AND PHYSIOLOGY. 



225 



aiding the return of blood towards the heart. For, as in the 

 aorta, its elasticity forces the blood, first against the semi- 

 lunar valves and then onwards through the arteries, so the 

 muscles, pressing upon the veins, urge the blood into the 

 pouches or valves on their inner coat, which, preventing re- 

 gurgitation, assist in returning it towards the heart. 



393. Respiration. — Respiration is another cause that 

 greatly aids in emptying the veins. In some persons a dis- 

 tinct fullness or pulsation of one of the veins of the neck is 

 noticed during each inspiration. This is produced by the 

 partial vacuum made by the act of inhaling air ; that is, as a 

 pressure is produced on all parts of the body by the atmos- 

 phere, not only a rush of air is made into the mouth, but the 

 blood is forced into the heart by the same cause. And could 

 we examine all the large veins of the body during inspiration, 

 without doubt we should see the blood returning rapidly in 

 them at each inhalation of air. 



394. Affinity of Venous Blood for Oxygen, -The affinity 

 or desire of venous blood for the oxygen, and the arterial for 

 the tissues, are important causes in the circulation, and es- 

 pecially in the capillaries. It is a principle well known in 

 physics, that if two fluids of different degrees of affinity for a 

 third fluid meet each other in a capillary tube, the fluid hav- 

 ing the strongest affinity for the third substance will either 

 partially or wholly force out the other fluid. This takes place 

 equally well through porous substances, membranes, or capil- 

 lary tubes. 



395. So that when pure air is present upon one side of the 

 membrane of the lungs and venous blood on the other, the 

 latter u' ges itself onward to meet the oxygen, and thus forces 

 that which is already purified into the pulmonary vessels, and 

 thence into the heart. 



396. The same thing is seen in the capillaries of the ex- 



393. How does the respiratory act aid the circulation of the blood? 394. How does 

 chemical affinity aid in the circulation ? 



