DEGENERATION AND REGENERATION OF NERVES. 353 



of the medullary sheath to the cells which compose the nucleated sheath of Schwann, 

 and the same view has been taken by other observers ; but it must nevertheless be 

 regarded as possible that it is actually formed by the axis-cylinder, or by the proto- 

 plasm which forms the peripheral layer of the axis-cylinder in its embryonic con- 

 dition, and for the following reasons, viz. (1) that in the central nervous system the 

 medullated nerve-fibres never at any time possess this nucleated sheath ; (2) that in 

 regenerating nerve-fibres, a thin medullary sheath appears around the growing axis- 

 cylinders before these are surrounded by their special nucleated sheath. At the 

 same time it is possible to suppose that the cells of Schwann's sheath may influence 

 the deposition of myelin. The neurolemma or sheath of Schwann on the other 

 hand appears certainly to be formed by cells which have applied themselves to 

 and have become flattened out around the preformed axis-cylinder, but whether 

 they are to be regarded as of mesoblastic origin (Vignal, Kolliker), or whether 

 they have passed out from the nerve-centre along with the processes of the 

 neuroblasts, and are therefore like the latter epiblastic in nature, is a question 

 which requires further investigation. On the whole, although we have no certain 

 information on the subject, it is probable that the medullary sheath has an origin 

 in common with the axis-cylinder, but that the primitive sheath is different, and 

 not improbably mesoblastic in origin. 



The formation of the medullary sheath occurs, not simultaneously over the whole 

 nervous system, but in regular order along definite tracts, and the knowledge of this 

 in the hands of Flechsig, has proved an important means of tracing the course of 

 certain strands of fibres in the nervous centres, as will be noticed when the subject 

 of the continuity of the fibres in those centres is dealt with. 



The fact that the nerve-segments orjnternodes of the peripheral nerves are con- 

 siderably shorter in the young animal, points to the existence of an interstitial as well 

 as a terminal growth of nerve-fibres. Besides such expansion of the internodes, 

 Vignal has described another method of growth in length of nerve-fibres ; mesoblast 

 cells similar to those which originally produced the nucleated sheath, applying them- 

 selves to the axis-cylinder of the nodes, and determining first an increase in length 

 of the nodal axis-cylinder, and then a formation of myelin upon this, so that a 

 short segment becomes intercalated at the node. These short segments soon grow 

 so as to attain the length of the remaining segments of the nerve-fibre. 



Degeneration and regeneration of nerves. The divided ends of a nerve 

 that has been cut across readily reunite by cicatricial tissue, but the cut ends of the 

 fibres themselves do not thus unite. On the contrary, soon after the section, a 

 process of degeneration begins in the peripheral or severed portion of the nerve. 

 The nuclei become multiplied, and the protoplasm about them largely increased in 

 amount, the segments taking on to some extent their embryonic condition. At the 

 same time the medulla of the white fibres degenerates into a granular mass consist- 

 ing of fatty molecules, and is then totally removed, and eventually the axial fibre 

 also disappears (fig. 416, A, B, and C). 



In regeneration the new fibres grow afresh from the axial fibres of the central 

 end of the divided nerve-trunk (often more than one from each) ; and, penetrating 

 into the peripheral end of the trunk, grow along this as the axis-cylinders of 

 the new nerves, becoming after a time surrounded with medullary substance (fig. 

 416, D). 



To this brief summary the following details may be added : In warm-blooded 

 animals the first changes in the peripheral part of the nerve are seen twenty-four 

 hours after the section. The nuclei underneath the primitive sheath are everywhere 

 found hypertrophied, the primitive sheath is distinctly visible, and protoplasm is 

 found to have accumulated at the expense of the medullary sheath, both in the 

 immediate neighbourhood of the nuclei, at the nodes, and also at other points in 



