136 



THE CIRCULATION OF THE BLOOD AND LYMPH 



piece of artery lying between them, with the blood that flows in it, is 

 connected up as one of the resistances in a Wheatstone's bridge (p. 726). 

 The secondary coil of a small inductorium, arranged for giving an inter- 

 rupted current, and with a single Daniell or dry cell in its primary, is 

 substituted for the battery, and a telephone for the galvanometer, 

 according to Kohlrausch's well-known method for the measurement of 

 the resistance of electrolytes. It is well to have the induction machine 

 set up in a separate room and connected to the resistance-box by long 

 wires, so that the noise of the Neefs hammer may be inaudible. The 

 bridge is balanced by adjusting the resistances until the sound heard 

 in the telephone is at its minimum intensity, the secondary coil being 

 placed at such a distance from the primary that there is no sign of 

 stimulation of muscles or nerves in the neighbourhood of the electrodes 



Fig. 61. Measurement of the Pulmonary Circulation -Time in Rabbit by Injection 

 of Methylene Blue. 



when the current is closed. A definite, small quantity of the salt solu- 

 tion is now allowed to run into the vein by turning the stop-cock of the 

 burette. It moves on with the velocity of the blood, and reaching the 

 artery on the electrodes causes a diminution of its electrical resistance 

 (p. 26). This disturbs the balance of the bridge, and the sound in the 

 telephone becomes louder. The time from the beginning of the injec- 

 tion to the alteration in the sound is the circulation - 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. 62). 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. 731). But this is less convenient. 



