4 68 NER VE. 



increased in intensity as in Hering's experiment, and this may excite the 

 nerve. 



According to Uexkull, all modes of mechanical stimulation are 

 effective only in proportion as they shatter the nerve, and there is good 

 ground for the belief that the withdrawal of water caused by drying, 

 and by many chemical stimuli, act in a similar manner. 



The method of mechanical stimulation eliminates all error due 

 to the spread of the exciting agency to other portions of tissue, 

 such as occurs when electrical modes are employed. The methods of 

 Uexkull, or modifications of these, are likely to be of great service 

 in the future, as their employment is not accompanied by nerve 

 injury. 



The conditions which determine the efficiency of a stimulus. — 

 The excitation of a nerve by an external stimulus is favoured by two 

 entirely independent sets of conditions, — one of these, it need hardly 

 be said, is physiological, and is expressed by the term maintenance of 

 high excitability ; the circumstances which play upon this physiological 

 state will be detailed in a succeeding section. The other conditions are 

 incident to the exciting cause, and involve the consideration of the 

 special characters which the external change must possess in order to 

 render it an effective stimulus. 



1. The intensity of the physical change. — The most obvious of 

 these characters is that of the force of the exciting agency, in other 

 words, the intensity of the stimulus, although it is a familiar fact that 

 there are minimal and maximal limits to its potency. Thus, an electrical 

 current of definite character, if very weak, is inadequate ; but when 

 gradually increased in intensity, without any alteration in its time 

 relations, it will, at some given point, become sufficiently potent to 

 stimulate. 



It is, however, by no means a simple matter to determine for any 

 one agent the point when its intensity has sufficiently developed to 

 arouse a response in a nerve. What is the indication that such a 

 response has been aroused ? In the case of a motor nerve, the con- 

 traction of the attached muscle shows that it has been aroused sufficiently 

 to produce an excitatory change of such extent that, when transmitted 

 through the nerve-endings, it evokes the muscle response. But it is 

 certain that the nerve itself must be affected by external agents which 

 are inadequate to cause an indirect muscular contraction. The only 

 physical indication of the excitatory change is the electrical response of 

 the nerve. 



In ascertaining, therefore, the relation between the intensity and 

 the exciting value of any stimulating agent, the electrical response 

 is the most reliable index of effective excitation. 



Such comparison has been made by Waller, who recorded the effect 

 produced when the strength of the exciting agent was gradually 

 increased. 1 The record consisted of a series of galvanometric excursions, 

 photographed on a slowly moving surface, each being the instrumental 

 summation of a number of electrical responses of a nerve, evoked by a 

 definite number of electrical stimuli (induction currents). The stimula- 

 tion was repeated at regular intervals of one minute, and was gradually 

 increased in intensity. The electrical nerve changes were contrasted 

 with the records of the muscle lift due to the nerve excitation by the 



1 Brain, London, 1895, vol. xviii. p. 209. 



