MECHANICAL RESPONSE OF NERVE 513 



the chamber. Observing these precautions, I have been 

 able to obtain responses from a given nerve for as much as 

 three hours continuously, whereas, without this care, they 

 would have come to a stop in a very short time. By a 

 modification of this arrangement, we are also enabled to 

 study the effect on the excitability of the nerve of various 

 gases and vapours contained in a second vessel, V. A series 

 of responses is first taken, under normal conditions that 

 is to say, when the nerve is surrounded simply by a moist 

 atmosphere. On now turning a three-way tap in a given 

 direction, the water-vapour can be made to pass through the 

 vessel v, filled with the given gas or vapour, before reaching 

 the nerve-chamber. The series of responses then obtained 

 will show either the immediate or the after-effect of the 

 reagent at will. For it is easy, by means of the three-way 

 cock, to shut off the gas and re-establish the first or normal 

 condition, after which the responses will afford an indication 

 of the nature of the after-effect 



The lower end of the nerve, as has been said, is attached 

 to the arm of the lever which passes through the fulcrum-rod. 

 A light mirror is fixed on the .fulcrum-rod, its face being 

 downwards. The pull caused by the excitatory contraction of 

 the nerve causes rotation of the fulcrum-rod, and this in turn 

 gives rise to a deflection of the spot of light reflected from 

 the mirror. A responsive relaxation of the nerve would 

 give rise, on the other hand, to a deflection of the spot of 

 light in the opposite direction. The long arm of the lever, 

 it will be noticed, is here the ray of light. The responsive 

 movement of the spot of light is recorded on a moving 

 photographic plate vertically below the mirror, and whose 

 movement, regulated by clockwork, is in a direction at right 

 angles to that of the spot of light. The photographic plate, 

 or the film wrapped round the drum, moves under a fixed 

 wooden cover, not shown in the figure, which is provided with 

 a narrow incised slit. The length of this is parallel to the 

 direction of the movement of light, and at right'angles to that 

 of the plate or film. The advantage of having the plate 



