334 VASCULAR CHANGES IN MUSCLES. [BOOK i. 



vessels of so large a part of the abdominal viscera, there is abundant 

 evidence of the presence of vaso-constrictor fibres. Division of this 

 nerve leads to a widening of the blood vessels of the abdominal 

 viscera, stimulation of the nerve to a constriction ; and as we shall 

 see, since the amount of blood vessels thus governed by this nerve 

 is very large indeed, interference either in the one direction or the 

 other with its vaso-motor functions produces very marked results, 

 not only on the circulation in the abdomen but on the whole 

 vascular system. There is some evidence that the splanchnic 

 nerves also contain vaso-dilator fibres, but this evidence is of a 

 more or less indirect character, and in any case, the number of such 

 fibres must be small. 



So far as we know, the vaso-motor fibres contained in the 

 sciatic and the like spinal nerves are distributed chiefly at 

 least to the blood vessels of the skin. Though so large a part of 

 the fibres of these nerves end in the muscles, the evidence of 

 vaso-motor fibres passing to the blood-vessels of the muscles is by 

 no means clear and undisputed. The blood vessels of a muscle 

 undoubtedly may change in calibre. For instance, when a muscle 

 contracts there is always an increased flow of blood through 

 the muscle ; this may be in part a mere mechanical result of the 

 change of form, the shortening and thickening of the fibres 

 opening out the minute blood vessels, but is also, if not chiefly, 

 due to the widening of the arteries by muscular relaxation. Such 

 a widening may be seen when a thin muscle of a frog is made, 

 in the living body, to contract under the microscope. But this 

 widening has not been proved beyond dispute to be due to the 

 action of vaso-dilator fibres. Indeed it has been argued that 

 when a muscle contracts, some of the chemical products of 

 the metabolism of the muscle may, by direct, local action on 

 the minute blood vessels, lead to a widening of those blood 

 vessels. And in some other organs, the brain and the kidney 

 for instance, we find functional activity accompanied by a widening 

 of the blood vessels under circumstances which seem to preclude 

 the possibility of the widening being due to vaso-dilator impulses 

 reaching the organ from without ; in such instances it is suggested 

 that the widening is due to a local effect of the products of the 

 activity of the organ. To this point we shall return. With 

 regard to vaso-constrictor fibres also the evidence that they are 

 supplied to muscles is, in like manner, not beyond dispute. 

 Section or stimulation of the nerves induces it is true changes in 

 the temperature of the muscles as it does in that of the skin. 

 But, as we urged just now, to argue from this that changes in the 

 blood supply have taken place is not wholly safe ; moreover the 

 changes in temperature observed are slight. Again, the fact that 

 when the nerve of a muscle is divided the blood vessels of the 

 muscle widen, somewhat like the blood vessels of the ear after 

 division of the cervical sympathetic, has been brought forward as 



