COURSE OF POST-GANGLIONIC FIBRES. 627 



tion; indeed, if the vessels are already contracted, there may be no 

 observable primary contraction, but dilatation only. And when, as first 

 shown by Ostroumoff, slowly repeated shocks about 1 in 2 seconds to 1 

 in 5 seconds are used as the stimulus, there may be primary dilatation. 

 The most obvious conclusion from these results is, that the dilatation of 

 the vessels is due to the presence of the vaso-dilator fibres ; but it may 

 perhaps be doubted whether the evidence is conclusive that in these 

 cases the stimulation does not lower the irritability and conductivity 

 of the peripheral vasomotor nerve fibres, and thus prevent tonic vaso- 

 motor impulses from coming into play. Lastly, we must mention a line 

 of evidence first put forward by Schiff, 1 and adopted by Ostroumoff and 

 others. This consists in showing that the dilatation of the vessels of 

 a given region of the skin which can be produced by impulses from the 

 central nervous system for example, by excitation of afferent fibres is 

 greater than that produced by section of the vaso-constrictor and all 

 other nerves to the region. Unfortunately for the conclusiveness of 

 these observations, the reflex dilatation has not in most cases, at any 

 rate been compared with the maximal dilatation caused by section, but 

 with the dilatation existing after the vessels have recovered some degree 

 of tone. 



Course of post-ganglionic fibres. The sympathetic fibres which 

 run to the skin leave the sympathetic chain by the grey rami, they 

 run to the spinal nerves, and follow the cutaneous branches to the skin. 2 

 It has been generally believed that some sympathetic fibres make their 

 way to the periphery along the sheaths of the arteries. It is easily 

 shown that this is not the case with the pilo-motor nerves, for if the 

 dorsal cutaneous nerves are cut, the sympathetic is no longer able to 

 cause movement of the hairs. Similarly, after section of the sciatic, the 

 sympathetic is no longer able to cause secretion from the sweat glands 

 of the foot. There is some difference of opinion with regard to vaso- 

 motor fibres. For it has been found by some observers that, after 

 section of the sciatic, changes take place in the calibre of the vessels of 

 the foot, which can only be due to impulses coming from the central 

 nervous system. But in these cases the crural nerve was left out of 

 account. And it has been shown that sometimes, at any rate, the 

 crural nerve contains vasomotor fibres for the foot. 3 In mammals, 

 then, there is no evidence that sympathetic fibres pass to the skin of the 

 trunks or limbs by way of the arteries, and, taking all the facts into 

 account, we may conclude that they all pass by way of the spinal nerves. 

 In the frog, experiments have been made upon the vasomotor nerves to 

 the hind-limb ; the general result of which is the same as in mammals, 

 although the statement that the fibres accompany the arteries has been 

 more positively put forward. 



In this connection there is one point to keep in mind the sym- 

 pathetic sends some fibres direct to the aorta, and these appear to spread 

 some way down the large arteries. It is not unlikely that these nerve 



1 Schriften d. Benier Naiurf. Gesellsch., 1856, S. 69. 



2 Langley, Journ. Physiol., Cambridge and London, 1891, vol. xii. p. 368 (sweat fibres) ; 

 ibid., p. 375 (vasomotor fibres); ibid., 1893, vol. xv. p. 199 (pilo-motor fibres); ibid., 

 1894, vol. xvii. p. 296 (secretory and vasomotor fibres). Of. these papers also for origin 

 and nerve-cell connection of the sympathetic fibres to the skin. 



3 Lewaschew, Arch. f. d. ges. PhysioL, Bonn, 1882, Bd. xxviii. S. 431 ; Dastre and 

 Morat, " Systeme nerveux vaso-moteur," Paris, 1884, p. 262 ; Spallitta and Consiglio, Arch, 

 ital. de biol., Turin, 1897, tomo xxviii. p. 257. 



