xvi] 



OF WARREN'S TRUSS 



981 



the heel is now more a mere lever than a pillar of support; it is 

 little more than a stiffened rod, with compression-members and 

 tension-members in opposite bundles, inosculating orthogonally at 

 the two ends*. 



In the bird the small bones of the hand, dwarfed as they are in 

 size, have still a deal to do in carrying the long primary flight- 

 feathers, and in forming a rigid axis for the terminal part of the 

 wing. The simple tubular construction, which answers well for 

 the long, slender arm-bones, does not suffice where a still more 

 efficient stiffening is required. In all the mechanical ^ide of anatomy 



Fig. 465. Metacarpal bone from a vulture's wing; stiffened after the manner 

 of a Warren's truss. From 0. Prochnow, Formenkunst der Natur, 



nothing can be more beautiful than the construction of a vulture's 

 metacarpal bone, as figured here (Fig. 465). The engineer sees in 

 it a perfect Warren's truss, just such a one as is often used for a main 

 rib in an aeroplane. Not only so, but the bone is better than the 

 truss; for the engineer has to be contefit to set his V-shaped struts 

 all in one plane, while in the bone they are put, with obvious but 

 inimitable advantage, in a three-dimensional configuration. 



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

 and compression, we have omitted to speak of a third very im- 

 portant factor in the engineer's calculations, namely what is known 

 as "shearing stress." A shearing force is one which produces 



* Cf. Fr. Weidenreich, Ueber formbestimmende Ursachen am Skelett, und die 

 Erblichkeit der Knochenform, Arch. f. Entw. Mech. li, pp. 438-481, 1922. 



