336 



MOTION IN ANIMALS AND PLANTS. 



known excitatory responses — all electrical changes which are concomi- 

 tants of action— may be compared Avith one of these types. 



(jfjf^gQ l^ — Response to a continuous stimulation. A difference of 

 potential comes into existence at the contacts at the time t^ and persists 

 long enough to produce its full effect on the column. (Diagram 1.) 



Case 2. — Series of short continuous stimulations. The column moves 

 in alternately opposite directions. (Diagram 2.) 



Case 3. — Response to a single instantaneous stimulation. A differ- 

 ence of potential comes into existence abruptly, and subsides abruptly 

 at first, afterwards less rapidly. (P' in diagram 4.) 



Now, I have found that in the study of my experimental results it 

 is of great advantage to proceed a priori. Let us assume that there 

 are three types of stimulation, and that each has its form of response. 

 We can best begin by inquiring to which of these three forms the 



24 26 



observed variation belongs, and then determine in what respects it 

 conforms with, or differs from, the type. 



In the diagrams, I have shown the types of photographic curves 

 which correspond to the three forms of response to stimulation I have 

 indicated. The faint lines represent photographic curves; the strong, 

 variations of potential difference. In each diagram the strong and the 

 faint lines have been drawn in their true mathematical relation to each 

 other, i. e., so that the vertical distance apart of strong from faint is 

 everywhere proportional to the gradient or slope of the photographic 

 curve, the proportion being such that if the electro-motive force of the 

 current acting on the electrometer varied according to the strong line, 

 the movement of the head of the mercury column would be expressed 

 by the faint line. We shall see as we proceed that one or other of 

 the three forms of photographic curve, which correspond to the three 

 forms of electrical change, just designated as typical, presents itself 



