340 PHYSIOLOGICAL DEVELOPMENT. 



not receive any extra nutrition did no other action come into 

 play. But if we consider how intermittent pressures must 

 act on cartihige, we shall see that there will result extra 

 nutrition of the concave side also. Squeeze between two 

 pieces of glass a thin bit of caoutchouc that has a hole 

 through it. While the caoutchouc spreads out away from 

 the centre, it also spreads inwards, so as partially to close the 

 hole. Everywhere its molecules move away in directions of 

 least resistance ; and for those near the hole, the direction of 

 least resistance is towards the hole. Let this hole stand for 

 the transverse section of one of the capillaries passing 

 throuo^h cartilao^e, and it will be manifest that on the side of 

 the unossified bone made concave in the way described, the 

 compressed cartilage will squeeze the capillaries traversing 

 it ; and in the absence of perfect homogeneity in the 

 cartilage, the squeeze will cause extra exudation from the 

 capillaries into the cartilage. Thus every additional strain 

 will give to the cartilage it falls upon, an additional supply 

 of the materials for growth. So that presently the side 

 which, by yielding more than any other, proves itself to be 

 the weakest, will cease to be the weakest. What further will 

 happen ? Some otlier side will yield a little — the bends will 

 take place in some other plane ; and the portions of cartilage 

 on which repeated tensions and pressures now fall will be 

 strengthened. Thus the rate of nutrition, greatest at the 

 place where the bending is greatest, and changing as the 

 incidence of forces changes, will bring about at every point a 

 balance between the resistances and the strains. Thus, too, 

 there will be determined that peripheral induration which we 

 see in bones so circumstanced. As in a shoot we saw that the 

 woody deposit takes place towards the outside of the cylinder, 

 where, according to the hypothesis, it ought to take place ; 

 so, here, we see that the excess of exudation and hardening, 

 occurring where the strains are most intense, will form a 

 cylinder having a dense outside and a porous or hollow 

 inside. These processes will be essentially the same 



