HISTOLOGY OF THE FROG 93 



The neurilemma forms a continuous covering to the nerve- 

 fibre, but the medullary substance is broken up into segments 

 by constrictions, placed at regular intervals, and known as the 

 nodes of Ranvier ; and, in this respect, and in others, the simile 

 of the telegraph wire will help us no further. Each node of 

 Ranvier is formed by a constricting band, in the form of an 

 annular ring extending inwards from the neurilemma to the 

 axial-fibre. The nature of this constricting band is not very 

 well understood, but its staining properties suggest that it 

 belongs to the class of intercellular substances, and if this view 

 be correct each internode would appear to be formed as a 

 single cell. This view is strengthened by the fact that in 

 each internode there is a single nucleus, surrounded by a small 

 quantity of granular matter, and lying underneath the neuri- 

 lemma, between it and the medullary sheath. But the axial- 

 fibre is continued without interruption across each node of 

 Ranvier, and it has been shown, both in development and in 

 the regeneration of a nerve-trunk which has been divided, 

 that the axial-fibre, which we must regard as the essential part 

 of a medullated nerve-fibre, is formed as an outgrowth of a 

 nerve-cell, and is therefore to be regarded as a very much 

 elongated process of a nerve-cell, and not as a product of a 

 number of cells represented by the nuclei of the internodes. 

 This much, at any rate, is certain : that every nerve-fibre, con- 

 tinuous as it is with a process of a nerve-cell, is dependent on 

 that cell for its nutrition, and dies if it is divided off from the 

 cell. It is furthermore certain that, in the course of develop- 

 ment, nerve-fibres first appear as pale fibres, destitute both of 

 medullary sheath and neurilemma. The next thing to make 

 its appearance is the neurilemma, which is nucleated, and is 

 known at this stage as the nucleated sheath of Schwann, and 

 lastly, the medullary sheath makes its appearance compara- 

 tively late in embryonic life. It is supposed by some authori- 

 ties that the medullary sheath is formed by the activity of the 

 cells composing the nucleated sheath of Schwann, and that, in 

 the adult nerve-fibre, the medullary sheath and neurilemma of 

 each internode represent the results of the activity of a single 

 cell derived from the nucleated sheath of Schwann, whilst the 

 axial-fibre which they surround is something different namely, 

 a process of the nerve-cell proper to the fibre. Other authors, 

 however, hold that the medullary sheath is formed from the 



