SECTION XI 



THE CIRCULATORY CHANGES DURING MUSCULAR 



EXERCISE 



IN the preceeding sections we Lave studied separately a number* of 

 mechanisms by which the heart or the vessels react to this or that con- 

 dition, in order to bring about an appropriate modification of the circulation. 

 In so doing we have analysed somewhat artificially the factors which are 

 normally involved simultaneously in the adaptation of the circulation to 

 the necessities of the body, as determined by the exigencies of its 

 environment. This adaptation is in fact a necessary condition of the 

 survival of the individual in the struggle for existence. Our view of the 

 working of the circulation as a whole is imperfect until we can effect a 

 synthesis of these isolated mechanisms, and trace out the chain of events 

 concerned in that intimate co-operation of all parts of the circulation with 

 all other systems and organs of the body which must be involved in every 

 act of life. For this purpose we cannot do better than take as an example 

 the complex of adaptations which are involved in muscular exericse. Though 

 for purposes of experiment the exercise may be that involved in working a 

 stationary bicycle, we must remember that it is the same series of processes 

 as are brought into play in the supreme struggle for life against an enemy 

 or rival, or in the chase for food which is necessary to avoid death by 

 hunger. For the analysis of the different events in the circulation, we have 

 hitherto had large recourse to animal experiments ; but with the facts thus 

 gained at our disposal, we can proceed to investigate the subject in man 

 himself, with the added advantages of his voluntary co-operation and of 

 the absence of abnormal conditions such as anaesthetics, etc. 



On initiating such experiments in man we meet at once with a new fact 

 viz. that under normal circumstances a reflex and automatic adaptation 

 of the heart and vessels is preceded and reinforced by the active intervention 

 of impulses proceeding from the brain. Thus the willed effort, or the emotion 

 of fear or anger which normally initiates extensive muscular movements, 

 gives rise at the same time to impulses starting in the brain centres, which 

 excite changes in the circulatory and respiratory systems of the same character 

 as those which will be later excited reflexly or automatically as a result of 

 the exercise. Thus during muscular movements we find the respiratory 

 exchanges and the ventilation of the lungs increased, the blood pressure 

 raised, and the' pulse quickened. With a man seated on a stationary bicycle 

 the mere question " Are you ready ? " evokes increase of muscular tone in the 

 act of attention, increased pulmonary ventilation, and a rise of [pulse rate 



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