en. XXI.] THE RATE OF BLOOD-FLOW. 265 



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

 case it was found that the time occupied was 2 7 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 Prof. Stewart 

 by improved methods, which have shown that the circulation time 

 is about 15 seconds, that 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 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 

 sodixim chloride solution is injected into the opposite jugular 

 vein. As soon as the salt reaches the carotid artery, the resist- 

 ance 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 illumi- 

 nated. 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 circula- 

 tion ; the longest circulation times were found in the kidney, the 

 portal system and the lower limbs. 



None of these methods, however, give the true time of the 

 entire circulation ; they give merely the shortest possible time 

 in which any particle of blood can travel through the shortest 

 pathway. The blood that travels in the axial current, or which 

 takes a broad pathway through wide capillaries, will arrive far 

 more speedily at its destination than that which creeps through 

 tortuous or constricted vessels. The direct observations of 

 Tigerstcdt on the output of the left ventricle show that the 

 circulation time of the whole blood is at least five times as long 

 as the period arrived at by the Hering method. It is therefore 

 fallacious to use the circulation times arrived at by Hering's or 

 Stewart's methods as a basis for calculating the total amount of 

 the blood in the body. 



