470 Royal Society : — 



whole of fig. 1 on the new plane by lines parallel to Cc, Dc?. The 

 projection so obtained will be the figure of the arch and abutments 

 required. Moreover, if the lines K, R, fig. 1, represent in length, 

 direction, and position, the resultants of the pressures of the abut- 

 ments on their foundations in the original arch, then will r, r, fig. 2, 

 the projections of R, R, represent the corresponding resultants in 

 the new arch ; and in like manner, the thrust at a is the projection 

 of the thrust at A. 



Glasgow, 18th February, 1856. W. J. MacquoRN RankiNE. 



Note. The horizontal foundation courses in fig. 2 do not form part 

 of the projection of fig. 1, but are supposed to be added after the 

 completion of the projection. 



March 13. — Sir Benjamin C. Brodie, Bart., V.P., in the Chair. 



The following communication was read : — 



" On the presence of fibrils of soft tissue in the Dentinal Tubes." 

 By John Tomes, Esq., F.R.S. 



Referring to the structural characters of dentine, and to the pre- 

 vailing behef that the dentinal tubes in the normal condition contain 

 fluid, the author goes on to show that the recognized histological 

 characters fail to account for the high degree of sensibility exhibited 

 by the dentine when diseased, or when suddenly exposed by the 

 removal of the enamel. 



It is found, moreover, that the dentine is not uniformly sensitive 

 throughout, but possesses a much higher degree of sensibihty at the 

 peripheral distribution of the dentinal tubes than deeper in the sub- 

 stance of the tooth ; and it is urged that these facts cannot be 

 accounted for by the presence of a fluid in the dentinal tubes, nor 

 by supposing that the hard unyielding dentine is intrinsically en- 

 dowed with sensation. This view of the matter is borne out by the 

 fact, that all sensibility is at once lost if the pulp of the tooth be 

 destroyed. 



Finding that the dentine owed its sensibihty to the presence of 

 the dentinal pulp, and knowing that the tubes have open extremities 

 in contact with the pulp, the author was induced to examine care- 

 fully the contents of the tubes. The investigation resulted in disco- 

 vering that the dentinal tubes, instead of containing fluid only, give 

 passage to fibrils of soft tissue, which pass from the pulp into the 

 tubes where these open upon the surface of the pulp-cavity, and from 

 thence may be traced into the branches. The fibrils may be demon- 

 strated by fracturing a perfectly fresh tooth, and then with a sharp 

 knife taking very thin sections from the dentine near the edge of 

 the pulp-cavity. The dentine will, when cut, break up into small 

 fragments, and from the edges of these the fibrils may be seen 

 extending. Sometimes a small portion of the pulp will be found 

 adherent, in which case the fibrils may be seen to extend from that 

 tissue into the dentine. The fibrils may be shown in a more striking 

 manner by decalcifying a section, and then, when it is placed upon a 

 slide^ tearing the specimen across the direction of the tubes. By 



