MICROSCOPY. 185 
pass with great rapidity. The part of these nerve cords capable 
of transmitting nervous impressions is generally conceded to be 
the axis cylinder, a thin, thread-like cord of extremely simple 
structure, never resembling the terminal network, and always sur- 
rounded by the medullary sheath, a white, fatty, albuminous sub- 
stance of at: least ten times its diameter, which seems calculated 
to insulate and protect it. This medullary sheath, or white sub- 
stance of Schwann, is also little permeable to aqueous or albumi- 
nous solutions, and would preserve a uniform degree of moisture 
in the axis cylinder. The axis cylinder seems almost like an 
elongated band of white fibrous tissue. But little structural pecu- 
liarity has been demonstrated in it, and it is probably most re- 
markable for the perfect continuity of its parallel strata. The 
author believes that whatever changes take place in it might occur 
in other forms of tissue; indeed that such changes do occur in all 
tissues, but that only here are they so insulated that their varia- 
tions become evident. If the axis cylinder could be replaced by 
a long filament of ordinary fibrous tissue, he would feel almost 
justified in expecting the nerve current to be as well conducted as 
by the axis cylinder itself. 
That the nerve current is some unknown form of energy, dif- 
ferent from heat, electricity, etc., but correlated with them, is men- 
tioned as the prevalent belief of physiologists. It is deemed 
unphilosophical to explain phenomena by some conjectural force 
rather than by those we know something about; and the excellent 
opportunity for the author’s favorite tilt at the physicists is taken 
advantage of with undisguised enthusiasm. 
The chemical theory of the nerve current is still less admissible. 
The axis cylinder is a firm, tough, fibrous-like band, evidently of 
slow growth, little prone to rapid change, and only in imagination 
capable of rapid disintegration and reconstruction. Its action 
cannot be performed by chemical decomposition of its particles, 
especially as it is surrounded by ten times its thickness of myelin 
(medullary sheath) one of the least permeable substances in the 
body, and one of the least suitable media through which to take 
up new material or get rid of products of decay. 
The vibratory theory is equally inconsistent with the structure 
of the axis cylinder, which is not well calculated to propagate 
motor impulses and which varies greatly in different parts of its 
Course. The thickness of the medullary sheath, and its greater 
