250 THE HUMAN BODY. 



The Influence of Gravity. Under ordinary circum- 

 stances this may be neglected, since in parts of the Body 

 below the level of the heart it will assist the llow in the ar-. 

 teries and impede it equally in the veins, while the reverse 

 is the case in the upper parts of the Body. In certain cases, 

 however, it is well to bear these points in mind. A part 

 "congested" or gorged with blood should if possible be raised 

 so as to make the back-flow in its veins easier; and sometimes 

 when the heart is acting feebly it may be able to drive blood 

 along arteries in which gravity helps, but not otherwise. Ac- 

 cordingly in a tendency to fainting it is best to lie down, and 

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

 bloodlessness of which is the cause of the loss of consciousness 

 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 dimi- 

 nution in the resistance opposed to the arterial flow may be 

 of importance. The head of a person who has fainted should 

 accordingly never be raised until he has undoubtedly recov- 

 ered, 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 move- 

 ments of the Body, acts as a pump to force on the blood in 

 them. 



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 metre (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 col- 

 umn of blood is distributed and does not all bear on any one 

 point. 



