MUSCULAR ACTIVITY 337 



with the increase in the velocity of the blood passing through 

 the pulmonary circulation, a direct linear relationship 

 being estabhshed between them. Incidentally the in- 

 creased pulmonary movement, and especially the ascent 

 and descent of the diaphragm, reinforce the pumping action 

 of the muscles in driving blood to the heart. 



By the stimulating action of the increased hydrogen ion 

 concentration of the blood upon the vaso-motor centre, 

 impulses passing along the sympathetic fibres cause vaso- 

 constriction of the visceral organs, whereby the general 

 blood-pressure is raised and blood is diverted in greater 

 quantity to the organs which require it — the brain, the 

 heart and the skeletal muscles. 



These several factors combine greatly to increase the 

 oxygen supply to the active tissues. The increased H. ion 

 concentration of the blood facihtates the dissociation of 

 oxyhgemoglobin, and therefore causes the blood to part 

 the more readily ^vith its oxygen as it passes more quickly 

 through the capillaries. 



So far we have considered the adaptation in the circu- 

 latory and respiratory mechanism only in so far as they 

 are produced by the increased muscular activity. Were 

 this the only causative factor, such adaptation would 

 take some time to establish itself. Experience shows, 

 however, that the increased blood-pressure, the deeper 

 respiration and the quickened pulse-rate occur within a 

 second of the beginning of exercise. In the mental pro- 

 cesses of concentration, therefore, impulses pass from the 

 cerebral cortex influencing directly the medullary centres. 

 When in an animal the lower hmbs are tetanised this 

 initial adaptation does not occur. 



As the temperature of the body is raised, changes occur 

 in the skin — dilatation of blood-vessels and secretion of 

 sweat — which have the effect of preventing the body 

 temperature from rising excessively. 



It is well known that the maximum physical effect of 

 which the body is capable depends upon the degree of 

 22 



