232 



PHYSIOLOGY 



when the muscle is loaded with about 30 grm. This increase of 

 work with increased load shows that the amount of external work 

 performed by a muscle is not a constant quantity, nor one determined 

 solely by the strength of stimulus, but is essentially conditioned by 

 the tension under which the muscle contracts. The muscle is in fact 

 endowed with a certain power of adaptation, so that it can respond 

 with increased efforts or expenditure of energy when it has more work 

 set it to do. It might be thought that the increased mechanical 

 energy evolved under these conditions had its origin at the expense 

 of some other form of energy, such as heat or electrical changes, but 

 it is found that increased tension augments all the processes of muscle, 



FIG. 67. Curve showing the length of a muscle under various loads in the 

 contracted condition B, and uncontracted condition A. The double 

 lines a &, &c., represent the contracted muscle, while the long single 

 lines a c, &c., show the length of the inactive muscle. 



including chemical changes and the production of heat. This excita- 

 tory effect of tension on skejetal muscle is aided in all the higher 

 animals by impulses which pass through the central nervous system, 

 the nature of which we shall have to discuss later on when dealing 

 with the question of so-called " tendon reflexes." The phenomenon, 

 however, is common to all forms of contractile tissues, and is indeed 

 much better marked in such forms as the heart-muscle and the 

 unstriated muscular fibres of the viscera. One may occasionally find 

 that the application of a slight load to a skeletal muscle actually 

 increases the height of the contraction, especially if the muscle be 

 not after-loaded. In the heart-muscle an increase of tension within 

 physiological limits causes invariably increased contraction a fact 

 of very great importance for the physiology of compensation in heart 

 disease. This excitatory influence affects not only the strength of 

 contraction but also the automatic, rhythmic, and conducting power 

 of the muscle ; and in some cases, as in the snail's heart, the rate of 

 beat is absolutely determined by the tension, the heart stopping 

 altogether if the tension be reduced to nothing. 



