174 
On the Active Part of the Nerve Fibre, 
peripheral ramifications are probably far more subtle, succeed one 
another with much greater rapidity, and are excited with far greater 
facility. 
In this communication I propose to limit myself principally to 
the consideration of the structure and action of the intervening 
part of the nerve cord, which is known as the axis cylinder, and is 
surrounded by and imbedded in a considerable thickness of the so- 
called white substance of Schwann, or medullary sheath of the 
nerve fibre. The latter material, which is ten or more times the dia- 
meter of the axis cylinder, consists of a fatty albuminous substance 
known as myelin or protagon. It forms a tolerably uniform layer 
round the core, which it probably insulates and protects, as the 
gutta-percha of the electric cable insulates and protects the copper 
wire which transmits the electric current, and thus prevents damage 
and loss of conducting property by the contact of adjacent objects. 
Not only so, but the constitution of the white substance of Schwann 
or medullary sheath is such as to interfere with the free passage of 
fluid. It is not very permeable to aqueous or albuminous solutions, 
and it is probable that by this investment the access of an undue 
proportion of fluid is prevented, and a uniform degree of moisture 
in the axis cylinder is preserved. Upon this thin thread-like 
impression -transmitting axis cylinder all our interest centres. 
We desire to ascertain how it acts, and we want to know by what 
means it conducts or transmits impressions direct from centre to 
periphery, or in the opposite direction. These questions have been 
often asked, but never conclusively answered. Indeed, in the pre- 
sent state of our knowledge, a conclusive answer is probably not 
to be given ; but it will not be an unprofitable waste of time to 
consider to what conclusions upon this question we are led by 
contemplating the facts revealed by careful microscopical investi- 
gation. 
There is hardly a tissue in the body concerning the structure and 
action of which we do not possess more positive information than 
we have been able to gain as regards the exact structure and mode 
of action of the axis cylinder. A strange mysteriousness still hangs 
over the question of structure of the axis cylinder. Even the ultra- 
physicists and chemists, who assert that all the phenomena of living 
beings are physical and chemical, affirm little of a very positive 
character upon the mode of action of the axis cylinder. Confident as 
they are in expressing themselves upon most physiological and many 
medical questions, there seems to be some doubt — even a little 
diffidence — just the slightest hesitation — concerning the manner in 
which this little cord carries to us intelligence from the outside world, 
and brings us into relation with the many objects that surround us. 
Its arrangement, they admit, looks very like that of an insulated 
wire ; but then it is such a bad conductor of electricity that the 
