well-defined line. The superficial layer 

 consisted of cement, in which the lacunae and branching canaliculi were large and distinct. 

 The deeper layer was more opaque, and required a very thin section to determine its 

 structure. It consisted of a granulated matrix traversed by numerous canals, which were 

 for the most part arranged perpendicularly to the surface of- the fang, so as to extend from 

 the dentine to the cement. To some extent they branched, and adjacent canals communi- 

 cated with each other (PI. III. fig. 18). 



The pulp-cavity was lined in the greater part of its extent by a well-defined layer, 

 having a maximum thickness of nearly 1-lOth of an inch. This lining could be seen with 

 the naked eye to extend into the crown of the tooth, reaching on one wall of the cavity to 

 within 1-1 0th of an inch of its apex, on the other to about 2-lOths of an inch. It had 

 a brown colour, and the surface next the pulp-cavity was marked by numerous shallow 

 grooves and small foramina. Examined microscopically this lining had essentially the same 

 structure as the deeper layer of the outer covering of the fang. The canals were, however, 

 of greater calibre in the inner lining, and passed obliquely from the surface next the pulp- 

 cavity to that next the thin layer of dentine (fig. 18). The minute foramina on the free 

 surface of this lining were the openings of these canals. Although, to the naked eye, the 

 dentine, which formed the apex of the pulp-cavity, did not appear to have any of this 

 substance in contact with it, yet, when examined microscopically, the part next the 

 cavity was observed to be discoloured brown, and with its proper dentine tubes often 

 indistinct, whilst some tubes of a larger calibre were seen in it. 



The cement, as capable of recognition by the presence of lacunee and canaliculi, 

 terminated about 2-lOths of an inch from the edge of the cleft-like opening of the pulp- 

 cavity, and previous to its termination it became very thin. The structure which formed 

 the wall of the cleft was directly continiious with the deeper layer of the outer investment 

 of the fang, and with the substance lining the pulp-cavity. It had a similar microscopic 

 appearance, but the part next the cavity had in addition to the obhquely-divided canals, 

 many canals divided transversely. There can, I think, be no doubt, that in the living 

 tooth these canals had contained blood-vessels. In size they approximated to the 

 Haversian canals in bone. 



The question now arises, What is the nature of this very vascular substance, which 

 formed the lining of the pulp-cavity, and the deeper layer of the external investment of 

 the fang 1 If one could have traced its development, one would have had no difficulty in 

 answering this question, as the several tissues of a tooth arise from definite structures. 

 Thus the enamel proceeds from the enamel organ, the cement from the alveolo-dental 

 periosteum, and the dentine, with its modifications termed vaso-dentine and osteo-dentine, 

 from the pulp. The absence, however, of both dentine tubes and lacunae and canaliculi in 

 its matrix, and the presence of vascular canals, leave doubts, from a structural point of view, 

 whether this substance ought to be regarded as a modified cement or a modified vaso- 



