FORCE OF ASPIRATION FROM THE THORAX. 103 



ditions. The thorax can therefore exert a powerful influence upon the hepatic cir- 

 culation ; for it is only the flaccidity of the walls of the vessels which prevents this 

 influence from operating throughout the entire venous system. Although this must be a 

 very important element in the production of the circulation in the liver, the fact that the 

 blood circulates in this organ in the foetus before any movements of the thorax take 

 place shows that it is not absolutely essential. All the influences which we have thus 

 far considered are merely supplementary to the action of the great central organ of the 

 circulation. 



A farther proof, if any were needed, of the suction force of inspiration is found in an 

 accident which is not infrequent in surgical operations in the lower part of the neck. 

 When the veins in this situation are kept open by a tumor or by induration of the sur- 

 rounding tissues, an inspiratory effort has occasionally been followed by the entrance of 

 air into the circulation, an accident which is liable to lead to the gravest results. This 

 occurs only when a divided vein is kept patulous ; and the accident proves both the 

 influence of inspiration on liquids in the veins near the chest and its restriction to the 

 vessels in this particular situation by the flaccidity of their walls. 



A full discussion of the subject of air in the veins, which is of great pathological inter- 

 est, does not belong to physiology. The blood is capable of dissolving a certain quantity 

 of atmospheric air; and a small quantity, very gradually introduced into a vein, can be 

 disposed of in this way. When, however, a considerable quantity suddenly finds its way 

 into the venous system, the patient experiences a sense of mortal distress and almost 

 immediately falls into a state of insensibility. A peculiar whistling sound is heard when 

 the air passes in ; and, if the ear be applied to the chest, we distinguish the labored 

 efforts of the heart, accompanied by a loud churning sound. On opening the chest after 

 death, the right cavities of the heart are invariably found distended with air and blood, 

 the blood being frothy and florid. Generally the left side of the heart is nearly or quite 

 empty. 



The production of death from air in the veins is purely mechanical. The air, finding 

 its way to the right ventricle, is mixed with the blood in the form of minute bubbles and 

 is carried into the pulmonary artery. Once in this vessel, it is impossible for it to pass 

 through the capillaries of the lungs, and death by suffocation is the inevitable 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 time is absorbed with- 

 out producing any injury. 



Aside from the pressure exerted by the contraction of muscles and the force of as- 

 piration from the thorax, the influences 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, this should operate at 

 all on the blood-current, beyond the simple influence of the reduction of the caliber of 

 the vessel. There is a slight contraction in the vense cavse in the immediate proximity 

 of the heart, which is very much more extended in many of the lower vertebrate ani- 

 mals 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 elevatvd 

 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 mount up against the force 

 of gravity. 



