NATURE OF STIMULUS 183 



Some nerves have a sheath or coat composed of un saturated 

 fatty acids and lecithin (and allied lipoids). The function of 

 this medullary sheath has not been discovered. It is formed 

 from separate cells, but it must retain some connection with the 

 nerve cell, as it dies and disintegrates when dissociated from it. 



1. Structure. The neuron, like any other cell, is a colloidal 

 fluid mass. This may be demonstrated by examination of the 

 living nerve by means of the ultra-microscope, when particles in 

 Brownian movement will readily be seen. Some of these particles, 

 at times, clump together to form local large aggregates which 

 again dissociate. Further, Carlson has shown that nerves may 

 be stretched without altering their efficiency, judged by rate of 

 conduction of an impulse. Macallum states that alterations in 

 surface tension can be detected especially in the growing nerve. 

 It has been urged by Gothlin that, as a nerve is doubly 

 refracting to a slight extent just like muscle it must have a 

 similar composition. These facts all go to prove that nerve is 

 of a liquid nature. 



2. Its function is to conduct. One cannot lay too much stress 

 on the fact that it does not conduct an impulse originating outside, 

 as a telephone wire conducts current from a battery. The battery 

 is an integral part of the neuron. 



3. The nature of the stimulus seems immaterial. Mechanical, 

 electrical or chemical stimuli all cause the nerve to propagate the 

 same kind of impulse. Furthermore, the excitatory result of the 

 propagated impulse depends not on the nature of the " trigger " 

 stimulus but on the nature of the effector mechanism to which 

 the nerve goes. That is, stimulation of the sciatic nerve by 

 electrical, mechanical, thermal or chemical means always causes 

 contraction of the gastrocnemius muscle ; stimulation of the 

 vagus nerve by any means always slows the heart, stimulation 

 of the chorda tympani causes the salivary glands to secrete, no 

 matter how the stimulation is effected. Miiller's law of the 

 specific energies of the senses states that, by whatever manner 

 a sensory nerve is stimulated, the resulting sensation is always 

 that produced by this nerve when stimulated by its specific 



' trigger." That is, the receptor organ of a sensation-carrying 

 nerve controls the sensation received. To be concrete, stimulation 

 of a taste nerve (chorda tympani), by pinching, sudden heating 

 or cooling, electrical shock or . chemical reagents, produces the 

 sensation of taste and nothing else. Similarly, stimulation of 

 the eye nerves by any means makes one see light. Keith Lucas 



