THE CIRCULATION. 165 



mere elasticity of the muscular walls gives a slight suction, 

 much as a sponge unnaturally compressed would as soon 

 as the pressure was relieved, spring to its more natural 

 dimensions. The suction due to this elastic expansion of 

 the walls is, however, a very small factor, and the main 

 power of suction is derived from the aspiration of the thorax 

 itself. As everyone knows the pressure in the thorax is 

 less than on the outside, and a wound through the chest wall 

 would at once suck air into the chest. 



In the inspiration the suction action becomes perfectly 

 evident, and a big stream of air rushes through nostrils and 

 mouth into the thorax. If the individual so breathing were 

 in a medium of water, the water would be sucked into the 

 chest with the same relative ease as air. But not only 

 does the windpipe lead into the chest, but the veins do also. 

 Consequently, when the chest enlarges there is a sucking 

 action not only upon the outside air but also upon the out- 

 side blood, and the air through the trachea and blood 

 through the veins are sucked heartwards. In very forced 

 expiration the condition of things may be reversed and the 

 veins instead of being sucked may be compressed. But even 

 in this compression the blood is pushed on towards the heart 

 as the valves in the veims interfere with its backward flow. 

 Thus, whether the veins are sucked or compressed, the re- 

 sult is much the same. 



That the sucking action of the chest has much to do 

 with drawing the blood into the heart is made evident in the 

 filling of the big veins and the stagnation of the venous 

 stream which occurs when we hold our breath for some 

 time. A person unable to get his breath becomes, as we 

 say, black and blue in the face. He does so because he is 

 no longer able to suck the venous blood issuing from the 

 veins of his skin into the empty heart. As, however, 

 the heart even when the chest wall is cut open, still has 

 venous blood flowing into it in slight quantities, it shows 

 that a small amount of the sucking action must be due to 

 the elastic expansion of the heart itself. It is interesting 



