44 



ELECTRO-PHYSIOLOGY 



CHAP. 



w 



8- 



ufl"l 



1 



This swelling of the medullary sheath is naturally most 

 obvious at points where, e.g. at a cross section, the myelin 

 is in immediate contact with the entering fluid. Here, along 

 with the formation of characteristic " myelin figures," there 

 is often a regular outflow of medulla from the sheath, which 

 may extend far beyond the cross - section. 

 Very singular figures are produced by treating 

 medullated nerve - fibres with hot alcohol and 

 ether, when a great part of the medullary sub- 

 stance goes into solution, leaving a delicate 

 network of a highly refractile substance, which 

 gives a chemical reaction analogous with that of 

 keratin, and therefore termed nenrokeratin by 

 Kiihne and Ewald (Fig. 154). It is not known 

 whether these reticulate " horny sheaths " are pre- 

 formed as such within the normal medullary 

 sheath. All changes produced in the aspect of 

 medullated nerve-fibres by different reagents must 

 be accepted with great caution as to structural 

 conclusions, owing to the extreme instability of 

 the medullary sheath. 



We said that all medullated fibres of ver- 

 FIG. 154. Nerve-fibre tebrates are at first destitute of a medullary 

 aLho? b in' l the sheath, which only appears at a given stage of 

 centre is the twisted development. How this occurs, and how the iierve- 



axis -cylinder; , ,. ,. -. 



between this and fibres themselves are developed, is a disputed 

 the sheath of ma tt er . It is certain that nerve-fibres arise 



Schwann the net- . 



work of neuro- under all circumstances from special cells (nerve- 



keratin. (KOlliker.) e rearded 



PT 



t j le j r prolongations J 



this K< >lliker and His have established for the roots of the spinal 

 nerves also. Both anterior and posterior roots at first appear 

 as bundles of naked axis -cylinders, springing in the former 

 from the motor cells of the anterior horn, while in the pos- 

 terior roots some of the fibres run inwards from the cells of the 

 spinal ganglia to the cord, and some outwards to the periphery. 

 Later on, the cells which arise from the mesoblast form first a 

 sheath which invests the entire bundle of unmedullated fibres, 

 and subsequently a special sheath to each fibre (sheath of Schwann). 

 This secondary origin of the sheath of Schwann appears still 

 more plainly in the developing nerves of the tadpole's tail 



