26 



COMPARATIVE ELECTRO-PHYSIOLOGY 



FIG. 17. Excitatory Response to Tension 

 and Compression 



When E is connected with the upper point A 

 a sudden bending down causes tension of 

 A. When connection is made with A' the 

 same flexure causes compression of A'. 

 Both induce galvanometric negativity. 



response of negativity. The electrical connection at A is| 

 next removed to the lower side of the specimen, at a point A'. 

 On now repeating the sudden flexure, A' undergoes com- 

 pression instead of ten- 

 sion. The result is a 

 similar negative elec- 

 trical response. 



Excitation may, 

 again, be produced by 

 means of a sudden blow 

 at a point. This blow 

 may be delivered by 

 means of a spring-tapper 

 (fig. 1 8), in which S is 

 the spring proper and 

 the attached rod R carries 

 at its end the tapping 

 head T. A projecting 

 rod the lifter L passes through S R. It is provided with 

 a screw-thread, by means of which its length, projecting 

 downwards, is regulated. By means of this the height or 



intensity of the stroke may 

 be varied. As one of the 

 spokes of the cog-wheel C 

 is rotated past L, the spring 

 is lifted and released, and 

 T delivers a sharp tap. 

 The height of the lift, and 

 therefore the intensity of 

 the stroke, is measured by 

 a graduated scale not 

 shown in the figure. We 

 can increase the intensity 



of this stroke through a wide range, first, by augmenting the 

 projecting length of the spring by a sliding catch. We may 

 give isolated single taps, or superpose a series in rapid 

 succession according as the wheel is rotated slowly or quickly. 



FIG. 1 8. The Mechanical Tapper 



