978 ON FORM AND MECHANICAL EFFICIENCY [ch. 



overhung by the loaded head, is the "compression-member"; the 

 outer side is the "tension-member"; the pressure-Unes, starting 

 from the loaded surface, gather themselves together, always in the 

 direction of the resultant pressure, till they form a close bundle 

 running down the compressed side of the shaft : while the tension- 

 lines, running upwards along the opposite side of the shaft, spread 

 out through the head, orthogonally to, and linking together, the 

 system of compression-lines. The head of the femur (Fig. 462) is 



Fig. 463. Crane-head and femur. After Culmann and J. Wolff. 



a little more complicated in form and a little less symmetrical than 

 Culmann's diagrammatic crane, from which it chiefly differs in the 

 fact that its load is divided into two parts, that namely which is 

 borne by the head of the bone, and that smaller portion which 

 rests upon the great trochanter ; but this merely amounts to saying 

 that a notch has been cut out of the curved upper surface of the 

 structure, and we have no difficulty in seeing that the anatomical 

 arrangement of the trabeculae follows precisely the mechanical 

 distribution of compressive and tensile stress or, in other words, 

 accords perfectly with the theoretical stress-diagram of the crane. 



