684 ON FORM AND MECHANICAL EFFICIENCY [ch. 



Mutatis mutandis, the same phenomenon may be traced in any 

 other bone which carries weight and is hable to flexure; and in 

 the OS calcis and the tibia, and more or less in all the bones of the 

 lower hmb, the arrangement is found to be very simple and 

 clear. 



Thus,, in the os calcis, the weight resting on the head of the 

 bone has to be transmitted partly through the backward-projecting 

 heel to the ground, and partly forwards through its articulation 

 with the cuboid bone, to the arch of the foot. We thus have, 

 very much as in a triangular roof-tree, two compression-members, 

 sloping apart from one another; and these have to be bound 



Fig. 336. Diagram of stress-lines in the human foot. (From Sir 

 D. MacAlister, after H. Meyer.) 



together by a "tie" or tension-member, corresponding to the 

 third, horizontal member of the truss. 



So far, dealing wholly with the stresses and strains due to 

 tension and compression, we have altogether omitted to speak 

 of a third very important factor in the engineer's calculations, 

 namely what is known as "shearing stress." A shearing force is 

 one which produces "angular distortion" in a figure, or (what 

 comes to the same thing) which tends to cause its particles to 



der Metatarsalien, A. f. E. M. xxiv, 1907; Triepel, Harm., Die trajectorielle 

 Structuren (in Einf. in die Physikalische Anatomie, 1908); Dixon, A. F., 

 Architecture of the Cancellous Tissue forming the Upper End of the Femur, 

 Journ. of Anat. and Phys. (3) xliv, pp. 223-230, 1910. 



