Johnston, Parts of the Neurone. ' 6ii 



The same idea that the function of the axone is to conduct is 

 impUed in the various efforts to discover a special conducting sub- 

 stance or structure in the neurone. Since Apathy's description 

 of neurofibrillae they have been made to do duty as the conducting 

 substance. Here further difficulties arise in the lamprey. If 

 neurofibrillae in a motor fiber are to supply a hundred muscle 

 fibers, they must be crowded into the motor fiber at its smallest 

 part. If the impulse is merely conducted by these, the impulse 

 itself must arise in the cell-body or dendrites and be measured in 

 strength by the potentiality of that part of the neurone. In the 

 lamprey the impulse starting from the cell is carried through a fiber 

 less than one micron thick and then distributed through a fiber 

 20 to 24/^ thick, and through a hundred branches to as many 

 muscle fibers. Each end-branch of the motor axone is thicker 

 than the axone itself within the spinal cord. I can not help 

 thinking that under these conditions the impulse, if merely con- 

 ducted, would become so subdivided and attenuated in strength 

 that it would not be effective. All its force would be spent on the 

 first few end branches or would be dissipated in the great expansion 

 of the nerve fiber. On the anatomical side, I can not see how the 

 neurofibrillae necessary for a hundred motor endings, and for 

 which presumably the large fibers are essential, could be packed 

 into the slender proximal portion of the fiber and still be function- 

 ally efficient. Yet Apathy, Bethe and others would have us 

 believe that the neurofibrillae are free and independent through- 

 out their course in the neurone. If the motor impulse were merely 

 conducted by neurofibrillae, we should expect the cell-body to be 

 large, the motor fiber to be thick at its proximal part, and the 

 combined thickness of the branches to be little if any greater than 

 that of the conducting fiber. 



On the other hand, if the neurofibrillae branch or form a net- 

 work and if the neuroplasm plays an essential part in conduction, 

 the existence of a slender portion in the axone will not be an 

 obstacle to the passage of an impulse, as will appear below. This 

 is not the place to enter into a discussion of the structural relations 

 of the neurofibrillae but it may be noted in passing that several 

 authors have described the neurofibrillae in the cell-body and in 

 the axone as branching, as connected by cross-fibrillae or as form- 

 ing an irregular netw^ork. 



How is the strength of the impulse delivered by a neurone 



