THE FLOW OF BLOOD IN THE VEINS 225 



which favor the flow. One such is the suction of the thorax already discussed 

 at page 176, as well as that of the heart itself. Besides these, several other 

 conditions in connection with the valves of the veins, operate to prevent stasis 

 of blood in the veins. 



The valves of the veins discovered by Fabricius ab Aquapendente in 1574, 

 are semilunar folds of the lining membrane, so arranged that they open toward 

 the heart but prevent the flow of blood in the opposite direction. Two such 

 valves as a rule stand opposite each other. 



When external pressure of any kind is exerted upon a vein, the backward 

 flow of blood is checked by the nearest valve, and it is compelled therefore 

 to move in the direction of the heart. As a result we find that with every 

 muscular contraction there is an increase in the quantity of blood flowing 

 from the corresponding vein. If the muscle be thrown into tetanus, there 

 follows at first an acceleration, then a retardation of the blood flow, which 

 lasts until the tetanus abates, and the pressure on the vein caused by it 

 ceases. 



Thus Burton-Opitz found the volume of flow in the femoral vein in one 

 experiment with a resting muscle to be 1.1 c.c. per second; with a tetanizing 

 stimulus of the sciatic nerve, lasting 8.1 seconds, the volume was 4.0 c.c. during 

 the contraction, 0.4 c.c. during complete tetanus, and after relaxation of the 

 muscle 1.3 c.c. 



Under ordinary circumstances clonic, cramplike contractions of the mus- 

 cles never occur, but with every movement of the body contraction and relaxa- 

 tion alternate. Because of the intermittent pressure upon the veins which 

 such an alternation produces, the usual muscular contractions must materially 

 favor the movement of the blood in them. 



Changing the attitudes of the body also is an important aid to the flow 

 of venous blood. 



The femoral vein under Poupart's ligament and in the fossa ovalis, becomes 

 empty of blood and collapses when the thigh is turned outward and at the same 

 time moved backward so as to stretch it as much as possible. It fills full again 

 as soon as the leg is brought back to its former position or is brought still fur- 

 ther forward or flexed as much as possible. These changes of position take place 

 with every step which we make (Braune). 



Finally, we have in the stretching movements of the body a means of 

 accelerating the blood in the veins. When a vein is elongated without at 

 the same time being compressed its cubic capacity is increased, and it then 

 exerts a suction on the blood column. For the venous system of the upper 

 extremities such a suction is obtained when with fists clinched and wrists 

 bent, the arms are stretched horizontally and moved backward in a certain 

 plane of rotation. A general state of relaxation and consequent stagnation 

 are obtained when with fingers stretched and the hand flexed dorsally the 

 arms are bent at the elbows and brought close to the thorax. The veins 

 of the lower extremities are stretched when the thighs are spread apart 

 and turned outward at the hip joint as far as possible, the knees and feet being 

 at the same time extended. Flexion, adduction and turning of the thighs 

 inward, bending of the knees and dorsal flexion of the feet bring about a 



