COURSE OF BLOOD IX THE VEINS. 101 



3. A possible influence from contraction of the coats of the vessels themselves. 

 This is marked in the veins near the heart in some of the inferior animals. 



4. The force of gravity, which operates only on vessels which carry blood from above 

 downward to the heart, and a slight suction force which may be exerted upon the blood 

 in a small vein as it passes into a larger vessel in which the current is more rapid. 



The obstacles to the venous circulation are : Pressure sufficient to obliterate the cali- 

 ber of a vessel, when, from the free communications with other vessels, the current is 

 simply diverted into another channel ; the expulsive efforts of respiration ; the contrac- 

 tions 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 in parts 

 of the trunk above the heart. 



Influence of Muscular Contraction. That the action of muscles has considerable in- 

 fluence on the current of blood in the veins situated between them and in their sub- 

 stance, 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 substance of muscles have been 

 compared by Chassaignac to a sponge full of liquid, vigorously pressed by the hand. It 

 must always be remembered, however, that, although the muscles are capable of acting 

 on the blood contained in veins in their substance with great vigor, the heart is fully 

 competent to carry on the venous circulation without their aid ; a fact which is exempli- 

 fied in a striking manner in the venous circulation in paralyzed parts. 



It has been shown by actual observations with the ha3madynarnometer, that muscular 

 action is capable of immensely increasing the pressure in certain veins. The first defi- 

 nite 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 

 galvanic current from a needle plunged into the cervical region of the spinal marrow to 

 one fixed in the muscles of the thigh. 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 raising the head increased it to 

 a little more than five inches, or nearly four times. These observations show at once 

 the great variations in the current and the important influence of muscular contraction 

 on the venous 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 neces- 

 sary 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 intermittent 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 con- 

 dition 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 mus- 

 cles, and that they do not exist in the veins of the cavities, which are not subject to the 

 same kind of compression. It is thus that the blood is prevented from passing back- 

 ward toward the capillary system ; and, when the caliber of a vein is reduced by com- 

 pression, 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 



