



50 ELECTRO-PHYSIOLOGY CHAP. 



other investments (sheath of Schwann, medullary sheath), also 

 possess a special delicate sheath (" axis-cylinder sheath "). In 

 isolated cases this certainly appears to exist, always, however, 

 as an excessively fine layer, hardly to be counted as a membrane 

 proper. 



The extreme instability of the substances of which the axis- 

 cylinder consists, leads, when it is treated with reagents, to the 

 appearance of many morphological changes which, without due 

 precautions, might easily lead to fallacies. Such is the marked 

 wrinkling induced even by physiological salt solution, and still 

 more by all strongly dehydrating methods of hardening, such as 

 alcohol, chromic acid and its salts, etc. 



For the same reason, stained sections of nerves, hardened in 

 chromic acid solutions, or salts, usually fail to give a correct 

 picture of the ratio between size of axis-cylinder and medulla, 

 since the axis-cylinder shrinks up within the swollen medullary 

 sheath, and forms in cross-section the well-known " sun-figures." 



The osmic acid method gives better, though still not un- 

 exceptionable, results. The ratio between axial space and 

 medulla was , estimated by M. Joseph in the electrical nerves of 

 Torpedo as 1 : 35. Within this great axial space the combined 

 osmic acid and alcohol method gives, both in longitudinal and in 

 transverse sections, a very delicate network (" axial reticulum " of 

 Joseph), which Joseph assumed to be preformed, and the meshes 

 of which should contain the axial fibrils, that do not appear in 

 preparations treated by this method. Joseph further asserted 

 that the " axial reticulum " is in direct connection with the 

 neurokeratin network of the medullary sheath; but this, as Kolliker 

 justly remarks, is rather evidence against preformation, since the 

 existence of the latter as a preformed constituent of the medullary 

 sheath is at least doubtful. The figures described by Joseph in 

 many respects resemble the structural relations predicated by 

 Btitschli of the axis-cylinder in its widest sense. The fibrillated 

 structure here as in muscle consists of a longitudinal series of 

 rods, the thicker lateral walls of which are united by very fine 

 cross-bridges. With a medium power this gives the appearance 

 of parallel longitudinal striation. It is uncertain whether this 

 rod-structure of Blitschli is really preformed, or is merely the 

 effect of reagents. In tissues of such excessive lability the last 

 hypothesis is always possible. Moreover, there are certain 



