. 318 PHYSIOLOGY. 



COURSE OF BLOOD IN THE VENOUS SYSTEM. 



When the blood has undergone within the general and pulmo- 

 nary capillaries the changes which result from processes of nutrition 

 and oxidation, it returns to the heart. It is the venous system which 

 is charged with this centripetal transportation. 



Has the action of the heart anything to do with the progression 

 of the venous blood? To-day, all the world recognizes that it is the 

 cardiac impulsion which, after having driven the blood through the 

 capillaries, still presides. That is, the venous blood-current is main- 

 tained primarily by the vis a tergo (force from behind). In other 

 words, it is what remains of the systolic energy of the heart trans- 

 mitted through the arteries and capillaries. The elasticity of the 

 venous walls themselves aids, to a slight extent, the movement of 

 the blood by their rather feeble contractions. Contraction of the 

 skeletal muscles, aspiration of the heart and thorax are factors also; 

 the last-named condition creates the vis a fronte. 



As the pulse- wave is normally caused to disappear in the capil- 

 lary network, so also the blood-pressure must suffer materially; in 

 fact, it continues falling even along the course of the veins until 

 the heart is reached. Nowhere along the venous system is the posi- 

 tive pressure more than the merest fraction of what is found along 

 the arterial tree. In the right side of the heart and the thoracic 

 portions of the great veins the pressure may even be negative; that 

 is, less than the atmospheric pressure. In the small venous radicles 

 coming from the capillary system the blood-current is more rapid 

 than in the capillaries themselves, but far from the speed of that 

 attained in the corresponding arterioles. 



There must of necessity be other influences exerted at this stage, 

 since the energy which the systole of the heart has put forth has been 

 greatly expended before it reaches the veins. 



At the head of the list of factors conducive to venous flow, other 

 than cardiac systole, stand the contractions of the skeletal muscles. 



The contraction of the muscles aids the passage of the venous 

 flow somewhat as follows : When pressure is brought to bear upon 

 the vein with its contents at any particular point naturally the con- 

 tained blood will endeavor to escape in two directions. That escap- 

 ing toward the capillary system is soon checked by the closing of 

 the first pair of valves, so that this portion of the vein becomes 

 swollen and distended, but firmly holds the blood. The closure of 

 the valves allows a current to be established in but one direction, 



