IV EFFECTS ON THE CIRCULATION 165 



of the pipes in a house may be extremely perilous to 

 life. 



12. The Influence of Respiration on the Circulation. 

 — Just as there are certain secondary phenomena which 

 accompany, and are explained by, the action of the heart, 

 so there are secondary phenomena which are similarly re- 

 lated to the working of the respiratory apparatus. Of 

 these the chief is the effect of the inspiratory and expi- 

 ratory movements upon the circulation. 



In consequence of the elasticity of the lungs, a certain 

 force must be expended in distending them, and this force 

 is found experimentally to become greater and greater 

 the more the lung is distended ; just as, in stretching a 

 piece of india-rubber, more force is requii'ed to stretch it 

 a good deal than is needed to stretch it only a little. 

 Hence, when inspiration takes place, and the lungs are 

 distended with air, the heart and the great vessels in the 

 chest are subjected to a less pressure than are the blood- 

 vessels of the rest of the body. 



For the pressure of the air contained in the lungs is 

 exactly the same as that exerted by the atmosphere upon 

 the surface of the body ; that is to say, fifteen pounds on 

 the square inch. But a certain amount of this pressure 

 exerted by the air in the lungs is counterbalanced by the 

 elasticity of the distended lungs. Say that in a given 

 condition of insjiiration a pound ^ pressure on the square 

 inch is needed to overcome this elasticity, then there will 

 be only fourteen pounds pressure on every square inch 

 of the heart and great vessels. And hence the pressure 

 on the blood in tliese vessels will be one pound per square 

 inch less than that on the veins and arteries of the rest of 

 the body, which lie outside the thorax. If there were no 

 aortic, or pulmonary, valves, and if the structure of the 



1 A " pound " is stated here for simplicity's sake. As a matter of fact 

 the pressure required is much less ttian tiiis, not more than 2 or 3 

 ounces. 



