xvi] 



THE STRUCTURE OF BONE 



685 



slide over one another. A shearing stress is a somewhat com- 

 pUcated thing, and we must try to illustrate it (however 

 imperfectly) in the simplest possible way. If we build up a pillar, 

 for instance, of a pile of flat horizontal slates, or of a pack of 

 cards, a vertical load placed upon it will produce compression, but 

 will have no tendency to cause one card to slide, or shear, upon 

 another; and in like manner, if we make up a cable of parallel 

 wires and, letting it hang vertically, load it evenly with a weight, 

 again the tensile stress produced has no tendency to cause one 

 wire to slip or shear upon another. But the case would have 



Fig. .3.37. Trabecular structure of the os ealcis. (From MacAlister.) 



been very different if we had built up our pillar of cards or slates 

 lying obliquely to the lines of pressure, for then at once there 

 would have been a tendency for the elements of the pile to slip 

 and slide asunder, and to produce what the geologists call "a 

 fault" in the structure. 



Somewhat more generally, if AB be a bar, or pillar, of cross-section a 

 under a direct load P, giving a stress per unit area = p, then the whole 

 pressure P = pa. Let CD be an oblique section, inchned at an angle 6 to the 

 cross-section; the pressure on CD will evidently be =^acos 0. But at any 

 point in CD, the pressure P may be resolved into the force Q acting along 

 CD, and N perpendicular to it : where N = P cos 6, and Q= P sin 6 = pa sin 6. 

 The whole force Q upon CD = q . area of CD, which is = q . a/cos, 6. 



