162 NERVES. 



sheath in the same way as the connective tissue-cells become filled with 

 fat in the development of the adipose tissue. At all events, in young 

 nerves the segments are shorter, and there is a layer of homogeneous or 

 finely granular protoplasmic substance outside the medullary sheath, 

 between it and the primitive sheath : as the nerves increase in size this 

 layer, being more and more encroached upon by fatty substance, eventu- 

 ally almost or entirely disappears, except in the immediate neighbour- 

 hood of the nuclei. In the brain and spinal cord at an early period 

 flattened cells are found surrounding the medullated fibres (Ranvier) ; 

 subsequently they disappear or become incorporated Avitli the inter- 

 stitial tissue of those organs, without having produced either a primitive 

 sheath or constrictions on the fibres. 



The fiict that the nerve segments of the peripheral nerves are consi- 

 derably shorter in the young animal points to the existence of an inter- 

 stitial as well as a terminal gro\vth of nerve-fibres. 



Another mode of formation of nerves has been described by Beale 

 and subsequently by Hensen, who state that a fibre may be produced 

 by the lengthening out of a connecting process between two cells, 

 the one of which remains in the central organ as a nerve-cell, whilst 

 the other becomes a peripheral terminal organ. For the details of 

 the description and for other observations on the development of the 

 nerves, the reader is referred to the original memoir.* 



Re-union and regeneration of nerves. — The divided ends of a nerve that has 

 been cut across readily reunite, and in process of time true nerve-fibres are 

 formed in the cicatrix, and restore the continuity of the nervous structure. The 

 conducting- property of the nerve, as regards both motion and sensation, is even- 

 tually re-established through the reunited part. But, immediately after the sec- 

 tion, 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 embiyonic 

 condition. At the same time the medulla of the white fibres degenerates into a 

 granular mass consisting of fatty molecules, and is then totally removed, and 

 eventually the axial fibre also disappears. 



The degeneration above referred to does not affect, at least to any great extent, 

 the part of the nerve remaining in connection with the nervous centre, which 

 seems to exert an influence in maintaining the nutrition of the nerve.f The 

 ganglia, as Avell as the brain and spinal cord, were shown by AValler to be 

 centres of this influence. He found that, in the central and undegenerated portion 

 of a divided spinal nerve, while the fibres belonging to the anterior root owe 

 their integrity to their connection with the spinal cord, those of the posterior 

 root are similarly dependent on the ganglion ; and that, if the posterior root be 

 cut between the ganglion and the spinal cord, not only will the fibres which belong 

 to it in the trunk of the nerve beyond the ganglion remain unchanged, but also 

 those above the ganglion, in the portion of the root left in connection with it ; 

 whereas the segment of the same root which remains connected with the cord 

 but severed from the ganglion degenerates. Section of the sympathetic nerve in 

 the neck is followed by degeneration of the cephalic segment as high as the 

 superior cervical ganglion, but no farther. 



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

 end of the divided nerve-ti-unk (often more than one from each) ; and, pene- 

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

 of the new nerves, becoming after a time surrounded with medullai-y substance. 



Beale, Phil. Trans., 1863. 

 + In the neighbourhood of the divided central end, the nuclei of the primitive sheath 

 multiply, and the white substance appears to break up into fat droplets, but the 

 axis-cylinder remains unaltered. (Ranvier, Compt. Rend. Ixxv. p. 1831.) 



