654 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



cicatricial tissue which forms early between the cut ends of the 

 nerves. 



The return of function to the sympathetic takes place before the 

 freshly formed fibres have become medullated. Thus, in the case 

 mentioned above, in which regeneration had occurred twenty-four days 

 after section of the cervical sympathetic, no medullated fibres were 

 found in the peripheral portion. 



Seven to eight weeks after section of the nerve, the medullated fibres in 

 the regenerated part of the nerve are still few in number. Even fifteen 

 months after section, the regenerated nerve has fewer medullated and more 

 non-medullated nerves than normal. 



Although the fibres from each spinal nerve usually take on during 

 regeneration the functions they possessed before section, they do not 

 always do so. Thus the first and second thoracic nerves may after 

 regeneration cause erection of hairs, and the fourth and fifth thoracic 

 nerves may cause dilatation of the pupil, effects which they do not 

 normally produce. The selective action of the nerve fibres has there- 

 fore its limits. And we may conclude that a sympathetic nerve fibre 

 of one class, as pupillo-dilator, is capable of becoming connected with a 

 nerve cell of another class as pilo-motor. 



Post-ganglionic fibres. In the accounts which have been given of 

 observations on the regeneration of the nerves to the limbs, mention is 

 occasionally made of recovery of vasomotor action. Since we have 

 seen reason to believe that the vasomotor fibres in limb-nerves are 

 post-ganglionic, we may conclude that post-ganglionic fibres can regener- 

 ate. Extremely few direct observations have, however, been made on 

 the matter. Tuckett l found, in a rabbit, that the anterior strands of 

 the superior cervical ganglion regenerated in 259 days. Stimulation 

 of the cervical sympathetic caused dilatation of the pupil, and treatment 

 of the internal carotid with dilute methylene-blue showed the presence 

 of regenerated non-medullated fibres. 



The problem already dealt with in relation to the pre-ganglionic nerves 

 obviously occurs also, in a slightly modified form, in relation to the post- 

 ganglionic nerves. The fibres from the several cells run to different tissues. 

 When the fibres re-grow, will they end in connection with the tissue with 

 which they were previously connected 1 I have, a's yet, made one experiment 

 only on this point. The anterior branches of the right superior cervical 

 ganglion were cut in a cat ; after nineteen weeks, the upper thoracic nerve roots 

 were stimulated on both sides. On the left side, the usual effects were 

 obtained. But on the right side, the first six thoracic nerves, instead of the 

 first three only, caused dilatation of the pupil. A part of the region of the face 

 in which the hairs are normally strongly erected, by stimulation of the fifth 

 and sixth thoracic nerves, was barely affected by stimulation of any nerve. 

 The explanation of this which at once presents itself, is that some of the 

 fibres which normally run to the hairs of the face, had, in the process of re- 

 growth, been carried out of their way, and had formed new nerve-endings in the 

 pupil. If this explanation is correct, it follows that a nerve cell does not 

 necessarily send its axis cylinder process to a particular kind of peripheral 

 tissue ; and it suggests that there is no specific difference between the different 

 classes of post-ganglionic fibres, and that the function which each serves is in 

 a way accidental, depending upon the kind of peripheral tissue with which it 

 first comes in contact. 



1 Journ. Physiol., Cambridge and London, 1896, vol. xix. p. 297. 



