244 THE HUMAN BODY. 



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. Accordingly 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 circum- 

 stances supposed, a slight diminution in the resistance op- 

 posed 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 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 subse- 

 quent flow through the vessel, but intermittent pressure, 

 such as exerted on many veins by muscles in the ordi- 

 nary movements 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 (p. 214) of the leg the weight of the whole 

 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 di- 

 minished, while that on the other vessels of the Body is un- 

 affected. In consequence blood tends to flow into the chest. 



