214 THE CIVIL ENGINEERS OE THE BODY 



One cannot say why cells in certain sitnations should have this 

 property of ossification. How far stresses and strains affect 

 the process is unknown. This we do know, however, that the 

 internal structure of the hones undergoes alterations to suit altera- 

 tions in the application of external forces. 



(ii.) Internal structure. In the earlier part of this chapter 

 mention was made of lines of stress, and it was there stated that as 

 long as sufficient strong material w^as present to include the course 

 of these lines it was an obvious economy to cut away as much as 

 possible of the matter in which there were no stress lines. If 

 these lines lie wholly in the structural material, then the danger 

 of rupture under shearing stress is eliminated. A shearing stress 

 is a force which tends to cause one part of a structure to slide over 

 another part. For example, a pile of coins compressed by a 

 force acting at right angles to the face of the coins effectively 

 resists the compression. If, however, the force were to act 

 obliquely to the face of the topmost coin, it would immediately 

 cause the pile to slip asunder. In othfr ivords, a shearing stress 

 is ineffective along the lines of maximum compression. The same 

 can be demonstrated for lines of maximum tension. For all other 

 lines, shearing stress has a definite value which is obviously at 

 maximum at 45°, i.e., half-way between the lines of tension and 

 compression. Professor Culmann, an engineer from Zurich, hap- 

 pened to see some drawings by Professor H. Meyer of the cancellous 

 tissue of the femur and at once noticed how the trabeculae of the 

 bone coincided wnth the lines of stress. He gave his class of 

 engineering students an outline of the femur and told them where 

 the stresses fell. He asked them to draw the internal structure 

 which would be necessary to meet these stresses. Fig. 51 shows the 

 result. Alongside this figure is given a diagram of the Fairbairn 

 crane — one of the best weight-lifting mechanisms known. The 

 similarity between the natural and the artificial structures is obvious. 

 It will be noticed that the lines of the trabeculae of the femur run 

 in two systems of curves. One system runs along the outer 

 convex side of the shaft, curves downwards as it opens out with 

 concavities downwards. The other system starts from the inner 

 side of the shaft and rises spreading outwards w ith the concavities 

 upwards. These systems correspond to the two kinds of lines 

 of stress present, e.g., tension and compression. The convex or 

 outer side has to resist tension, while the inner convex side, 

 overhung by the loaded head, is the compression member. The 

 head of the femur is a little more complicated than Fairbairn's 

 crane, in that the load is applied on two points, i.e., on the head of 

 the bone and on the great trochanter. This entails a division in 



