612 HOWARD. [Vov. XIX. 
konnte theoretisch annehmen, das die Stabchen an und fur 
sich nicht anders, als die markigen Nervenfasern wirken’”’ 
(p. 136). “Jene [Stabchen] waren aber wahrhaft positiv und 
das Mark von diesen [| Nerven] wahrhaft negativ” (p. 136). 
It is thus evident that Valentin believed that the optical 
axes of the rods and of the nerve fibers were not in agreement, 
but were at right angles to each other, and this opinion was 
accepted by Max Schultze (67), Krause (’92),* and Greet 
(3300))- 
It is not easy to account for Valentin’s statement that the 
axis cylinders of nerves are negatively anistropic, unless we 
assume that in consequence of the imperfect knowledge of 
nerve structure at his time he has recorded the reaction of 
the medullary sheath, which 1s negative, instead of that of the 
axis cylinder. Valentin’s work was done on Torpedo mar- 
morata and shows that his observations were made almost 
entirely upon medullated nerves. It is quite evident that 
what he refers to as sheaths of the nerve must have been the 
positively reacting connective tissue of the peripheral nerves, 
for he makes no mention whatever of the brilliantly con- 
spicuous medullary sheath as such. He does, however, speak 
of pressing out the retina of a frog with a cover-glass and 
finding fibers which he considers to be parts of the optic nerve. 
These, he states, also showed negative reactions, but there is 
no certainty that what he described were really optic nerve 
fibers. 
In my tests of nerves I found medullated fibers unsatis- 
factory objects for clear demonstration of optical properties 
in the axis cylinder, because of the strong predominance of 
the reaction color of the medullary sheath. The non-medul- 
lated fibers from invertebrates (crayfish) were more satis- 
factory, but even here the presence of the positive Schwann’s 
sheath, though comparatively thin, made conclusive observa- 
**“TDie Aussenglieder sind ferner positiv doppelbrechend; die optische 
Axe liegt in ihrer Langsrichtung und es ist bemerkenswert, dass sie 
sich entgegengesetzt wie das bekanntlich negativ Nervenmark verhalten.” 
(Krause, ’92, p. 150.) 
