vii STRUCTURE AND ORGANISATION OF NERVE 43 



ends : the internal relations of these were specially investigated 

 by Schiefferdecker. These secondary breaks in the continuity of 

 the medullary sheath are known, after their discoverer, Lantermann, 

 as " Lantermann' s indentations" (Fig. 157). On treating fresh 

 medullated nerve-fibres with silver nitrate, characteristic black 

 crosses appear at the constrictions ; the solution of silver enters 

 most rapidly at this point, and not only colours the ground- 

 substance of the constriction (Schiefferdecker's ZwiscJienscheibe 

 intermediary discs), but also penetrates a certain distance along the 

 axis-cylinder, spreading on either side between the latter and the 

 medullary sheath (" periaxial clefts") (Fig. 159). The long arms 

 of the cross are often discontinuous, and appear as a more or less 

 prolonged series of transverse striae, the so-called " silver lines of 

 Frommann," the origin of which has not yet been adequately 

 determined. 



During the death of the fibre many striking phenomena make 

 their appearance. We have said that the medullary sheath in 

 living nerve-fibre is smooth and homogeneous : in moribund nerve 

 it alters conspicuously. Even with the most favourable conditions, 

 in fluids that are as far as possible indifferent, the excessive 

 instability of the substance of the medullary sheath causes rapid 

 alterations, which are generally described as coagulation-phenomena, 

 or formation of " myelin figures." These are chiefly characterised 

 by a kind of folding and wrinkling of the medullary sheath, so 

 that the lateral border of the fibres, which was at first rectilinear, 

 becomes much undulated, while irregular lumps and knotty lines 

 and networks appear on the surface, and soon conceal the notches, 

 although the constrictions still remain visible. 



These changes are intimately associated with the chemical 

 constitution of the medullary sheath, lecithin and cholesterin 

 being among its chief constituents. It is owing to the former 

 that the medulla of the nerve, when treated with osmic acid, 

 stains a more or less deep black, so that even the finest medullary 

 sheaths can be detected by this reaction. Water, dilute acids, 

 and solutions of salts cause a swelling of the medulla, which occurs 

 most rapidly and distinctly in the central nerve-fibres, where the 

 sheuth of Schwann is wanting. Here, as also in the peripheral 

 medullated fibres, there is often a characteristic blistering of the 

 medullary sheath, beginning at the free end of the medullary 

 segment, and extending right along it. 



