STRUCTURE OF NERVE CELLS. 603 



of regenerating nerve, even after the posterior root ganglion bad been 

 removed. 



It is a remarkable but unexplained fact, that the regenerating hbres which 

 are growing along the old nerve sheaths find their way eventually to the same 

 structures (muscles, skin, blood vessels) that they were distributed to before 

 section and degeneration had occurred. In this connection, Langley's observa- 

 tions upon the regeneration of the cut cervical sympathetic of the cat 1 may be 

 noticed. These show that the nerve fibres in that nerve, which are of very 

 different functional distribution, some carrying impulses to the dilator pupilla?, 

 others to blood vessels, others to hair muscles, others to secretory cells, never- 

 theless, after section and regeneration, are found to effect connection with the 

 same cells in the superior cervical ganglion as before ; e.g., the fibres in the 

 first thoracic anterior root still give, on stimulation after regeneration, dilatation 

 of the pupil and movement of the membrana nictitans ; those in the third 

 thoracic root still mainly produce contraction of ear vessels, and so on. Even 

 more striking are the experiments of the same observer upon the results of join- 

 ing the central cut end of the vagus to the peripheral cut end of the cervical 

 sympathetic. These show that the vagus fibres, when regenerating under 

 these circumstances, make connections with the cells of the superior cervical 

 ganglion of the sympathetic, so that all the effects of stimulating the cervical 

 sympathetic can now be produced through the vagus. 



It is not difficult to understand why the part of the nerve fibre 

 which is cut off from the nucleated body of the parent cell, should 

 undergo degeneration, for this is a phenomenon which is exhibited by 

 all cells ; the nucleus being essential for the maintenance of nutrition 

 of all parts of the cell. But it is much more difficult to comprehend the 

 occurrence, under these circumstances, of the changes which have been 

 described under the name of chromatolysis, and the degeneration which 

 follows those changes in the case of certain cells. Actual degeneration 

 does not extend along the axis cylinder backwards for any great distance ; 

 the greater part of the central end of the nerve fibre remains to all 

 appearance unaltered. Nor can the chromatolysis be alone due to arrest 

 of function of the nerve cells, since, although this might be alleged for the 

 sensory cells, which no longer receive the usual nervous impulses from the 

 periphery, the same cannot be admitted for the motor cells, which must 

 receive impulses by reflex and voluntary paths as before. Perhaps 

 the explanation is to be found in the fact that, in the first instance, the 

 section of the nerve and the chemical and electrical changes thereby set 

 up, and, secondly, the inflammatory and cicatricial changes in the 

 wound, maintain a constant excitation of the cut ends of the nerve fibres, 

 and thus keep the cells in continual activity. The changes produced 

 (chromatolysis and swelling of the cells) are, in fact, changes which 

 accompany activity, and they can be also brought about by the produc- 

 tion of local inflammation, and by the application of a chemical stimulus 

 (salt) to nerve-endings, as well as by the section of the nerve. This 

 would not suffice to explain the difference which has been described 

 by v. Gehuchten between the ultimate behaviour of motor and sensory 

 nerve cells, the former of which undergo, after about fifteen days, a 

 gradual process of reparation ; whereas the latter, at least those of the 

 vagus ganglion, fail to become repaired, but, on the contrary, are 

 eventually completely degenerated. This may, however, well be due to 



1 Journ. Physiol., Cambridge and London, 1892, vol. xiii. ; and 1895, vol. xviii. p. 280. 

 See also this volume, article "Sympathetic," p. 691. 



