THE PHENOMENON OF CONDUCTION. 87 



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

 are concentrated solutions of the neutral salts of the allcalies, 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 tmnk 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 dehcate axis cylinder. 

 It may be mentioned that under certain conditions — 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 stimuh fail to do so. 



3. Thermal stimuli. A sudden change in temperature may 

 stimulate the nerve fibers. This method of stimulation is ver\ 

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

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

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

 a muscular contraction. On the other hand, the sensory nerv(^ 

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

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

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

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

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

 of several differences in reaction between motor and sensory fiber> 

 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 stimuli. 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 

 from the secondary coil of an induction apparatus or the so-called 

 static electricity from a Ley den jar or other source. In most experi- 

 mental work the induced current is used. The terminal wires from 

 the secondary coil are connected usually with platinum wires im- 

 bedded in hard rubber, forming what is known as a stimulating elec- 

 trode. (See Fig. 30.) By this means the platinum ends which now 

 form the electrodes, anode and cathode, can be placed close together 

 upon the nei ve tnmk, and the induced current passing from one to 

 the other through a short stretch of the nerve sets up at that point 



