THE CIRCULATION OF THE BLOOD 333 



make it easier for the heart to send blood up to the brain, defi- 

 ciency in its blood-supply being the cause of the loss of conscious- 

 ness in a fainting-fit. In fact, so long as the breathing continues, 

 the aspiration of the thorax will keep up the venous flow (see 

 below), while, in the circumstances supposed, a slight diminution 

 in the resistance opposed to the arterial flow may be of impor- 

 tance. The head of a person who has fainted should accordingly 

 never be raised until he has undoubtedly recovered, a fact rarely 

 borne in mind by spectators, who commonly rush at once to lift 

 any one whom they see fall in the street or elsewhere. 



The Influence of Transient Compression of the Veins. The 

 valves of the veins being so disposed as to permit only a flow 

 towards the heart, when external pressure empties a vein it assists 

 the circulation. Continuous pressure, as by a tight garter, is of 

 course bad, since it checks all subsequent flow through the vessel ; 

 but intermittent pressure, such as is exerted on many veins by 

 muscles in the ordinary movements of the Body, acts as a pump 

 to force on the blood in them. 



The value of this pumping of the blood out of the veins by 

 muscular movements is well illustrated by comparing two classes 

 of workers whose occupations require that they be upon their 

 feet continuously for hours. The condition of varicose veins, 

 which is a stasis of blood in the superficial veins of the lower 

 extremities, is very prevalent among motormen, and others who 

 must stand still for long periods, but is virtually unknown among 

 postmen, who are walking during the time spent on their feet. 



The valves of the veins have another use in diminishing the 

 pressure on the lower part of those vessels in many regions. If, 

 for instance, there were no valves in the long saphenous vein of 

 the leg the considerable weight of the column of blood in it, 

 which in the erect position would be about a meter (39 inches) 

 high, would press on the lower part of the vessel. But each set of 

 valves in it carries the weight of the column of blood between it 

 and the next set of valves above, and relieves parts below, and 

 so the weight of the column of blood is distributed and does not 

 all bear on any one point. 



Aspiration of the Thorax. Whenever a breath is drawn the 

 pressure of the air on the vessels inside the chest is diminished, 

 while that on the other vessels of the Body is unaffected. In 



