626 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



4. Flushing of the shin, or a rise of temperature in it, are at times 

 and in certain circumstances produced by stimulation of the sym- 

 pathetic ; and it is generally believed that the changes are due to the 

 presence of vaso-dilator nerves. 1 The evidence, on the whole, is in 

 favour of the presence of such nerve fibres, but it is, I think, premature 

 to regard the question as settled. 



Some of the statements made do not harmonise with well-established 

 facts. Thus it has been said that weak tetanising currents stimulate 

 vaso-dilator more than vaso-constrictor fibres, although, as we have 

 already mentioned, the cervical sympathetic in the dog, when stimulated 

 with weak currents, produces contraction, and when stimulated with 

 strong currents, produces dilatation of the vessels of the bucco-facial 



region. 



The strongest evidence for the presence of vaso-dilator fibres in the 

 sciatic is that afforded by the series of well-known experiments initiated 

 by GJ-oltz, 2 on the effect of stimulating the sciatic nerve some two to 

 four days after cutting it. The thermometric method, the plethys- 

 mography method, and direct observation have given similar results, 

 namely, that the vaso-constrictor action becomes less as time goes on, 

 and that in the last day or two, before irritability completely disappears, 

 the vascular dilatation is out of proportion to the preliminary contraction, 

 or occurs without any contraction at all. 



It is not an unfair deduction that the vascular dilatation is due to the 

 presence of vaso-dilator fibres, which become prepotent some days after section 

 of the nerve, in consequence of their preserving their conductivity rather 

 longer than the vaso-constrictor fibres do. But, on the other hand, it has not 

 been shown by experiments in typical dilator and constrictor fibres, that the 

 latter degenerate more slowly than the former, nor is it inconceivable that the 

 changes which take place in the nerve and the tissue after section of the 

 nerve should be of a nature enabling stimulation of constrictor fibres to cause 

 the described degree of dilatation. 



According to Dziediul, 3 the vaso-dilator fibres of the submaxillary gland 

 (chorda tympani) take eleven days to lose their irritability, and those of the 

 nervus erigens nine days. These observations are, I think, untrustworthy. 

 I have found the vaso-dilator fibres of the chorda tympani of the cat to be 

 completely without effect in five days. Anderson and myself 4 found that the 

 sacral nerves seven days after section had no effect of any kind. 5 



There is some evidence also for the presence of vaso-dilator fibres 

 in the sympathetic supply to the skin, in the effects produced by 

 stimulating fresh and normal nerves. Primary dilatation is com- 

 paratively rarely obtained with any strength or form of stimulation, but 

 not uncommonly the after-dilatation seems out of proportion to the 

 primary contraction. 6 When a part only of sympathetic supply to a 

 given region is stimulated, the dilatation may occur during the stimula- 



1 For reference to papers, in addition to those mentioned in this section, cf. article on 

 "Circulation " in this volume. 



3 Arch. f. d. ges. Physiol., Bonn, 1874, Bd. ix. S. 174. Of later papers, cf. especially 

 Ostroumoff, ibid., 1876, Bd. xii. S. 219; and Bowditch and Warren, Joum. Physiol., 

 Cambridge and London, 1886, vol. xii. p. 416. 



3 Jahresb. u. d. Fortschr. d. Anat. u. Physiol., Leipzig, 1880, pt. 2, S. 68. 



4 Joum. Physiol., Cambridge and London, 1896, vol. xix. p. 372. 



8 For a further criticism of the evidence relating to vaso-dilator fibres of the limbs, and 

 especially of the muscles, see p. 640. 



6 Cf. Dastre and Morat, Compt. rend. Acad. d. sc, Paris, 1878, tome lxxxvii. p. 880. 



