286 THE CIRCULATION IN THE BLOOD-VESSELS [dl. XXI 



vein, or from the corresponding vein of the other side. The sub- 

 stance injected is one that can be readily detected by a chemical 

 test (the prussian blue reaction). Vierordt improved this method 

 by collecting the blood as it flowed out, in a rotating disc divided 

 into a number of compartments. The blood was tested in each com- 

 partment, and the ferrocyanide discovered in one which in the case of 

 the horse received the blood about half a minute after the injection 

 had been made. The experiment was performed in a large number 

 of animals, and the following were a few of the results obtained : 



In the horse . . . .31 seconds. 



dog . . . 16 



cat . . . 6-5 



fowl . . . 5 



At first sight these numbers show no agreement, but in each case 

 it was found that the time occupied was 27 heart-beats. The dog's 

 heart, for instance, beats twice as fast as the horse's, and so the time 

 of the entire circulation only occupies half as much time. 



The question has recently been re-investigated by Stewart by 

 improved methods, which have shown that the circulation time is 

 considerably less than was found by the researches of Hering and 

 Vierordt. The great objection to the older method is the fact that 

 haemorrhage is occurring throughout the experiment, and this would 

 materially weaken the heart and slow down the circulation. Stewart 

 has employed two methods. In the first, the carotid artery is exposed, 

 and non-polarisable electrodes applied to it. These are placed in 

 circuit with a cell, a galvanometer and one arm of a Wheatstone's 

 bridge. After the resistances in the bridge have been balanced, and 

 the galvanometer needle brought to rest, a small quantity of strong . 

 sodium chloride solution is injected into the opposite jugular vein. 

 As soon as the salt reaches the carotid artery, the resistance of the 

 blood is altered, the balance of the Wheatstone's bridge is upset, and 

 the galvanometer needle moves. The period between the injection 

 and the swing of the needle is accurately noted. 



The second method used is even simpler, and gives practically the 

 same results; a solution of methylene blue is injected into the 

 jugular vein. The carotid artery on the opposite side is exposed, 

 placed upon a sheet of white paper, and strongly illuminated. The 

 time is noted between the injection and the moment when the blue 

 colour is seen to appear in the artery. Stewart has applied these 

 methods also for determining the time occupied by the passage of 

 blood through various districts of the circulation ; the longest circula- 

 tion times were found in the kidney, the portal system, and the 

 lower limbs. He calculates that the total circulation time in man 

 is about 15 seconds. 



