CH. XXII.] TIME OF A COMPLETE CIRCULATION 289 



changes has also been observed. Thus Lortet found that the carotid 

 flow is five or six times greater when the horse is actively masticating 

 than when it is at rest. After section of the cervical sympathetic, 

 the lessening of the peripheral resistance raised the velocity from 

 540 to 750 mm. per second. 



The Time of a Complete Circulation. 



Among the earliest investigators of the question how long an 

 entire circulation takes, was Hering. He injected a solution of 

 potassium ferrocyanide into the central end of a divided jugular 

 vein, and collected the blood either from the other end of the same 

 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 compart- 

 ment, and the ferrocyanide was 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 reinvestigated 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 carotid. 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. 



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