980 ON FORM AND MECHANICAL EFFICIENCY [ch. 



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 together by a "tie" 

 or tension-member, corresponding to the third, horizontal member 

 of the truss. 



It is a simple corollary, confirmed by observation, that the 

 trabeculae have a very different distribution in animals whose 

 actions and attitudes are materially different, as in the aquatic 

 mammals, such as the beaver and the seal*. And in much less 

 extreme cases there are lessons to be learned from a study of the 



Fig, 464. Diagram of stress-lines in the human foot. From 

 Sir D. MacAlister, after H. Meyer. 



same bone in different animals, as the loads alter in direction and 

 magnitude. The gorilla's heelbone resembles man's, but the load 

 on the heel is much less, for the erect posture is imperfectly achieved : 

 in a common monkey the heel is carried high, and consequently 

 the direction of the trabeculae is still more changed. The bear 

 walks on the sole of his foot, though less perfectly than does man, 

 and the lie of the trabeculae is plainly analogous in the two; but 

 in the bear more powerful strands than in the os calcis of man 

 transmit the load forward to the toes, and less of it through the 

 heel to the ground. In the leopard we see the full effect of tip-toe, 

 or digitigrade, progression. The long hind part (or tuberosity) of 



* Cf. G. de M. Rudolf, Habit and the architecture of the mammalian femur, 

 Jovrn. Anatomy, lvi, pp. 139-146, 1922. 



