CAUSES OF THE VENOUS CIRCULATION 77 



It has been shown by actual observations with the hemodynamometer, 

 that muscular action is capable of increasing the pressure in certain 

 veins. Bernard found that the pressure in the jugular of a horse, in re- 

 pose, was 1.4 inch (31.8 millimeters); but the action of the muscles in 

 raising the head increased it to a little more than five inches (127 milli- 

 meters), or nearly four times. Such observations show at once the great 

 variations in the current and the important influence of muscular contrac- 

 tion on the venous circulation. 



In order that contractions of muscles shall assist the venous circula- 

 tion, two conditions are necessary : 



1. The contractions must be intermittent. This is always the 

 case in the voluntary muscles. It is a view entertained by many physi- 

 ologists that each muscular fibre relaxes immediately after its contrac- 

 tion, which is instantaneous, and that a certain period of rest is necessary 

 before it can contract again. However this may be, it is well known 

 that all active muscular contraction, as distinguished from the efforts 

 necessary to maintain the body in certain ordinary positions, is intermit- 

 tent and not very prolonged. Thus the veins, which are partly emptied 

 by the compression, are filled again during the repose of the muscle. 



2. There should be no possibility of a retrograde movement of the 

 blood. This condition is fulfilled by the action of the valves. Ana- 

 tomical researches have shown, also, that these valves are most abundant 

 in veins situated in the substance of or between the muscles ; and they 

 do not exist in the veins of the cavities, which are not subject to the 

 same kind of compression. 



Force of Aspiration from the Thorax. During the act of inspiration, 

 the enlargement of the thorax, by depression of the diaphragm and eleva- 

 tion of the ribs, affects the movements of fluids in all the tubes in its 

 vicinity. The air enters by the trachea and expands the lungs so that 

 they follow the movements of the thoracic walls. The flow of blood into 

 the great arteries is somewhat retarded, as is indicated by a diminution 

 in the arterial pressure ; and finally, the blood in the great veins passes 

 to the heart with greater facility and in increased quantity. This last- 

 mentioned phenomenon can be readily observed, when the veins are 

 prominent, in profound or violent inspiration. The veins at the lower 

 part of the neck are then seen to empty themselves of blood during in- 

 spiration, and they become distended during expiration, producing a sort 

 of pulsation that is synchronous with respiration. This can always be 

 observed after exposure of the jugular in the lower part of the neck. 

 Direct observations on the jugulars, however, show that the influence of 

 inspiration can not be felt much beyond these vessels. They are seen 

 to collapse with each inspiratory act, a condition which limits this influence 



or THE . 

 lIWIVFDRfTY 1 



