RESPIRATION. 



207 



ished, and the intra-thoracic pressure returns to the normal, which is 

 not exactly equal to the atmospheric, pressure. The effect of this on the 

 veins is to increase their intra-vascular pressure, and so to diminish the 

 flow of blood into the left side of the heart, and with it the arterial ten- 

 sion, but this is almost exactly balanced by the necessary increase of 

 arterial tension caused by the increase of the extra-vascular pressure of 

 the aorta and large arteries, so that the arterial tension is not much 

 affected during expiration either way. Thus, ordinary expiration does 



one limb of a manometer with the pleural cavity. Inspiration begins at i and expiration at e. The 

 intra-thoracic pressure rises very rapidly after the cessation of the inspiratory effort, and then slow- 

 ly falls as the air issues from the chest; at the beginning of the inspiratory effort the fall becomes 

 more rapid. (M. Foster.) 







not produce a distinct obstruction to the circulation, as even when the 

 expiration is at an end the intra-thoracic pressure is less than the extra- 

 thoracic. 



The effect of violent expiratory efforts, however, has a distinct action 

 in preventing the current of blood through the lungs, as seen in the 

 blueness of the face from congestion in straining; this condition being 

 produced by pressure on the small pulmonary vessels. 



We may summarize this mechanical effect, therefore, and say that in- 

 spiration aids the circulation and so increases the arterial tension, and 

 that although expiration does not materially aid the circulation, yet under 

 ordinary conditions neither does it obstruct. Under extraordinary con- 

 ditions, as in violent expirations, the circulation is decidedly obstructed. 

 But we have seen that there 'IB no exact correspondence between the 

 points of extreme arterial tension and the end of inspiration, and we must 

 look to the nervous system for an explanation of this apparently contra- 

 dictory result. 



The effect of the nervous system in producing a rhythmical alteration 

 of the blood pressure is twofold. In the first place the cardw-inliibitory 

 centre, is believed to be stimulated during the fall of blood pressure, pro- 



