Chap, ii.] RESPIRATION. 509 



into the left ventricle and sent thence over the body is not 

 more arterial than usual ; at the same time the amount of blood 

 sent out at each heart stroke is less, often much less, than the 

 normal ; and the spinal bulb as well as the other tissues suffer 

 in consequence from a deficiency of oxygen. The deficient 

 supply to the bulb manifests itself in dyspnceic or at least in 

 laboured breathing, which sometimes through the mechanical 

 influences 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. 



§ 317. 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 pres- 

 ently see that the same condition has a notable effect in pro- 

 moting the perspiration ; it probably has a similar influence 

 over other secretions. On the other hand, as we have seen 

 § 303, 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 con- 

 nect 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 mus- 

 cular 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 defi- 

 ciency 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. 



