RATE OF MOVEMENT OF THE BLOOD 215 



the capillaries to the heart. By the time that the venules are reached, 

 owing to friction in the capillaries the blood will have lost most of the 

 force imparted to it by the heart action. Nevertheless, this remaining 

 vis a tergo must be considered as the basic cause for the movement of 

 the venous blood near the periphery. As the venules get larger, two 

 other factors come into play: the massaging influence of the muscles, 

 and the valves of the veins. By the movements of the muscles the veins 

 which lie between will be rhythmically compressed, and this will tend to 

 cause the blood to be moved forward and backward in them, the back- 

 ward movement being however prevented by the operation of the valves. 

 When the tonicity of the muscles is subnormal, as in conditions of ill 

 health, the absence of this massaging action permits the blood to stag- 

 nate in the veins > especially in those of the lower extremities in upright 

 animals, with the consequence that the veins become dilated, particularly 

 just above the valves, thus causing the condition known as varicose veins. 



As the thorax is approached, two other factors become operative: the 

 aspirating influence of the thorax during inspiration, and the negative 

 intraventricular pressure (see page 152). There is no doubt that the 

 former of these is of considerable importance in maintaining the venous 

 return near the heart, for although the change of pressure induced by in- 

 spiration amounts to only 5 millimeters of mercury, yet it acts so 

 slowly that it produces a considerable influence. The aspirating effect 

 of the ventricle at the beginning of diastole is, however, of no sig- 

 nificance in attracting blood to the heart, for although, as we have seen, 

 it may be considerable, yet it lasts for so short a time that it could not 

 overcome the inertia of the column of blood in the vena cava. Even if 

 the negative pressure did last for a longer period, it could not attract 

 more than a small amount of blood, because it would cause the thin 

 collapsible walls of the veins to come together and thus block the pas- 

 sage towards the heart. 



