THE CIRCULATION OF THE BLOOD AND LYMPH 123 



measure the interval between the injection and appearance of the 

 salt with considerable accuracy. Hermann made a further advance 

 by allowing the blood to play upon a revolving drum covered with a 

 paper soaked in ferric chloride, and by using the less poisonous 

 sodium ferrocyanide for injection. 



Even as thus modified, the method laboured under serious defects. 

 It was not possible to make more than a single observation on one 

 animal, at least without allowing a considerable interval for the 

 elimination of the ferrocyanide, and, further, the method was unsuited 

 for the estimation of the circulation time in individual organs. In 

 both of these respects the more recently introduced electrical method 

 presents considerable advantages ; for by its aid we can not only 

 obtain satisfactory measurements of the circulation time in such 

 organs as the lungs, liver, kidney, etc., but we can repeat them an 

 indefinite number of times on the same animal. 



A cannula, connected with a burette (or a Mariotte's bottle, or a 

 syringe), containing a solution of sodium chloride (usually a 1*5 to 

 2 per cent, solution), is tied into a vessel say, the jugular vein. 

 Suppose that the time of the circulation from the jugular to the 

 carotid is required that is, practically the time of the lesser or 

 pulmonary circulation. A small portion of one carotid artery is 

 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 

 resistances in a Wheatstone's bridge (p. 534). The secondary coil 

 of a small inductorium, arranged for giving an interrupted current, 

 and with a single Daniell 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 re- 

 sistance 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 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 electrodes 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 elecuodes causes a diminution 

 of its electrical resistance (p. 34). This disturbs the balance of the 

 oridge s and the sound in the telephone becomes louder. The time 

 from the beginning of the injection to the alteration in the sound is 



* The eiectrocies can easily be made bv 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. 



