LECTUEES ON BIOLOGY 



bridge, running exactly in the direction of the greatest pressure. 

 "Nature," aptly says J. Wolff, " has built the bones as the 

 engineer builds a bridge, obtaining a maximum of strength and 

 suitability of form with a mimimum of material." 



But it may be objected 

 that this instance refutes 

 our arguments and proves 

 what we are refuting. 

 Granted that the strength 

 of the bone rests on simple 

 mechanical laws ; but to 

 what cause is its perfect 

 construction due ? Does 

 not the very perfection 

 testify to the influence of 

 some mysterious source of 

 force ? Is it conceivable 

 that blind mechanical 

 forces can create perfec- 

 tion? The credit of hav- 

 ing shown the way out of 

 this difficulty to a natural 

 explanation of the appar- 

 ently inexplicable belongs 

 to Eoux and his Theory of 

 the Struggle of the Parts 

 in the organism. 

 We have already seen that constant use strengthens our 

 organs, whilst non-use leads to degeneration and atrophy. This 

 rule does not only apply to the muscles and other organs of the 

 body, but also, as everyone may ascertain in himself, to our 

 mental faculties, in particular the memory. 



Eoux stated that this strengthening is produced by a 

 specific function of the organ itself in other words, that it is 

 the functional stimulus which strengthens the organs. For 

 though the increased use and more intense activity cause an 

 increased metabolism, this is not only balanced, but there is even 

 a large credit balance, owing to increased power of assimilation. 

 It may therefore be said that the stimulus has a direct nutritive 



FIG. 14. LONGITUDINAL SECTION OF A 

 THIGHBONE. 



a, Marrow-shaft; b, spongy tissue. (After 

 v. Hanstein, " Naturgeschichte des Tierreichs.'') 



