CHAP, iv.] THE VASCULAR MECHANISM. 271 



do ordinary motor nerves ( 78). The result is perhaps even still 

 more striking if a mechanical stimulus, such as that of " crimp- 

 ing " the nerve by repeated snips with the scissors, be employed. 

 Exposure to a low temperature again seems to depress the con- 

 strictors more than the dilators ; hence when the leg is placed in 

 ice-cold water stimulation of the sciatic, even when 7 the nerve has 

 been but recently divided, throws the dilator only into action and 

 produces flushing of the skin with blood. Slow rhythmical stimu- 

 lation moreover of even a freshly divided nerve may produce dila- 

 tion. And there are other facts which support the same view 

 that the sciatic nerve (and brachial plexus) contains both vaso- 

 constrictor and vaso-dilator fibres which are differently affected by 

 different circumstances. 



In the splanchnic nerves which supply fibres to the blood ves- 

 sels 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 abdo- 

 minal 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 evi- 

 dence 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 un- 

 doubtedly 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 open- 

 ing out the minute blood vessels, but is also, if not chiefly, due to 

 the widening of the arteries by relaxation of their muscular walls. 

 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 



