1052 



PHYSIOLOGY 



and of blood pressure. And these changes are increased as soon as the word 

 " go " is given and the man starts to pedal, i.e. before the increased metabolic 

 changes in the muscles can have had time to affect the medullary centres, 

 or the muscular contractions the vigour of the circulation. The reinforcing 

 impulses from the cortex, which stimulate the medullary centres and put 

 these various mechanisms into action, are effective especially at the beginning 

 of muscular work. In any steady work produced without particular effort 

 or attention, the subsequent adaptations of the different organs of the body 

 are probably chiefly automatic, the central reinforcing impulses being of 

 especial importance when, under emotional stress of any description, the 

 animal has to put forth its maximum effort. 



L-t 

 16.00 



14.70 



13.40 



6.90 

 5.60 

 430 



24 

 22 



20 



"I 



| 

 16 Q 



c 







14 S 

 I 



12 

 10 

 8 



315 420 525 



Work per Minute 



630 735 640 945 1050 



FIG. 482. Chart showing the effect of increasing amounts 

 of muscular work on the total ventilation of the lungs V, 

 on the blood flow BF, and on the oxygen absorption 2 . 

 (From MEANS and NBWBURGH ) 



Increased work means increased metabolism. We have seen that the 

 oxygen intake and the C0 2 output may undergo a ten or twelve fold augmenta- 

 tion during violent muscular effort carried out for a short time, and a five fold 

 increase is not uncommon and may last for many hours. The muscles of a 

 warm-blooded animal become rapidly fatigued on being deprived of an 

 adequate arriount of oxygen. A necessary condition then of all muscular 

 exercise is that the muscles shall be supplied with oxygen in proportion to 

 their requirements. Since the arterial blood is under normal conditions 90 to 

 95 per cent, saturated with oxygen, no appreciable further amount can be 

 provided by increasing the saturation of the haemoglobin in the blood ; so 

 that an eight to twelve fold increase in the oxygen usage by the muscles must 



