324 CIRCULATION. 



and death by suffocation is the inevitable result, if the quan- 

 tity of air be large. It is because no blood can pass through 

 the lungs, that the left cavities of the heart are usually found 

 empty. 



Certain cases of entrance of air into the veins in surgical 

 operations, though presenting the most alarming immediate 

 symptoms, have terminated in recovery. In these instances, 

 the quantity of air is not sufficient to completely block up 

 the pulmonary capillaries, and it is gradually absorbed by 

 the blood. 



Air injected into the arteries produces no such serious ef- 

 fects as air in the veins. It is arrested in the capillaries of 

 certain parts, and in the course of time is absorbed without 

 having produced any injury. 



Aside from the pressure exerted by the contraction of 

 muscles, and the force of aspiration from the thorax, the in- 

 fluences which assist the venous circulation are very slight. 

 As far as the action of the coats of the vessels themselves is 

 concerned, their contraction, it must be remembered, is slow 

 and gradual, like the contraction of the arteries ; and it is 

 hardly possible that in the general venous system it should 

 operate at all on the blood-current, beyond the simple influ- 

 ence of the reduction of the caliber of the vessel. There is 

 a slight contraction in the vense cavae, in the immediate 

 proximity of the heart, which is very much more extended 

 in many of the lower vertebrate animals, and may be men- 

 tioned as having an influence, very insignificant it is true, 

 on the flow of blood from the great veins. 



In the veins which pass from above downwards, the force 

 of gravity favors the flow of blood. This is seen by the tur- 

 gescence of the veins of the neck and face, when the head is 

 kept for a short time below the level of the heart. If the 

 arm be elevated above the head, the veins of the back of the 

 hand will be much reduced in size, from the greater facility 

 with which the blood passes to the heart ; while they are 



