214 THE CIRCULATION OF THE BLOOD 



and more susceptible than that of the lungs to different conditions of 

 temperature. In a dog in which the pulmonary circulation time was 

 about 8.5 seconds, that of the spleen was about 11 seconds, and of the 

 kidney about 17.5 seconds. The shortest circulation time of all is of 

 course that in the coronary artery, but that through the retina can not fall 

 far behind it. 



To determine the total circulation time, we must know: (1) the average 

 amount of blood passing by each part in a given time, and (2) the average 

 circulation time of each part. From such computations, which however 

 are obviously subject to considerable error, it has been reckoned that the 

 total circulation time in man must lie somewhere between 1 and 1.25 

 minutes. 



MOVEMENT OF BLOOD IN VEINS 



Before leaving this part of our subject, a few words may be said con- 

 cerning the forces concerned in the movement of blood in the veins from 

 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- 

 Avard 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 



