31 S CIRCULATION. 



raising the head increased it to a little more than five inches, 

 or nearly four times. 1 These observations show at once the 

 great variations in the venous current, and the important 

 influence of muscular contraction on the circulation. 



In order that contractions of muscles shall assist the 

 venous circulation, two things are necessary : 



1. The contraction must be intermittent. This is always 

 the case in the voluntary muscles. It is a view entertained 

 by many that each muscular fibre relaxes immediately after 

 its contraction, which is instantaneous, and that a certain 

 period of repose is necessary before it can contract again. 

 However this may be, it is well known that all active mus- 

 cular contraction, as distinguished from the efforts necessary 

 to maintain the body in certain ordinary positions, is inter- 

 mittent, 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 move- 

 ment of the blood. This condition is fulfilled by the action 

 of the valves. Anatomical researches have shown that these 

 valves are most abundant in veins situated in the substance 

 of or between the muscles, and that they do not exist in the 

 veins of the cavities, wlrich are not subject to the same kind 

 of compression. It is thus that the blood is prevented from 

 passing backward toward the capillary system; and when 

 the caliber of a vein is reduced by compression, part of its 

 contents must be forced toward the heart. This action of 

 the valves constitutes their most important function. 



Milne-Edwards alludes to an important physiological 

 bearing of the acceleration of the venous circulation by con- 

 tractions of muscles, on their nutrition. 2 It is apparently 

 necessary that the supply of blood should be increased in a 

 muscle, in proportion to and during its activity ; for at that 



1 BERNARD, Lemons sur la Physiologic et la Palhologie du Systeme Nerveux, 

 Paris, 1858, tome i., p. 285. 



* Lefons sur la Physiologic, tome iv., p. 310. 



