THE PHENOMENON OF CONDUCTION. 81 



tions for instance, at one stage in the regeneration of 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 



Fig. 29. Stimulating (catheter) electrodes for nerves: b, Binding posts for attachment 

 of wires from the secondary coil; s, insulating sheath of hard rubber; p, platinum points 

 laid upon the nerve. 



indicate that there is some essential difference in structure or com- 

 position 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 Leyden 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. 29). By this means the platinum ends which now 

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

 upon the nerve trunk, and the induced current passing from one to 

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

 nerve impulses which then propagate themselves along the nerve 

 fibers. The induction current is convenient because of its intensity, 

 which overcomes the great resistance offered by the moist tissue ; be- 

 cause of its very brief duration, in consequence of which it acts as a 

 sharp, quick, single stimulus or shock, and because of the great ease 

 6 



