46 ELECTRO-PHYSIOLOGY CHAP. 



nucleus, which obviously corresponds with Schwann's nuclei in 

 medullated vertebrate nerves, although Ketzius disputes the 

 existence of the sheath of Schwann, not merely in the splanchnic 

 cord, but also in peripheral nerves. The myelin sheath runs 

 uninterruptedly from one constriction to the next, exhibits double 

 contours, and a shining, fatty appearance ; after treatment with 

 osmic acid the sheath becomes first gray, then black, exactly 

 like the medullary sheath of vertebrate nerve-fibres (6). 



THE AXIS-CYLINDER 



The finer structure of this, the functionally most important 

 part of the nerve-fibre, is again much disputed. Apart from 

 the undoubted difficulties of investigation, there is no doubt 

 that the most appropriate objects have not in many cases been 

 selected for experiment. On the one hand large elements are 

 required, on the other absence of thick sheaths which may 

 obscure the field of the microscope. It is a priori obvious 

 that medullated fibres must be less favourable objects than the 

 non-medullated fibres of vertebrates and invertebrates. And, 

 in fact, that theory of the structure of the axis-cylinder which 

 is most widely current, and appears morphologically and physio- 

 logically the best - grounded, is fundamentally derived from 

 observations on the nerve - fibres of invertebrate, and non- 

 medullated fibres of vertebrate animals. As early as 1843 Eemak 

 noted a bundle of fine fibrils in certain giant nerve-fibres of the 

 ventral cord of the crayfish, in place of the axis-cylinder, and 

 M. Schultze subsequently embraced the view of a uniform 

 fibrillated structure of the axis-cylinder in all nerve-fibres. He 

 pointed out that (more especially in the thick medullated fibres 

 from the lateral columns of the spinal cord, " in which, since 

 there is no sheath of Schwann, the axis-cylinder can be readily 

 isolated, either in the fresh state, or still better after maceration 

 in iodised serum ") a parallel striation and a finely-granulated 

 substance between the striae may be distinguished with a high 

 power, which can only indicate " a composition of fibrils and 

 interfibrillar substance." Even within the medullary sheath, 

 Schultze was able to detect the same structure of axis-cylinder 

 in the thick fibres from the brain of the torpedo. Very significant 

 again for the fibrillated structure of the axis-cylinder are the 



