432 THE PREFUNCTIONAL AS CONTRASTED WITH 



Conversely, when extra demands are made upon an intact organ, 

 it also may respond by increased growth. The excess growth of 

 striated muscle under the influence of heavy work is the most 

 familiar case. The heart, too, is an excellent example. In small 

 birds, the relative heart-size is greater in specimens from high lati- 

 tudes than in those of the same species from milder climates, owing 

 to the greater demands made upon the circulation in cold condi- 

 tions.^ 



In voluntary muscle, it is probable that the direction of the fibres 

 is also influenced by function, in the first instance by the tension 

 to which the muscle is exposed by the growth of the skeletal parts 

 to which it is attached.'^ The directive eflFect of stress has been ex- 

 perimentally demonstrated in connective tissues. By subjecting 

 thin tissue-cultures of fibroblasts to variations in surface tension 

 it has been possible to show that whereas in regions free from 

 directional stress, fibres are formed at random in all directions, in 

 regions subjected to directional tension the medium is condensed 

 along the lines of stress. The fibres orient themselves along these 

 condensations, and the cells multiply more rapidly in these regions^ 

 (fig. 21 1 ). This case falls perfectly into fine with the experiments on 

 regenerating tendons. If the achilles tendon of an animal is cut, 

 the space between the cut ends is filled with debris, blood, and 

 phagocytes, and resembles a tissue-culture. Fibroblasts soon grow 

 into it, and the fibres which they produce are at first chaotic ; next 

 they form a meshwork with diagonal interlacings ; and finally form 

 parallel bundles. The muscle, exerting a pull on one of the cut 

 ends of the tendon, sets up lines of stress in the ground-substance, 

 and this orientates the growth of the fibres. 



But if the muscle also is cut, so as to abolish the pulling eflFect, 

 no tendon is formed. If now a silk thread is drawn through 

 the regenerating tissue, in a direction at right angles to that of the 

 original tendon, constant gentle pulling on the silk thread will pro- 

 duce a bundle of fibres orientated according to the artificially- 

 produced lines of stress. A tendon has here been formed, but at 

 right angles to its normal direction.^ 



In respect of the orientation of the cells to the lines of stress, and 



^ Hesse, 1921. ^ Carey, 1921 a. 



^ Weiss, 1929, 1933. ^ Lewy, 1904; see also Nageotte, 1922. 



