124 A MANUAL OF PHYSIOLOGY 



the circulation-time between jugular and carotid, and it can be read off 

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

 on a revolving drum. 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 un- 

 polarizable electrodes (p. 542). 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 ad- 

 vantage. 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 substance, methylene white. The details of the method 

 are given in the Practical Exercises (p. 192). 



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 

 1 1 -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 1 2 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 exceed- 

 ing 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. 



