1906.J 



Regeneration of Nerves. 



261 



another. The elements forming the original chain are regarded as ectodermal 

 elements which have wandered out from the spinal cord rudiments. The 

 sheath is derived from those parts of the original cell chains which retain 

 their protoplasmic character. 



3. The nerve trunk is not a secondarily formed bridge between the spinal 

 cord and the peripheral organ, but exists from the first, and in subsequent 

 development it merely undergoes elaboration and increase of length as the 

 distance between the spinal cord and the periphery increases with the 

 increasing size of the animal. 



His study of the development of motor nerves in the fish Lepidosiren leads 

 Kerr to adopt the third view as regards the nerve trunk itself, and the 

 second view as regards its sheath. In early stages the motor trunk is naked, 

 but later masses of mesenchymatous protoplasm, laden with yolk, become 

 applied to the nerve trunk, at first over only a small portion of its length ; these 

 masses gradually spread over the whole trunk, from which they are clearly 

 distinguishable by staining reactions. As development goes on, the yolk in 

 these masses is used up, their protoplasm becomes less and less conspicuous, 

 and eventually is only to be detected in the immediate vicinity of the 

 nuclei. At first the central protoplasmic strand in each fibre is simply 

 granular, but later the passage of impulses finds expression in the marking 

 out of definite fibrillar tracks, some undifferentiated protoplasm remaining as 

 the inter-fibrillar substance. 



It is pretty clear that the great function of the sheath is as a nutritive 

 organ ; its protoplasm is at first laden with yolk, and this is gradually used 

 up as the nerve trunk develops within it. 



That the main function of nuclei, apart from reproduction, is to control 

 cytoplasmic metabolism is well recognised, and the nuclei of the sheath are 

 able to exercise this control over the active metabolism of the developing 

 nerve trunk, which is destitute of nuclei of its own. Connected with this 

 relation is, no doubt, the active multiplication of these nuclei observed in 

 early stages of nerve regeneration. In such regeneration it may well be 

 that the protoplasmic matrix of the nerve simply repeats the process of 

 its original development, increasing in size and then developing nerve fibrils 

 within itself. 



On this view the process which takes place in the peripheral segment of a 

 cut nerve would be somewhat as follows : — 



The fibrils, no longer subject to the stimulus of passing nerve impulses, 

 revert to their protoplasmic condition, and the sheath becomes highly active : 

 it increases in thickness, and its nuclei multiply ; its protoplasm digests the 

 remains of the medullary sheath. Supplied with nourishment by the 



