248 LECTURES TO SCIENCE TEACHERS. 



with it. 1 Let me point out to you further that the recording 

 tympanum is supported by a vertical rod which is of one piece 

 with the clock-work and cylinder, so that its distance from 

 the axis of the cylinder is invariable and that on this rod 

 there is a rack and pinion, by which the recording tympanum 

 can be fixed at any desired height. 



I will now perform a series of experiments before you, 

 each of which will consist in allowing a single induction 

 shock to pass through the nerve, and recording the resulting 

 muscular motion on the cylinder. And in order that the 

 result of each experiment may be distinguishable from that 

 of its predecessor, and at the same time readily comparable 

 with it, Mr. Page will by means of the pinion I have men- 

 tioned, so adjust the recording tympanum that each tracing 

 shall be a twenty-fifth of an inch below the one last made. 



We have now made ten records. I will next bring the 

 catch h on the cylinder into contact with the trigger, which 

 closes the primary circuit of the induction apparatus, so 

 that the cylinder shall remain for a few moments in the 

 exact position, which when revolving, it attains at the 

 moment that the induction shock passes through the nerve. 

 The muscle contracts and compresses the receiving tym- 

 panum. The consequent expansion of the recording 

 tympanum elevates the lever attached to it, and thereby 

 makes a vertical mark on the recording surface. From 

 this vertical mark I draw with the aid of the rack and 

 pinion a vertical line, which of course cuts all the ten tracings 

 at right angles. My object in doing so, is that the inter- 

 section may mark in each tracing, that point in it which 

 corresponds to the instant at which the nerve is excited. 



Let us now study the results. On the left side of the 

 vertical line, the record consists of ten horizontal lines, 

 one below the other at a distance of one millimetre each 

 from each. Beyond, i.e., to the right of the vertical, they 

 go on in the same direction at first, but at a distance of 

 about an eighth of an inch, each line curves upwards in a 

 sweep of which you will see that the form agrees precisely 

 in all the ten tracings. Having culminated, it descends to 

 its previous height with a more gradual curve than that by 

 which it ascended. 



1 In this experiment the opening shock is cut off by a mechanism 

 not here described. 



