THE PHENOMENON OF CONDUCTION. 85 



means at any point in its course. Artificial stimuli capable of 

 affecting the nerve fiber that is, capable of generating in it a nerve 

 impulse which then propagates itself along the fiber may be divided 

 into the following groups : 



1. Chemical stimuli. Various chemical reagents, when applied 

 directly to a nerve trunk, excite the nerve fibers. Such reagents 

 are concentrated solutions of the neutral salts of the alkalies, acids, 

 alkalies, glycerin, etc. This method of stimulation is not, however, 

 of much practical value in experimental work, since it is difficult or 

 impossible to control the reaction. 



2. Mechanical stimuli. A blow or pressure or a mechanical in- 

 jury of any kind applied to a nerve trunk also excites the fibers. 

 This method of stimulating the fibers is also difficult to control 

 and has had, therefore, a limited application in experimental work. 

 The mechanical stimulus is essentially a pressure stimulus, and the 

 difficulty lies in controlling this pressure so that it shall not actually 

 destroy the nerve fiber by rupturing the delicate axis cylinder. 

 Various instruments have been devised by means of which light 

 blows may be given to the nerve, sufficient to arouse an impulse, 

 but insufficient to permanently injure the fibers. The results ob- 

 tained by this method have been very valuable in physiology as con- 

 trols for the experiments made by the usual method of electrical 

 stimulation. It may be mentioned also that under certain condi- 

 tions for instance, at one stage in the regeneration of injured 

 nerve fibers mechanical stimuli may be more effective than 

 electrical, that is, may stimulate the nerve fiber when electrical 

 stimuli totally fail to do so. 



3. Thermal stimuli. A sudden change in temperature may 

 stimulate the nerve fibers. This method of stimulation is very 

 ineffective for motor fibers, only very extreme and sudden changes, 

 such as may be obtained by applying a heated wire directly to 

 the nerve trunk, are capable of so stimulating them as to produce 

 a muscular contraction. On the other hand, the sensory nerve 

 fibers are quite sensitive to changes of temperature. If a nerve 

 trunk in a man or animal is suddenly cooled, or especially if it is 

 suddenly heated to 60 to 70 C., violent pain results from the 

 stimulation of the sensory fibers in the trunk, while the motor 

 fibers are apparently not acted upon. We have in this fact one 

 of several differences in reaction between motor and sensory fibers 

 which have been noted from time to time, and which seem to 

 indicate that there is some difference in structure or irritability 

 between them. 



4. Electrical stimidi. Some form of the electrical current is be- 

 yond question the most effective and convenient means of stimulat- 

 ing nerve fibers. We may employ either the galvanic current that 

 is, the current taken directly from a battery or the induced current 



