6 10 Joiinwl of Comparative Neurology and Psychology. 



5. The volume of the neuraxone in the case of these motor 

 neurones greatly exceeds that of the cell-body and dendrites com- 

 bined. The dendrites of the motor neurones (as of other central 

 neurones in the lamprey) are slender smooth fibers, unlike those 

 in the brain of higher forms. The bodies of the motor cells in 

 the spinal cord are usually not greater in diameter than the largest 

 motor fibers at the point where they enter the muscles. In the 

 case of the largest motor neurones measured, I estimate the volume 

 of the cell body and dendrites as equal to that of a cylinder 15// 

 in diameter and .2 mm. in length, while the motor fibers are often 

 20/i thick and 2 mm. long. In mammals and man also the volume 

 of the neuraxone often exceeds that of the rest of the neurone, but 

 in this case the greater part of the volume is made up of a con- 

 ducting fiber. For example, in the case of the motor neurones 

 for the muscles of the lower leg and foot, the volume of the neurone 

 is found chiefly in the fiber connecting the cell-body in the lumbar 

 cord with the motor end plates in the muscles. This fiber is 

 fairly uniform in diameter throughout its length. In the lamprey 

 the portion of the fiber intervening between the cell-body and the 

 first end-branch is a small part of its length and the greatest part 

 of the volume of the neurone lies within the muscle where the 

 end branches are being gi\^n ofi^. The conducting portion of the 

 fiber grows as it proceeds, from a slenderto a very thick fiber: itisa 

 cone whose apex is at the cell of origin and whose base is in the 

 muscle. 



These facts seem to the writer to have a bearing on some impor- 

 tant problems of neurology. 



The nature of the nerve impulse. 



Is the nerve impulse something which is created in any specific 

 part of a neurone and thence merely conducted to its destination ? 

 This is implied in the ganglionic theory of the neurone, which 

 regards the cell-body as analogous to a battery which discharges 

 currents along the axone. Aside from the valid objections already 

 known, the facts quoted above are inconsistent with such a view. 

 We can scarcely conceive of a cell-body creating an impulse 

 which it discharges into a conductor as thick as itself and fifty 

 times as long. The thick axone must have some other signifi- 

 cance than that of a mere conductor. 



