MECHANICS OF THE CIRCULATION IN THE VESSELS 137 



The circulation-time of an organ like the kidney can be measured by 

 adjusting a pair of electrodes under the renal artery and another under 

 the renal vein, and reading off the interval required by the salt solution 

 to pass from the point of injection first to the artery and then to the 

 vein. The difference is the circulation -time through the kidney. 



For certain purposes, and particularly for measurements on small 

 animals like the rabbit, or on organs whose vessels are too delicate to . 

 be placed on electrodes without the risk of serious interference with 

 the circulation, another method may be employed with advantage. !. 

 depends on the injection of a pigment, like methylene blue, which at 

 first overpowers the colour of the blood and shows through the walls of 

 the bloodvessels, but is soon reduced to a colourless substance (Fig. 61). 

 . The details of the method are given in the Practical Exercises (p. 217.) 



It may be said in general terms that in one and the same animal 

 the time of the lesser circulation is short as compared with the total 

 circulation-time, relatively constant, and but little affected by changes 

 of temperature. In animals of the same species it increases with the 

 size, but more slowly, and rather in proportion to the increase of 

 surface than to the increase of weight. 



Thus a dog weighing 2 kilogrammes had an average pulmonary 

 circulation -time of 4*05 seconds, while that of a dog weighing ii'8 kilos 

 was 8*7 seconds, and that of a dog with a body- weight of 18*2 kilos only 

 1 0*4 seconds. It is probable that in a man the pulmonary circulation- 

 time is not usually much less than 12 seconds, nor much more than 

 15 seconds. 



The circulation-time in the kidney, spleen and liver is relatively 

 long and much more variable than that of the lungs, being easily 

 affected by exposure and changes of temperature (increased by 

 cold, diminished by warmth) . 



In a dog of 13-3 kilos weight the average circulation-time of the 

 spleen was 10-95 seconds; kidney, 13-3 seconds; lungs, 8-4 seconds. 

 The circulation-time of the stomach and intestines is (in the rabbit) 

 comparatively short, not exceeding very greatly that of the lungs, 

 but it is lengthened by exposure. The circulation-time of the 

 retina and that of the heart (coronary circulation) are the shortest 

 of all. 



The total circulation-time is properly the time required for the whole 

 of the blood to complete the round of the pulmonary and systemic 

 circulation. But there are many routes open to any given particle of 

 blood in making its systemic circuit. If it passes from the aorta through 

 the coronary*circulation it takes an exceedingly short route. If it passes 

 through the intestines and liver, or through the kidney, or through the 

 lower limbs, it takes a long route. So that to determine the total cir- 

 culation-time by direct measurement we must know (i) the quantity 

 of blood that passes on the average by each path in a given time, and 

 (2) the average circulation-time of each path. If the average weight of 

 blood in each organ be represented by WL w 2 , w 3 , etc.; and the average 

 circulation -times by t lt t z , t s , etc. ; and t be the total systemic circulation- 

 time; then tffr, w^ , w 3 :, etc., will represent the quantity of blood 

 passing through each organ in i seconds, since in the average circula- 



