158 APPLIED PHYSIOLOGY 



internal pressure than the arteries, and when the 

 pressure within them is much increased they easily be- 

 come overdistended, and lose their elasticity. Hence 

 it is that when the arterial pressure is brought to bear 

 upon a vein by the establishment of a communication 

 between it and an artery the vein becomes varicose 

 (aneurysmal varix). 



The flow of blood in the veins is determined by the 

 following factors : (1) the vis a tergo of the arterial 

 pressure ; (2) the vis a fronte of the ' thoracic pump '; 

 (3) the ' squeeze ' exerted by the contraction of the 

 muscles, and by the 'tissue-tension,' which, thanks to 

 the veins being provided with valves, drive the blood 

 always towards the heart. Gravity would exert a very 

 great retarding effect on the flow were it not for the 

 valves with which every vein subjected to its action is 

 provided, but if these fail to act the flow is greatly 

 impeded, as happens in cases of varicosity. 



The influence of the vis a tergo will depend upon the 

 degree to which the potential energy exerted by the 

 force of the arterial pressure is obliterated by the peri- 

 pheral resistance in the capillaries. If, owing to high 

 viscosity of the blood or great contraction of the arte- 

 rioles, the potential energy of the arterial system be 

 greatly reduced, the velocity of the flow in the veins will 

 be proportionately diminished, and in cases of heart- 

 failure these factors help to bring the circulation to a 

 standstill. 



The influence of the vis a fronte exerted by the 

 thoracic pump is almost as important as that of the 

 arterial pressure, but will be fully dealt with when 



