628 RESPIRATION AND MUSCULAR WORK. [BOOK n. 



it probably has a similar influence over other secretions. On the 

 other hand, as we have seen 373, there are reasons for thinking 

 that the activity of the respiratory centre and so the energy of the 

 whole respiratory act is influenced by chemical changes, other than 

 the decrease of oxygen and increase of carbonic acid, brought 

 about in the blood by the activity of the skeletal muscles. 



The closeness and the intricacy of the ties which thus connect 

 the respiratory system with almost all parts of the body may be 

 illustrated by considering the effects of muscular work on the 

 body, and the conditions which, apart from the capacity of the 

 muscles themselves and of the motor nervous apparatus which 

 puts them to work, determine the power of the body to do work. 

 During work, especially arduous work, the muscular contractions 

 rob the blood of much oxygen and load it with much carbonic 

 acid. This change in the blood would itself increase the activity 

 of the respiratory centre and the energy of the respiratory 

 movements, and might be sufficient to secure such an increase of 

 these movements that the deficiency of oxygen and increase of 

 carbonic acid should never overstep certain limits. But, as we 

 have said, apparently other products of muscular metabolism act so 

 potently in stimulating the respiratory centre that the respiratory 

 movements are more than sufficient to compensate the changes in 

 the gases of the blood. The efficacy of the augmented respiratory 

 movements is much increased by a concomitant increase in cardiac 

 activity and a swifter or fuller stream of blood through the lungs ; 

 indeed unless backed up by the cardiac increase the mere increase 

 of the pulmonary ventilation might prove inadequate. 



Hence the capacity for arduous muscular labour is determined 

 not by the respiratory mechanism alone, nor by the vascular 

 system alone, but by both, and especially by both working together 

 in harmony and concert. The increased ventilation would be idle 

 unless it were accompanied by a quicker circulation, and the 

 quicker circulation would similarly be of comparatively little use 

 unless accompanied by increased ventilation. To a bystander the 

 working of the respiratory pump is much more obvious than that 

 of the vascular system, and indeed the subject himself is much 

 more directly conscious of changes in the former than of changes 

 in the latter. Hence when the organism ceases to be able to 

 meet the demands which the labour is making upon it, the 

 subject is said to be "out of breath," though in a large number of 

 cases the failure lies much more at the door of the vascular than 

 of the respiratory system. And, as a rule, it may perhaps be said 

 that when two men differ in their capacity for strenuous work, 

 such as running a race, the difference, though it is often familiarly 

 spoken of as one of " wind " or power of breathing, is in reality not 

 a difference in ventilating capacity but a difference in the power 

 of the heart to keep up to and work in harmony with the increased 

 respiratory movements. 



