218 THE VASCULAR MECHANISM. 



bility of the nerve. For example, if the sciatic be divided, and some days 

 afterward, by which time the flushing and increased temperature of the foot 

 following upon the section has wholly or largely passed away, the peripheral 

 stump be stimulated with an interrupted current, a renewed flushing and 

 rise of temperature is the result. We are led to conclude that the sciatic 

 nerve (and the same holds good for the brachial plexus) contains both vaso- 

 constrictor and vaso-dilator fibres, and to interpret the varying results as due 

 to variations in the relative irritability of the two sets of fibres. The con- 

 strictor fibres appear to predominate in these nerves, and hence constriction 

 is the more common result of stimulation ; the constrictor fibres also appear 

 to be more readily affected by a tetanizing current than the dilator fibres. 

 When the nerve after division commences to degenerate, the constrictor 

 fibres lose their irritability earlier than the dilator fibres, so that at a certain 

 stage a stimulus, such as the interrupted current, while it fails to affect the 

 constrictor fibres, readily throws into action the dilator fibres. The latter, 

 indeed, in contrast to ordinary motor nerves ( 81), retain their irritability 

 after section of the nerve for very many days. The result is, perhaps, even 

 still more striking if a mechanical stimulus, such as that of " crimping " the 

 nerve by repeated snips with the scissors, be employed. Exposure to a low 

 temperature again seems to depress the constrictors more than the dilators ; 

 hence, when the leg is placed in ice-cold water stimulation of the sciatic, even 

 when the nerve has been but recently divided, throws the dilator only into 

 action and produces flushing of the skin with blood. Rhythmical stimula- 

 tion, moreover, of even a freshly divided nerve produces dilatation. 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. We may point 

 out that the case of the vagus of the frog is a very analogous one ; in it are 

 both cardiac inhibitory (true vagus) and cardiac augmentor (sympathetic) 

 fibres, but the former, like the vaso-constrictor fibres in the sciatic, are 

 predominant, and special means are required to show the presence of the 

 latter. 



In the splanchnic nerve (abdominal splanchnic), which supplies fibres to 

 the bloodvessels of so large a part of the abdominal viscera, there is abun- 

 dant evidence of the presence of vaso-constrictor fibres, but the presence of 

 vaso-dilator fibres has not clearly been shown. Division of this nerve leads 

 to a widening of the bloodvessels 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 vasomotor functions produces very 

 marked results, not only on the circulation in the abdomen, but on the whole 

 vascular system. 



In nerves going to muscles vaso-dilator fibres predominate ; indeed, in 

 these the presence of any vaso-constrictor fibres at all has not at present been 

 satisfactorily established. 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 bloodvessels, but is not wholly, and prob- 

 ably not even largely, thus produced. A notable feature of vasomotor 

 fibres is that, in very many cases at all events, their action is not affected by 

 small or moderate doses of urari such as render the motor nerves of striated 

 muscle powerless. Thus, in a frog placed under the influence of a moderate 

 amount of urari, stimulation of a nerve going to a muscle will produce 

 vasomotor effects unaccompanied and unobscured by any contraction of the 

 striated fibres. By placing a thin muscle of a frog, such as the mylo-hyoid, 



