984 ON FORM AND MECHANICAL EFFICIENCY [ch. 



equilibrimn, or minimal disturbance ; but if it be inclined obliquely 

 to the pressure-lines, the shearing force will at once tend to act 

 upon it and move it away. This is neither more nor less than what 

 happens when we comb our hair, or card a lock of wool : filaments 

 lying in the direction of the comb's path remain where they were ; 

 but the others, under the influence of an obUque component of 

 pressure, are sheared out of their places till they too come into 

 coincidence with the Unes of force. So straws show how the wind 

 blows — or rather how it has been blowing. For every straw that 

 lies askew to the wind's path tends to be sheared into it; but as 

 soon as it has come to lie the way of the wind it tends to be 

 disturbed no more, save (of course) by a violence such as to hurl 

 it bodily away. 



In the biological aspect of the case, we must always remember 

 that our bone is not only a living, but a highly plastic structure; 

 the httle trabeculae are constantly being formed and deformed, 

 demohshed and formed anew. Here, for once, it is safe to say that 

 "heredity" need not and cannot be invoked to account for the 

 configuration and arrangement of the trabeculae: for we can see 

 them at any time of hfe in the making, under the direct action 

 and control of the forces to which the system is exposed. If a bone 

 be broken and so repaired that its parts lie somewhat out of their 

 former place, so that the pressure- and tension-lines have now a new 

 distribution, before many weeks are over the trabecular system will 

 be found to have been entirely remodelled, so as to fall into line 

 with the new system of forces. And as Wolff pointed out, this 

 process of reconstruction extends a long way off from the seat of 

 injury, and, so cannot be looked upon as a mere accident of the 

 physiological process of healing and repair; for instance, it may 

 happen that, after a fracture of the shaft of a long bone, the 

 trabecular meshwork is wholly altered and reconstructed within the 

 distant extremities of the bone. Moreover, in cases of transplantation 

 of bone, for example when a diseased metacarpal is repaired by 

 means of a portion taken from the lower end of the ulna, with 

 astonishing quickness the plastic capabilities of the bony tissue are 

 so manifested that neither in outward form nor inward structure 

 can the old portion be distinguished from the new. 



Herein then lies, so far as we can discern it, a great part at least 



