98 CIRCULATION OF THE BLOOD IN THE VESSELS. 



The cause of death from air hi the veins is purely mechanical. The air, 

 finding its way to the right ventricle, is mixed with the blood in the form of 

 minute hubbies and is carried into the pulmonary artery. Once in this ves- 

 sel, it is impossible for it to pass through the capillaries of the lungs, and 

 death by suifocatioii is the result, if the quantity 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. 



Air injected into the arteries produces no such serious effects as air in the 

 veins. It is arrested in the capillaries of certain parts and in the course of 

 a short time is absorbed. 



Aside from the pressure exerted by the contraction of muscles and the 

 force of aspiration from the thorax, the influences which assist the venous 

 circulation are very slight. There is a slight contraction in the venae cavae 

 in the immediate proximity of the heart, which is much more extended in 

 many of the lower vertebrate animals and may be mentioned 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 downward, the force of gravity favors 

 the flow of blood. This is seen by the turgescence 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 distended when the hand is allowed to 

 hang by the side and the blood has to rise against the force of gravity. 



Some physiologists are of the opinion that the right ventricle exerts an 

 active suction force during its diastole ; but experiments on animals do not 

 fully sustain this view, and if such a force be exerted, its effect upon the cir- 

 culation, even in the veins near the heart, must be very slight. In the great 

 irregularity in the rapidity of the circulation in different veins, it must fre- 

 quently happen that a vessel empties its blood into another of larger size, 

 in which the current is more rapid. In such an instance, as a physical neces- 

 sity, the more rapid current in the large vein exerts a certain suction force 

 on the fluid in the smaller vessel. 



USES OF THE VALVES OF THE VEINS. 



It is evident that the principal use of the valves of the veins is to present 

 an obstacle to the reflux of blood toward the capillaries ; and it remains only 

 to study the conditions under which they are brought into action. 



There are two distinct conditions under which the valves of the veins may 

 be closed. One of them is the arrest of circulation, from any cause, in veins 

 in which the blood has to rise against the force of gravity ; and the other, 

 compression of veins, from any cause generally from muscular contraction 

 which tends to force the blood from the vessels compressed, into others, when 

 the valves offer an obstruction to a flow toward the capillaries and necessitate 

 a current in the direction of the heart. In the first of these conditions, the 

 valves are antagonistic to the force of gravity, and when the caliber of any 



