562 HOWARD. [VoLt. XIX. 
Stibchen selbst anlangt, so scheint mir aus ihrem Verhalten 
im frischen Zustande, ihrer leichten Veranderlichkeit und ihrer 
Reaction gegen Wasser und andere Substanzen unwiderleglich 
zu folgen, dass dieselben mit andern, blassen Nervenrohren, 
namentlich den Opticusfasern in der Retina, auf eine Stufe zu 
stellen sind und die Natur von zarten, mit einem zahflussigen 
eiweissreichen und auch fettfuhrenden Inhalt erfullten Rohren 
besitzen.”’ And he further adds (p. 329): “Eine wesentliche 
Differenz zwischen den Stabchen und blassen Nervenrohren 
kenne ich nicht.” 
Kolliker by this declaration took the position, held jor years 
by Henle, of identifying the rods with the axis cylinder (“Ner- 
venrohren”’).. Leydig (755; ’64) also declared the rods and 
cones of vertebrates to be “eigenthtmliche Umwandlungen der 
Nervensubstanz.”’ The investigations of Max Schultze (66; 
71) cleared up many points in the structure of the rods and 
cones. Through these researches the contention of H. Muller 
and Kolliker was sustained. 
The results of Schultze’s researches and those of the workers 
up to his time, with regard to the structure of the visual cells, 
is well summarized in the following account, taken in the 
main from Hoffmann (’75). The description is based upon 
the amphibian rods and cones, which on account of their large 
size have yielded more results to investigation than those of 
any other animals. 
The Rods consist of two parts, chemically and physically 
different, named the inner and outer segments or limbs. The 
outer segment is strongly refractive while the inner one con- 
sists of a homogeneous substance much less refractive. The 
segments are clearly distinguishable one from the other. What 
is evident in the fresh condition is especially marked after 
treatment with osmic acid. This reagent colors the outer seg- 
ment black, while the inner one remains colorless or assumes 
a greyish shade. The reaction to (or behavior in) water, dilute 
acids, and alkalis is totally different for the two segments. 
Valentine (’62) and Max Schultze (’67) found that the 
