THE CIRCULATION OF THE BLOOD AND LYMPH 125 



electrical resistance (p. 25). This disturbs the balance of the bridge, 

 and the sound in the telephone becomes louder. The time from the 

 beginning of the injection to the alteration in the sound is the circula- 

 tion-time between jugular and carotid. It can be read off by a 

 stop-watch, or more accurately by an electric time-maker writing on 

 a revolving drum (Fig. 52) . Instead of the telephone a galvanometer 

 may be used, the equal and oppositely directed induction shocks being 

 replaced by a weak voltaic current and the platinum by unpolarizable 

 electrodes (p. 625). But this is less convenient. 



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. 

 It 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 sub- 

 stance (Fig. 51). The details of the method are given in the 

 Practical Exercises (p. 203). 



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 

 11*8 kilos was 8-7 seconds, and that of a dog with a body-weight 

 of 1 8' 2 kilos only 10-4 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 rela- 

 tively 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 intes- 

 tines 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 



