652 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



Degeneration of Sympathetic Nerve Fibres after Section — 



Duration of Irritability. 



The niedullated nerve fibres of the sympathetic undergo the same 

 series of changes in degeneration as other medullated fibres. But the 

 changes are in most cases more rapid ; since most of the nerves are 

 small, and the medulla, which has to be absorbed, is less in amount. 

 Thus, after section of the sympathetic trunk, all the fatty portion of the 

 medulla may be absorbed in two or three weeks, and the degeneration 

 which has taken place may only be indicated by a few rows or small 

 clumps of granules. 



In non-medullated nerves, section produces extremely little histo- 

 logical change, when the nerves are treated with most reagents x ; but 

 Tuckett 2 has shown that the core of the non-medullated nerve in about 

 two days ceases in the fresh state to stain with methylene-blue. The 

 conducting core appears to become granular and then to be absorbed, 

 the sheath still remaining. 



Broadly speaking, sympathetic nerves lose their irritability in the 

 dog, cat, and rabbit, about the fourth day after section, sometimes a 

 little earlier, sometimes a little later. There is no satisfactory evidence 

 that there is any constant difference in the time after section at which 

 different classes of nerve fibres lose their irritability (cf. above, p. 626). 

 The time is longer in old animals than in young ; it varies in different 

 animals, and with the state of nutrition of the animal ; it varies also 

 with the size of the nerve cut. In the rabbit, Tuckett found that the 

 branches of the superior cervical ganglion which run to the internal 

 carotid artery, lose their irritability two days after section ; but it may 

 be doubted whether early degeneration is characteristic of non-medul- 

 lated fibres, since the vaso-constrictor fibres of the sciatic of the dog 

 retain their irritability up to about the fourth day. 



Waller 3 was the first to make observations on the duration of irritability 

 of the cervical sympathetic. He found that, at the end of the third day, stimu- 

 lation of the peripheral (upper) end no longer caused dilatation of the pupil. 

 Budge, 4 as the result of experiments on rabbits, stated that the irritability of 

 the pupillo-dilator fibres of the sympathetic lasts four to six days, and 

 occasionally even longer. 



Eegeneration of Sympathetic Fibres. 



Pre-ganglionic fibres.— In the earlier observations on the regenera- 

 tion of nerve fibres, pre-ganglionic fibres were at times the subject of 

 experiment, but no distinction was made between them and any other 

 nerve fibres. Some attention was directed to observing differences in the 

 time of return of function to nerve fibres of different classes, as motor and 

 sensory, vaso-constrictor and vaso-dilator, but the inherent differences 

 attending regeneration in the pre-ganglionic and post-ganglionic fibres 

 were not even mentioned. Waller 5 found that, two months after section 

 of the cervical sympathetic, dilatation of the pupil could be obtained by 



1 Langley and Anderson, Joum. Physiol., Cambridge and London, 1894, vol. xvii. 

 p. 183. 



2 Ibid., 1896, vol. xix. p. 291. 



3 Compt. rend. Acad. d. sc, Paris, 1851. 



4 " Ueber die Bewegung der Iris," Braunschweig, 1855. 



5 Compt. rend. Acad. d. sc, Paris, 1853, p. 378. 



