STRUCTURE OP NERVOUS TISSUE. 347 



the white substance of Schwann is subsequently deposited around it, separating 

 it from the membranous tubular envelop. The statements of Schwann and 

 Kolliker respecting the origin of the peripheral plexuses, have been already re- 

 ferred to ( 343). It is believed by the last-named observer (loc. cit.), that 

 the fibres of the trunks with which these plexuses become connected, originate 

 in cells which become fusiform by elongation, and which then coalesce at their 

 extremities; and these seem to increase, after the first formation of the trunks, 

 by the longitudinal subdivision of fusiform cells which had not previously un- 

 dergone complete metamorphosis into fibres, or by the development of cells de 

 novo. The first development of the vesicular substance appears to take place on 

 the same plan with its subsequent renewal; that is, from free nuclei, around 

 which the intervening granular matter seems to collect, the cell-walls being of 

 subsequent formation. 



349. The regeneration of Nervous tubuli which have been destroyed, takes 

 place in continuity with those which have been left sound. This is readily proved, 

 on the one hand, by the return of the sensory and motor endowments of the 

 part whose nerves have been separated; and on the other, by microscopic exa- 

 mination of the reunited trunks themselves. All our knowledge of the func- 

 tions of the Nervous System leads to the belief, that perfect continuity of the 

 Nerve-tubes is requisite for the conduction of an impression of any kind, whether 

 this be destined to produce motion or sensation; and various facts, well known 

 to Surgeons, prove that such continuity may be re-established after it has been 

 completely interrupted. In the various operations which are practised for the 

 restoration of lost parts, a portion of tissue removed from one spot is grafted 

 (as it were) upon another; its original attachments are more or less completely 

 severed, frequently altogether destroyed, and new ones are formed. Now, in 

 such a part, so long as its original connections exist, and the new ones are not 

 completely formed, the sensation is referred to the spot from which it was taken : 

 thus, when a new nose is made, by partly detaching and bringing down a piece 

 of skin from the forehead, the patient at first feels, when anything touches the 

 tip of his nose, as if the contact were really with the upper part of his forehead. 

 After time has been given, however, for the establishment of new connections 

 with the parts into whose neighborhood it has been brought, the old connections 

 of the grafted portion are completely severed, and an interval ensues during 

 which it frequently loses all sensibility; but after a time its power of feeling is 

 restored, and the sensations received through it are referred to the right spot. 

 A more familiar case is the regeneration of Skin, containing sensory nerves, 

 which takes place in the well-managed healing of wounds involving loss of sub- 

 stance. Here there must obviously be, not merely a prolongation of the nerve- 

 tubes from the subjacent and surrounding trunks, but also a formation of new 

 sensory papillae. A still more striking example of the regeneration of Nervous 

 tissue, however, is to be found in those cases (of which there are now several on 

 record) in which portions of the extremities, that have been completely severed 

 by accident, have been made to adhere to the stump, and have, in time, com- 

 pletely recovered their connection with the Nervous as with the other systems, 

 as is indicated by the restoration of their motor and sensory endowments. The 

 evidence is still more satisfactory, however, when it is furnished not merely by 

 the return of functional activity, but by structural examination of the divided 

 part. This investigation has been recently pursued by M. Brown-Sequard, with 

 very surprising results. Having divided the Sciatic nerve of a Guinea-pig, he 

 observed indications of a return of sensibility in the limb supplied by it, as 

 early as a month after the operation ; within another month, the sensibility had 

 decidedly augmented, though it was still much inferior to that of the sound 

 limb, and the muscles then began again to act; six months after the section, the 

 sensibility appeared to have completely returned, and the motor power to have been 



