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A MANUAL OF PHYSIOLOGY 



isolated, and laid on a pair of hook-shaped platinum electrodes,* 

 covered, except on the concave side of the hook, with a layer of 

 insulating varnish. To further secure insulation, a bit of very thin 

 sheet-indiarubber is slipped between the artery and the tissues. By 

 means of the electrodes the piece of artery lying between them, 

 with the blood that flows in it, is connected up as one of the resist- 

 ances in a Wheatstone's bridge (p. 617). The secondary coil of a 

 small inductorium, arranged for giving an interrupted 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 Kohl- 

 rausch'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, 



FIG. 51. MEASUREMENT OF THE PULMONARY CIRCULATION-TIME IN RABBIT BY 

 INJECTION OF METHYLENE BLUE. 



so that the noise of the Neef's 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 elec- 

 trodes when the current is closed. A definite, small quantity of the 

 salt solution 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 

 * The electrodes can easily be made by beating out one end of a piece 

 of thick platinum wire to a breadth of 5 or 6 mm., and then bending the 

 flattened part into a hook, or by bending pieces of stout platinum foil. 



