232 THE PHYSIOLOGY OF MUSCULAR WORK 



the motor cortex of the brain down to the muscles the inhibitory 

 action is diminished and the accelerator nerve is stimulated. 

 This is important, for it throws light upon the great influence of 

 training upon the orderly contraction of the heart, and in relation 

 to that subject it will need further consideration later. In addi- 

 tion to this nervous control, it would appear that other influences 

 are at work. The products of muscular activity act upon the 

 heart directly and indirectly through the nervous system ; carbon 

 dioxide is one of the most important of these metabolites, and it 

 quickens the beat of the heart f 25 ). Muscular work raises the 

 pressure of carbon dioxide in the alveolar air of the lungs, as 

 Haldane and Priestley l have shown ; the breathing becomes 

 more rapid or deeper for the maintenance of a constant level in 

 the pressure of that gas in the alveolar air and presumably in the 

 blood of the lungs. Thus there is a mutual dependence of the 

 heart and lungs upon one another, and an insight is afforded into 

 the close association between the heart and lungs which is ex- 

 pressed by a " good wind." Sarcolactic acid is another metabolite 

 formed during muscular activity, and directly or indirectly may 

 influence the heart. 



The temperature of the body rises during muscular activity, 

 it may be as high as 102 F. in a healthy man, and it is known 

 that the heart responds to an increased temperature by a more 

 rapid beat. In addition vaso-motor changes occur, more blood 

 goes to the active muscles and more to the skin when the 

 temperature rises, and in some cases at least a more rapid beat 

 of the heart appears to be an effort of compensation for the fall 

 of blood pressure which may arise from the dilatation of the vessels 

 in those parts. 



The response of the heart to muscular work and to rest is so 

 extremely rapid that it would appear to be due in the first place 

 to nervous influences ; the effects of metabolites and of the rise in 

 the temperature of the body develop more slowly and persist for 

 a considerable time after the work has ceased. 



The heart is a muscle, and resembles the skeletal muscles in its 

 response to work. Gradually increasing work with good nutrition 

 and periods of rest strengthens it. There is an increased growth, 

 a physiological hypertrophy which is more or less proportional to 

 the development of the skeletal muscles. The untrained muscle 



1 See page 240. 



