690 ON FORM AND MECHANICAL EFFICIENCY [ch. 



That is to say, the origin, or causation, of the phenomenon would 

 seem to lie, partly in the tendency of growth to be accelerated 

 under stram : and partly in the automatic effect of shearing 

 strain, by which it tends to displace parts which grow obliquely 

 to the direct lines of tension and of pressure, while leaving those 

 in place which happen to lie parallel or perpendicular to those 

 lines : an automatic effect which we can probably trace as working 

 on all scales of magnitude, and as accounting therefore for the 

 rearrangement of minute particles in the metal or the fibre, as 

 well as for the bringing into line of the fibres themselves within 

 the plant, or of the little trabeculae within the bone. 



But we may now attempt to pass from the study of the 

 individual bone to the much wider and not less beautiful problems 

 of mechanical construction which are presented to us by the 

 skeleton as a whole. Certain problems of this class are by no 

 means neglected by writers on anatomy, and many have been 

 handed down from Borelli, and even from older writers. For 

 instance, it is an old tradition of anatomical teaching to point 

 out in the human body examples of the three orders of levers*; 

 again, the principle that the limb-bones tend to be shortened in 

 order to support the weight of a very heavy animal is well under- 

 stood by comparative anatomists, in accordance with Euler's law, 

 that the weight which a column liable to flexure is capable of 

 supporting varies inversely as the square of its length ; and again, 

 the statical equilibrium of the body, in relation for instance to 

 the erect posture of man, has long been a favourite theme of the 

 philosophical anatomist. But the general method, based upon 

 that of graphic statics, to which we have been introduced in our 

 study of a bone, has not, so far as I know, been applied to the 

 general fabric of the skeleton. Yet it is plain that each bone plays 



* E.g. (1) the head, noddmg backwards and forwards on a fulcrum, represented 

 by the atlas vertebra, lying between the weight and the power; (2) the foot, raising 

 on tip-toe the weight of the body against the fulcrum of the ground, where the 

 weight is between the fulcrum and the power, the latter being represented by the 

 tendo Achillis; (3) the arm, lifting a weight in the hand, with the power (i.e. the 

 biceps muscle) between the fulcrum and the weight. (The second case, by the way, 

 has been much disputed; cf. Haycraft in Schafer's Textbook of Physiology, p. 251, 

 1900.) 



