CHAP, ii.] RESPIRATION. 659 



discussed above has the happy result of improving the pulmonary 

 circulation and so produces compensating effects. When the 

 pulmonary artery is suddenly plugged with a clot the primary and 

 urgent symptom is "want of breath," though air enters freely into 

 the chest; and "cardiac dyspnoea" is a common symptom of 

 cardiac disease. 



390. Other systems of the body are also related to the 

 respiratory system, though by ties less striking than those which 

 bind to it the vascular system. We have seen that deficient 

 arterialization of the blood stirs up the muscles of the alimentary 

 canal to increased activity, and we shall presently see that the 

 same condition has a notable effect in promoting the perspiration ; 

 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 



