512 



COMPARATIVE ELECTRO-PHYSIOLOGY 



glass cover. The nerve is placed vertically within this, and 

 held, at its upper end, by a clamp. The lower end of the 

 nerve is connected with the short arm of the Optical Lever 

 by means of a thread, which passes through a hole in the 

 floor of the chamber. A second thread of cotton moistened 

 with saline solution hangs loosely from the end of the nerve, 

 and is connected with the electrode E'. When the electrodes 

 E and E' are put in connection with the secondary of an 

 induction coil R, the entire length of the nerve is subjected to 

 direct excitation. When, on the other hand, we wish to study 



the effect of transmitted excitation, 

 the nerve is lightly clamped at B 

 (fig. 315). Excitation is then induced 

 in the portion of the nerve A a, and 

 after transmission through the inter- 

 vening tract, causes the motile effect 

 in the responding portion of the 

 nerve B c. 



One precaution which I find to be 

 very necessary is the maintenance of 

 the properly humid condition in the 

 nerve-chamber. This is specially 

 important in the warm weather which 

 characterises the greater part of the 

 year in India. The usual means of 

 keeping the chamber moist, by a large 

 quantity of blotting-paper soaked in water, is not sufficient 

 to bring about the maintenance of the normal excitability of 

 the nerve for any length of time. This need was met by 

 keeping moist vapour in uniform circulation through the 

 nerve-chamber. An air-bag is kept under suitable pressure, 

 and the air, bubbling through water in the vessel W, is made 

 to enter the nerve-chamber through an entrance-pipe, and 

 to escape by an exit-pipe. In warm weather it is well to 

 keep fragments of ice in the water- vessel. By proper mani- 

 pulation of the stop-cock of the air-bag, a gentle stream of 

 cooled and humid air is kept in constant circulation through 



FIG. 315. Diagrammatic 

 Representation of Ar- 

 rangement for Obtaining 

 Transmitted Effect of 

 Stimulus. L, indicating 

 lever 



