THE PHYSIOLOGY OF MUSCULAR WORK 233 



is overstretched and injured by severe and prolonged exertion, 

 but the damage is more easily done and less easily repaired in 

 the case of the heart. The skeletal muscles become fatigued and 

 painful during movement, and the enforced rest gives them time 

 to recover ; the heart cannot take such a profound rest, it must 

 continue to beat, if life is to be maintained, and can rest only by 

 beating less frequently and less vigorously. In the case of the 

 untrained heart over-exertion produces palpitation, overdistension, 

 and dilatation. The duration of systole to diastole, that is, of 

 work to rest, may be less than 1 : 1 in the fatigued heart, whereas 

 in the normal heart the proportion is 1 : 3. 



The increase in the rate of the heart beat is within certain 

 narrow limits proportional to the work done. The pulse rate 

 may be readily doubled by running down and up stairs for thirty 

 seconds, but such vigorous exercise rapidly produces dyspnoea 

 and cannot be maintained. The heart not only works more 

 rapidly, but it has to pump the blood into the aorta at a pressure 

 which may be one-half greater than the pressure at rest. Thus a 

 healthy well-trained man who had performed such an exercise 

 showed the following changes in the pulse and blood pressure ; 

 the pulse was increased from 56 to 124, and the blood pressure in 

 the brachial artery from 126 to 142 mm. Hg. A high blood pressure 

 is not maintained, for during the continuation of exercise the 

 cutaneous arterioles dilate. 



The limit of efficiency as regards the rate of the heart beat 

 appears to be reached at about 160 per minute, but there are 

 doubtless individual differences. 



The influence of marching with a load has been especially 

 investigated by Zuntz and Schumberg ( 26 ) ; they not only found an 

 increase in the rate of the contraction of the heart but also evidence 

 of engorgement of the liver and heart with blood. The frequency 

 of increased hepatic and cardiac dulness on percussion was 56, 70, 

 and 87 per cent., when the soldiers marched carrying loads of 

 22, 27, and 31 kg. respectively. The average increase of cardiac 

 dulness was 1 cm., that of the liver 2'3 cm. ; the highest figures were 

 3*5 and 5 cm. respectively. The cardiac dilatation involved the right 

 ventricle on sixty-two occasions, and the left also on thirty-one 

 occasions. The engorgement did not coincide with the increase in 

 the rate of the pulse, but with the increased breathing and the rise 

 in the temperature of the body. In such experiments the belts and 



