338 Proceedings of Royal Society of Edinburgh. [sess. 
take an adult healthy hone and then a growing hone. The osteo- 
blasts lay down a matrix of formed material. The more active the 
cells, within certain limits, the greater the relative amount of 
matrix. This matrix may now he looked upon as inert or dead 
organic matter, and we hold that it corresponds to a membrane 
through which dialysis may take place, or rather the layers near the 
two surfaces may he so considered, and as the molecular com- 
binations of the phosphoric acid and lime and the carbonic acid and 
lime take place around the osteoblasts (which, as above stated, 
during their active formative changes give off the carbonic acid to 
render the lime for the time insoluble) there is a continuous process 
by dialysis of separation of these lime salts, which first make their 
appearance in the centre of the matrix trabeculse where the two cur- 
rents meet, as it were ; from this point the calcification extending 
towards the surface. We look upon the formed matrix then as play- 
ing the part of a dialysing membrane, that serves to separate the lime 
salts prepared in its immediate neighbourhood by the carbonic acid 
forming cells, this carbonic acid causing a throwing down of phos- 
phates of lime with a small proportion of lime in which the phosphoric 
acid is usually replaced by carbonic acid. It should be observed 
in this connection that the carbonic acid is, when acting on the 
lime solution, in a nascent condition, and therefore in a much 
better position to combine with any lime already held by the 
phosphoric acid. That the phosphate of lime and the alkaline V 
earths generally are deposited merely mechanically in the bones, 
is very evident, as Lehmann points out in his Physiological 
Chemistry , vol. i. p. 414, because we can so thoroughly deprive 
them of all mineral constituents by dilute hydrochloric acid, and 
as we have before pointed out, because the matrix in which they 
are deposited is vitally and chemically inert, though like many 
animal membranes it may still retain the power of allowing of 
dialysis/ In the case of newly formed bones or parts of bones 
there is, according to Valentin (quoted by Lehmann, loc. cit ., vol. i. 
p. 417), “.always a greater quantity of carbonate of lime before they 
are provided with their proper quantity of phosphate.” This bears 
out our theory, because it is whilst the cells are most active in 
building up the matrix that the largest proportion of nascent car- 
bonic acid will be present, and therefore the period at which a 
