414 MESSRS. F. GOTCH AND V. HORSLEY 



Relation of Effect to Strength of Stimulus. 



To ascertain this relationship, the practice we employed was to use an initial 

 strength of stimulus which was just sufficient to produce a cord electrical effect when 

 applied to either nerve with the animal efficiently narcotised, then to follow with a 

 stimulus of double that intensity, and observe the now increased value, and finally to 

 proceed by doubling the strength of stimulus until a point was reached when the effect 

 was only slightly altered. Such an experiment is the following one made upon the 

 Cat. An initial excitation for 5 seconds with the secondary coil at a distance of 500 

 (see Chapter III. on experimental method and apparatus) gave a cord effect indicated 

 by a galvanometric deflection of 65 ; similar excitation of twice the intensity, 1000, 

 gave a cord effect of 122, and an excitation of four times the intensity, 2000, gave a 

 cord effect of 152. In the Rhesus Monkey a similar result was observed; thus an 

 initial excitation for 5 seconds of the sciatic nerve, with the coil at 2000, gave a cord effect 

 of 32 (galvanometer deflection), whereas twice the intensity of stimulation, 4000, 

 gave 60, and three times the intensity, 6000, gave 78. These selected individual 

 examples furnish data which are strictly in accordance with the average effect as 

 deduced from all the observed instances which occur in our experimental records. 

 Thus if we select all the unexceptional readings of galvanometric cord effects obtained 

 in both Cat and Monkey with the initial strength of stimulus, the average of seventeen 

 is 5G ; similar readings (twelve instances) with twice this strength of stimulus give 

 an average of 100 ; whilst the average of sixteen instances with four times the initial 

 strength is 168. It is thus evident that the cord effect increases with the intensity 

 of the stimulus at first in direct proportion to the strength, but afterwards in one 

 less directly related with this factor. A point is ultimately reached at which no 

 increase, but a decrease, in cord effect occurs. This, it need hardly be said, is partly 

 dependent upon the damaging effect of the stronger exciting current upon the 

 stimulated nerve, and partly upon the fatigue of the cord cells through the arrival in 

 the cord of the very intense nerve impulses evoked by the strong stimulus. In these 

 experiments, as in all others to be referred to in this section, the number and 

 characters of the successive stimuli employed were the same in each case. (See 

 Chapter III. Section 3.) 



It is remarkable how close the amounts of the different cord readings obtained in 

 various individuals of one species are to one another, and how often, especially in later 

 experiments, the initial intensity of stimulus required with different nerves and different 

 animals was the same (500 coil), thus showing the similarity of the narcosis. We may 

 here draw attention to the minimum and maximum effect obtained in the Cat, the first 

 with the coil at 500 and the second at 2000 ; the minimum being 20, the maximum 340. 

 So far then as the relation between the size of the cord effect and the strength of the 

 nerve stimulus is concerned, there is a strong resemblance between a nerve-to-cord 

 excitatory electrical change and that in a nerve ; the ether effect before-mentioned 



