MULTIPLE RESPONSE OF NERVE 533 



This magnification, of the order of a hundred thousand 

 times, may be accomplished in either of two different 

 ways. A magnified image of the end of the long lever 

 may, in the first place, be thrown on a distant screen. 

 By this compound magnification the sensitiveness of the 

 record may be raised to the extent desired. Or, in the 

 second place, we may employ a battery of two levers in 

 series. The first of these gives a magnification, say, of 

 five hundred times, and is connected with a second optical 

 lever, by which a multiplying magnification of two hundred 

 times is easily obtained. It is unnecessary to point out that 

 special care should in this second case be taken to ensure 

 the steadiness of the support. With due precautions it is, 

 however, not difficult to secure the entire elimination of all 

 disturbing elements. 



When the spot of light from the second lever is thus 

 thrown on a distant screen, it is very interesting to watch the 

 various changes induced in the nerve by the environmental 

 conditions. An isolated nerve in a moist chamber, cut off 

 from its natural sources of energy, becomes increasingly 

 sub-tonic. This process is attended by an abnormal relaxa- 

 tion, which causes a steady movement of the spot of light 

 in one direction. When the nerve has become .very sub- 

 tonic, the effect of stimulus, as that of electric shocks, is to 

 enhance the tonic condition, and by this the downward 

 drift of the spot of light is retarded or arrested. In 

 cases of extreme sub-tonicity there is no further response, 

 beyond this arrest. But where the sub-tonicity is less 

 pronounced, stimulus will induce the abnormal positive 

 response ! by a sudden positive variation of the drift, which 

 is followed by recovery in the opposite direction. The 

 after-effect of absorption of stimulus is further effective in 

 causing the gradual retardation and final arrest of the 

 downward drift. By such absorption of stimulus the tonic 



1 The abnormal positive response is also obtained from nerve in ordinary 

 tonic condition, it should be remembered, by the application of excessively 

 feeble stimulus. 



