986 ON FORM AND MECHANICAL EFFICIENCY [ch. 



of this, by next day they sustained 300 gm., and a few days later 

 even 400 gm.* 



The kneading of dough is an analogous phenomenon. The 

 viscosity and perhaps other properties of the stuff are affected by 

 the strains to which we have submitted it, and may thus be said 

 to depend not only on the nature of the substance but on its 

 history f . It is a long way from this simple instance, but we stretch 

 across it easily in imagination, to the experimental growth of a 

 nerve-fibre within a mass of clotted lymph : where, when we draw 

 out the clot in one direction or another we lay down traction-lines, 

 or tension-lines, and make of them a path for growth to follow J. 



Such experiments have been amply confirmed, but so far as I am 

 aware we do not know much more about the matter: we do not 

 know, for instance, how far the change is accompanied by increase 

 in number of the bast-fibres, through transformation of other tissues ; 

 or how far it is due to increase in size of these fibres; or whether 

 it be not simply due to strengthening of the original fibres by some 

 molecular change. But I should be much inclined to suspect that 

 this last had a good deal to do with the phenomenon. We know 

 nowadays that a railway axle, or any other piece of steel, is weakened 

 by a constant succession of frequently interrupted strains; it is 

 said to be "fatigued," and its strength is restored by a period 

 of rest. The converse effect of continued strain in a uniform direc- 

 tion may be illustrated by a homely example. The confectioner 

 takes a mass of boiled sugar or treacle (in a particular molecular 

 condition determined by the temperature to which it has been 

 raised), and draws the soft sticky mass out into a rope; and then, 

 folding it up lengthways, he repeats the process again and again. 

 At first the rope is pulled out of the ductile mass without difficulty ; 

 but as the work goes on it gets harder to do, until all the man's 

 force is used to stretch the rope. Here we have the phenomenon 



* Op. (it. Hegler's results are criticised by 0. M. Ball, Einfluss von Zug aiif die 

 Ausbildung der Festigungsgewebe, Jb. d. wiss. Botanik, xxxix, pp. 305-341, 1903, 

 and by H. Keller, Einfluss von Belastung und Lage auf-die Ausbildung des Gewebes 

 in Fruchtstielen, Inaug. Diss. Kiel, 1904. 



t Cf. R. K. Schofield and G. W. S. Blair, On dough, Proc. R.S. (A), cxxxviii, 

 p. 707; cxxxix, p. 557, 1932-33; also Nadai and Wahl's Plasticity, 1931. For 

 analogous properties of hairs and fibres, see Shorter, Journ. Textile Inst, xv, 

 1824; etc. 



X Cf. Ross Harrison's Croonian Lecture, 1933. 



