XVI] THE STRUCTURE OF BONE 687 



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 obhque component of pressure, are sheared 

 out of their places till they too come into coincidence with the 

 lines of force. So straws show how the wind blows — or rather 

 how it has been blowing. For every straw that Ues 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 re- 

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

 structure; the little trabeculae are constantly being formed and 

 deformed, demolished 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 life, 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-hnes 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 of the physical causation of what at first sight strikes us as 

 a purely functional adaptation : as a phenomenon, in other words. 



