NERVES SUPPLYING SKELETAL MUSCLES. 641 



if they exist, are much more easily paralysed by curari than the other 

 known vaso-dilator fibres. In view of this, it is impossible to avoid the 

 suspicion that the dilatation of the vessels, which was observed when a 

 minimal dose of curari was given, was accompanied by an unnoticed 

 slight contraction of the skeletal muscular fibres. 



Heidenhain experimented on the sciatic nerve. He found that when 

 the sciatic nerve was stimulated four days after it had been cut, a rise 

 of temperature was produced in the gastrocnemius. The rise, however, 

 was slow and slight. It was produced on completely curarised animals, 

 and, according to Gaskell, curari paralyses the vaso-dilator nerves to 

 muscle. 



It is obvious that the conclusive form of experiment, as regards vaso- 

 dilator fibres of sympathetic origin, is to observe the effects of stimulat- 

 ing the abdominal sympathetic. Gaskell found a slight decrease of the 

 blood-How through the muscles. Heidenhain found a slight fall of 

 temperature. Neither found, on direct stimulation, evidence of the 

 presence of vaso-dilator fibres. The absence of vaso-dilator action has 

 been attributed to the antagonistic action of the vaso-constrictor fibres. 

 What we know of the antagonism of vaso-constrictor and vaso-dilatation 

 fibres, makes it permissible to assume that a few vaso-dilator fibres may 

 be present in the abdominal sympathetic, but is directly opposed to the 

 assumption that there are sufficient vaso-dilator fibres to produce the 

 very large increase of blood-flow which occurs during contraction of a 

 muscle. It was pointed out by Gaskell that the increase of blood-flow 

 accompanying contraction, however produced, is comparable to the 

 increased blood-flow produced elsewhere by the chorda tympani and by 

 the nervus erigens. If a few vaso-constrictor fibres are stimulated 

 simultaneously with either of these nerves, there is either vaso-dilatation 

 at once or after a preliminary constriction ; if many vaso-constrictor 

 fibres are stimulated with either of these nerves, there is complete 

 vaso-constriction, with or without an after-dilatation. In no case are 

 results obtained similar to those so far obtained by stimulating the 

 abdominal sympathetic, namely, with all strengths of stimulus a slight 

 constriction without after-dilatation. 



Further, on Heidenhain's view that both the vascular effects in 

 muscle on stimulating a muscle nerve are due to fibres coming from 

 the abdominal sympathetic, it is inexplicable why the muscle nerve and 

 the sympathetic should produce such very different results. 



There is, then, no satisfactory evidence that the sympathetic sends 

 vaso-dilator fibres to the skeletal muscle, although it is possible that 

 it sends a few. As a provisional hypothesis, we may suppose that 

 the increased blood-flow which takes place in muscle, on stimulation of 

 its motor nerve, is brought about by changes of some kind in the 

 muscle fibres. 



In the frog, it was shown by Gaskell 1 that the branch of the fifth 

 nerve which supplies the mylo-hyoid muscle, contains vaso-dilator fibres 

 for it. This muscle, however, is more related to the muscles of the 

 tongue — to which in mammals vaso-dilator fibres run — than to the 

 muscles of the limbs, so that the behaviour of the mylo-hyoid does not 

 justify us in drawing any conclusion as to the innervation of the limb 

 muscles, or of the skeletal muscles of the body in general. It may be 

 noticed the vaso-dilator fibres of the mylo-hyoid are not paralysed by 



1 Journ. Anat. and Physiol., London, 1877, vol. xi. p. 720. 

 VOL. II. — 41 



