Johnston, Parts of the Neurone. 613 



tion of mere conduction. The obvious adaptation of the dendrites 

 to the receptive function should lead us to look to the axone for 

 the means of adapting the strength of the impulse to its object. 



Can the axone determine the strength of the impulse so as to 

 secure the adaptation to function ? The affirmative answer is 

 at once suggested by the facts at hand. In the lamprey the single 

 motor neurone must innervate a hundred muscle fibers. There 

 is nothing about the cell-body or dendrites to suggest special 

 adaptation to this enormous task. The axone, however, increases 

 in diameter until when it enters the muscle it appears quite equal 

 to the work expected of it. If the strength of the impulse is pro- 

 portional to the caliber of the fiber through which it travels, the 

 effective stimulation of the muscles of the lamprey can readily be 

 understood; otherwise not. Each end branch of a motor fiber 

 is conspicuously thicker than the whole axone at its most slender 

 part in the spinal cord, while the area of the cross section of the 

 thickest part of the motor fiber is more than four hundred times 

 that of its slender portion in the spinal cord. We can not conceive 

 of the motor axone of less than one micron diameter being divided 

 into one hundred branches without increase in diameter. If there 

 were no increase in thickness of nerve fiber and no increase in 

 volume of motor impulse between the spinal cord and the myo- 

 tomes, I can not understand how eff'ective impulses could be 

 delivered to the muscle fibers. The conclusion reached, then, is 

 that the strength of the impulse is determined by the caliber of the 

 axone and that this is adapted to the work to be performed. 



If this statement is correct for the lamprey, is it not substantially 

 true for motor fibers generally .^ Is not the combined diameter of 

 the end branches of a motor fiber greater than that of its conduct- 

 ing portion "^ Above all, would not the motor end plate serve 

 especially the function of increasing the strength of the nerve 

 impulse so that it may be eff^ective as it enters the muscle fiber .^ 

 Miss Dunn (1902) has shown reason to believe that in the leg of 

 the frog there is a conical diminution of nerve fibers toward the 

 periphery. It does not appear, however, whether this diminution 

 takes place in motor or sensory fibers, or both. There is no con- 

 flict between her results and the hypothesis here ofi^ered, because 

 this hypothesis would be satisfied if a conical enlargement took 

 place in the combined diameter of the end branches, regardless 

 of the thickness of the fiber in its conducting portion. 



