164 THE ACTION OF THE LIVING CELL 



volvement, nervous impulses are not carried to the 

 affected limb and the muscles receive no stimuli from 

 the central nervous system. Therefore, unless moved 

 by external manipulation, the muscles of the injured 

 member participate in little or no movement, and 

 after a time they undergo degeneration or atrophy. 

 Consequently the injured limb may dwindle to a size, 

 measured in terms of tissue mass, considerably less 

 than that of the opposite limb. The essential point to 

 be observed here is that inaction leads to atrophy. No 

 factor other than enforced inaction is necessary to 

 bring about atrophy; it may be accomplished simply 

 by splinting a limb in one fixed position for a few 

 months. 



Just why the degenerative changes described by the 

 word atrophy should be caused by muscular inaction 

 has never been entirely clear in the past, although it 

 usually has been attributed to the impairment of circu- 

 lation consequent to inaction. This is due to the fact, 

 as first suggested by Borelli in the seventeenth cen- 

 tury, that the venous return of blood to the heart is 

 owing not to the heartbeat, but to the extrinsic circu- 

 latory mechanism of the veins; for their muscle con- 

 tractions, by exerting a pressure upon the underlying 

 veins and capillaries, force the blood by virtue of the 

 unidirectional valves towards the abdomen and trunk. 

 When the muscles are completely inactive this proc- 

 ess cannot proceed at all; or when partially inactive, 

 the magnitude of the venous return to the heart is 

 seriously diminished. There follows then, as the case 

 may be, either a complete or a partial stagnation of 

 blood in the tissues of the immobilized part. Such 

 being the case, the tissues suffer a lack of foodstuffs and 



