CAUSES OF THE VENOUS CIRCULATION. 317 



free communication with other vessels, the current is simply 

 diverted into another channel ; the expulsive efforts of res- 

 piration ; the contractions of the right side of the heart ; and 

 the force of gravity, which operates, in the erect posture, on 

 the current in all excepting the veins of the head, neck, and 

 parts of the trunk above the heart. 



Influence of Muscular Contraction. That the action of 

 muscles has a considerable influence on the current of blood 

 in the veins situated between them, and in their substance, has 

 long been recognized. It is exemplified in the operation of 

 venesection, when it is well known that the jet from the vein 

 may be very much increased in force by contraction of the 

 muscles below the opening. This action is so marked, that 

 the parts of the venous system which are situated in the sub- 

 stance of muscles have been compared by Chassaignac to a 

 sponge full of liquid, vigorously pressed by the hand. 1 It 

 must always be remembered, however, that though the 

 muscles are capable of acting on the blood contained in veins 

 in their substance with great vigor, the heart is fully capable 

 of producing the venous circulation without their aid ; a fact 

 which is exemplified in a striking manner in the venous cir- 

 culation in paralyzed parts. 



It has been shown by actual observations with the hemo- 

 dynamometer, that" muscular action is capable of immensely 

 increasing the pressure in certain veins. The first definite 

 experiments on this subject were made by Magendie, who 

 showed a pressure of over two inches of mercury produced 

 by a general muscular contraction, on the passage of a gal- 

 vanic current from a needle plunged into the cervical region 

 of the spinal marrow to one fixed in the muscles of the thigh. 2 

 The experiments of Bernard have shown this more accurately. 

 This physiologist found that the pressure in the jugular of a 

 horse, in repose, was 1/4: inch ; but the action of the muscles in 



1 BERARD, op. cit, tome iv,, p. 57. 



8 MAGENDIE, Phenomenes Physiques dela Vie, Paris, 1842, tome iii., p. 163. 



