UOUX: FUNCTIONAL ADAPTATION 325 



with the least possible waste of the propelling power through 

 frictional resistance. So, too, the spongy substance of the 

 long bones is arranged in lamella; which take the direction of 

 the principal stresses and strains which fall upon the bones 

 in action. 



Functional structure may be formed either in the first or 

 in the second period of development, may be either inherited 

 or acquired, but it reaches its full differentiation only in the 

 second period, i.e., under the influence of functioning. 

 Practically speaking, functional structure is directly 

 dependent for its full development and for its continued 

 conservation upon the exercise of the particular function 

 which it serves. In the second period, but not in the first, 

 increased use leads to hypertrophy of the functional structure, 

 disuse to atrophy. 



From functional structure is to be distinguished non- 

 functional structure, which has no relation to the bodily 

 functions is neither adapted to perform any of these, nor 

 has arisen as a by-product of functional activity. " To this 

 category belong, for example, among typical structures, 

 the triangular form of the cross-section of the tibia, 

 the dolicocephalic or brachycephalic shape of the skull, 

 most of the external characters distinguishing genera and 

 species, many of the external features of the embryo which 

 change in the course of development, besides most of the 

 abnormal forms shown by monstrosities, tumours, etc." (p. 74, 

 1910). Non-functional structure is not affected by 

 functional adaptation, and may accordingly be left out of 

 consideration here. 



Now the influence of functioning upon the form and 

 structure of an organ is twofold. There is first the immediate 

 change brought about by the very act of functioning for 

 example, the shortening and thickening of skeletal muscles 

 when they act. This is a purely temporary change, for the 

 organ at once returns to its normal quiescent state as soon 

 as it ceases to function. Such temporary functional change, 

 brought about in the moment of functioning, is usually 

 dependent for its initiation upon some neuro-muscular 

 mechanism, though it may be elicited also by a chemical 

 stimulus. It is thus always a phenomenon of " behaviour." 



