988 ON FORM AND MECHANICAL EFFICIENCY [ch. 



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 Hne of 

 the fibres within the plant, or of the 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 understood 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 foi 

 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 J know, been apphed to the 

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

 a part in relation to the whole body, analogous to that which a little 

 trabecula, or a little group of trabeculae, plays within the bone 

 itself: that is to say, in the normal distribution of forces in the 

 body the bones tend to follow the lines of stress, and especially 

 the pressure-Hnes. To demonstrate this in a comprehensive way 

 would doubtless be difficult ; for we should be dealing with a frame- 

 work of very great complexity, and should have to take account of 



* E.g. (1) the head, nodding 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. Hay craft in Scha,{eT'B Textbook of Physiology, 1900, 

 p. 251.) Cf. (int. al.) G. H. Meyer, Statik u. Mechanik der menschlicken Knochen- 

 geriiste, 1873, pp. 13-25. 



