176 THE CIVIL ENGINEERS OF THE BODY 



change. It is significant that bone ash contains about 84 per cent, 

 of the former and only 7-6 per cent, of the latter salt. 



One cannot say why cells in certain situations 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 bones undergoes alterations to suit alter- 

 ations 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 was 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 other words, 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. Prof. Culmann, an engineer from Zurich, happened 

 to see some drawings by Prof. H. Meyer of the cancellous tissue 

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

 coincided with 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. 31 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 with 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, 



