No. 440.] STRUCTURE OF OUTER SEGMENTS. 



einen wahrhaft negativen Korper vor sich hat und die ganze 

 Erscheinung nur von clem Markc hcrriihrt " (Valentin, '62, p. 

 123). "Man konnte theoretisch annehmen, das die Stabchen 

 an und fur sich nicht anders, als die markigen Nervenfasern 

 wirken" (p. 136). " Jene (Stabchen) vviiren aber wahrhaft 

 positiv und das (Nerven) Mark von diesen wahrhaft negativ " 

 (P. 136). 



It is thus evident that Valentin believed that the optical axes 

 of the rods and of the nerve fibres were not in agreement but 

 were at right angles to each other, and this opinion was accepted 

 by Max Schultze ('67), Krause ('92), 1 and Greeff (: 00). 



It is not easy to account for Valentin's statement that the 

 axis cylinders of nerves are negatively anisotropic unless we 

 assume that in consequence of the imperfect knowledge of nerve 

 structure at his time he has recorded the reaction of the med- 

 ullary sheath, which is negative, instead of that of the axis cylin- 

 der. Valentin's work was done on Torpedo marmorata 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 con- 

 nective tissue of the peripheral nerves, for he makes no mention 

 whatever of the brilliantly conspicuous medullary sheath as such. 

 He does, however, speak of pressing out the retina of a frog 



reactions, but there is no certainty that what he described were 

 really optic nerve fibres. 



In my tests of nerves I tound medullated fibres unsatisfactory 

 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-medullated fibres from 



