12 



The Structure of Nerve-fibres. 

 By J. S. Macdonald, the University of Sheffield. 



(Communicated hy Professor Sherrington, F.E.S. Eeceived July 31, — 

 Eead November 22, 1906.) 



Frog's nerve-fibres present very different appearances in different portions 

 of their length, when teased and examined in Einger's solution. The main 

 distinctions affect the contents of the myelin sheath. To avoid mis- 

 understanding I propose to speak of this portion of the nerve-fibre as the 

 " intramyelin material," since the term more generally used (" axis-cylinder ") 

 has now become so completely identified with new appearances produced in 

 this material by the action of reagents. 



Briefly summarised the main differences observed are as follows : — 



(a) In places the intramyelin material exhibits the perfect transparency 

 and homogeneity of a clear solution. 



(b) In places this limpidity is clouded by a fine punctate granulation. 



(c) At others the granular appearance is much more marked. The 

 individual granules are of different sizes, and vary from obvious spherules 

 to just perceptible points. 



(d) In other places again large hemispherical vacuoles make their appear- 

 ance, their bases upon the myelin sheath and their convex surfaces pointing 

 inwards towards the central axis of the fibre. These marginally placed 

 vacuoles are situated at irregular distances from one another, and are of 

 irregular size. They lie now on this side, now on the other side of the fibre. 

 The remainder of the intramyelin material, the more central portion, is here 

 less translucent and more granular. 



(e) The vacuoles here have extended, and by their more numerous 

 formation and communication with one another have joined to form an 

 irregularly spiral channel. The central and less translucent portion of the 

 intramyelin material also presents the appearance of a spiral, as it bends 

 now to this point, now to that point upon the myelin sheath at places where 

 a separation from the myelin sheath has not yet occurred. The curves of 

 this irregular spiral are now short, now long. The material of which it is 

 formed also varies in diameter, more swollen here, more condensed there. 

 The central mass is much longer than the straight tubular myelin sheath 

 within which it lies. 



(/) In other places this central coagulum is of more uniform diameter and 

 forms a straighter line. It has undergone a diminution in length and in 

 thickness. 



