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IV. — On the Active Part of the Nerve Fibre, and on the probable 
Nature of the Nerve Current. By Lionel S. Beale, Fellow 
of the Boyal Society and of the Royal College of Physicians, 
Physician to King’s College Hospital. 
Most observers will agree in the opinion that the particular por- 
tion of the nerve fibre concerned in the transmission of nervous 
impressions is in all probability the axis cylinder. This view will 
be generally admitted, at least as regards those extended nerve 
cords, which in man and the higher animals intervene between the 
central origin and the peripheral ramifications of nerves. This inter- 
mediate portion of the nerve cord varies much in length in different 
cases, and it may be regarded as non essential, because in all animals 
we have parts of the nervous system which are active in which this 
portion of the nerve does not exist at all ; while in some animals, 
the part of the nerve fibre under consideration is altogether absent, 
and in none is it to be found at a very early period of life, when 
nerve action is nevertheless certainly taking place. This inter- 
mediate portion of the conducting cord then may vary wonderfully 
in length, like an electric wire, without any great difference in the 
action resulting. In the smallest mammal it cannot be more than 
half an inch in length, while in the whale it extends to many feet. 
Nevertheless, impressions pass from one end to the other with great 
rapidity in both instances. 
Now the active part of the nerve fibre distributed to the peri- 
pheral nerve organ which receives the impressions, exhibits the same 
general structure and anatomical arrangement in all cases. It is 
invariably a pale, very transparent, faintly granular, but in the 
natural state perfectly invisible cord, composed of still finer fibres, 
or consisting of a soft material which is traversed by currents which 
take several different lines or paths along it, as may be distinctly 
demonstrated. This has been regarded as an extension of the axis 
cylinder, and in an anatomical sense the conclusion is nearly true. 
The statement is not, however, perfectly accurate, because in many 
specimens it is evident that the so-called medullary sheath of the 
nerve and the axis cylinder gradually become altered until the 
delicate terminal ramifications of the nerve fibres are reached. 
The fibres of this terminal portion of the nerve network never 
resemble in structure the central part of the extended cord already 
referred to. Indeed, in the adult mammal these two parts of the 
nervous system exhibit such remarkable contrast in minute struc- 
ture, and in chemical composition and physical characters, that 
it is difficult to believe they are of the same nature. Whatever 
may be the real nature of the change in the part of the cord mid- 
way between centre and periphery, the changes in the more delicate 
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