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



furrows, and of a l^rowuisli colour. The surfaces of the protruded part again were com- 

 paratively smooth, and of a yellow colour. A longitudinal section was then made through 

 the shaft from the alveolar border to the upper end close to the base of the denticle. 

 The shaft was then seen to be solid thi'oughout, except for a minute mesial chink 1-1 0th 

 of an inch long, and admitting only the point of a fine needle, which was situated 7-lOths 

 of an inch from the upper end of the shaft. 



To the naked eye the shaft consisted in the greater part of its length of an external 

 cortical part investing a central band. The cortical part was of a dull yellow colour ; at 

 the alveolar end it formed a thin lamina on each surface of the tooth, but at and near the 

 line of emergence from the gum it was 2-lOths of an inch thick, and on the extruded 

 part of the shaft it averged about 1-1 0th of an inch in thickness. The cortical layer 

 consisted of cement containing well-marked lacunae and canaliculi. In the centre of many 

 of the lacunae a minute solid particle was situated, apparently the dried and shrivelled 

 mass of nucleated protoplasm which occupies the lacuna in a living tooth.^ 



Sections through Haversian canals were occasionally seen in the cement, more 

 especially in its deeper part. The surface of the section through the cement was marked 

 by numerous lines placed pai'aUel to each other, and to the surface of the tooth, which 

 gave it a laminated appearance. 



In the alveolar part of the tooth, and in the larger portion of the protruded part of 

 the shaft, the cortical layer was in apposition with the central band, which had an opaque 

 white appearance, and varied in breadth from 2-lOths to 4-lOths of an inch. This band 

 was traversed by canals, some of which were continuous with those of the cement, though 

 others were divided transversely and obliquely. The matrix between these canals had a 

 granulated appearance. The opaque central band had, therefore, the structural characters 

 of the modified vaso-dentine described in the young tooth. 



The upper end of the shaft, in proximity to the base of the denticle, was complex in 

 structure, and consisted of several layers (fig. 19) ; a, the most supei'ficial, consisted of 

 cement, in which, however, no Haversian canals were seen. Immediately subjacent to a 

 was the layer h, thicker than the cement, and of an opaque white appearance : it had the 

 same general structure as the central band of the shaft, and the chief vascular canals were 

 directed perpendicularly to the surface of the tooth. The next layer, c, was from ^d to 

 ^th the thickness of b, and was even more opac[ue ; some vascular canals were seen to pass 

 at intervals from it into the layer h. Subjacent to c was the layer d, which was about equal 

 to it in thickness : it was very translucent, and contained undulating and branched 

 dentine tubes, which ran outwards to the layer c. In one or two places c was less opaque 

 than usual, and could be seen to contain closely aggregated tubes, not unlike dentine 

 tubes, in addition to the vascular canals already referred to ; c may, therefore, be regarded 

 as vaso-dentine, whilst d is pure dentine. As these two layers were traced from the 



' See fig. 207, p. 756, of my Introduction to Human Anatomy, for an illustration of the contents of the lacunse of a tooth. 



