CHAP, iv.] THE VASCULAR MECHANISM. 333 



So far the results are quite like those obtained by division and 

 stimulation of the cervical sympathetic, and we might infer that 

 the sciatic nerve and brachial plexus contain vaso-constrictor fibres 

 only for the vessels of the skin of the hind limb and fore limb, 

 vaso-dilator fibres being absent. But sometimes a different result 

 is obtained ; on stimulating the divided sciatic nerve the vessels 

 of the foot are not constricted but dilated, perhaps widely dilated. 

 And this vaso-dilator action is almost sure to be manifested when 

 the nerve is divided, and the peripheral stump stimulated some 

 time, two to four days, after division, by which time commencing 

 degeneration has begun to modify the irritability of the nerve. 

 For example, if the sciatic be divided, and some days after- 

 wards, 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 result as due to 

 variations in the relative irritability of the two sets of fibres. The 

 constrictor fibres appear to predominate in these nerves, and hence 

 constriction is the more common result of stimulation ; the con- 

 strictor fibres also appear to be more readily affected by a tetanizing 

 current than do the dilator fibres. When the nerve after division 

 commences to degenerate the constrictor fibres lose their irrita- 

 bility 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, appear to retain their irritability after section 

 of the nerve for a much longer time than do ordinary motor nerves 

 ( 83). 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. Slow rhythmical stimulation moreover of even a 

 freshly divided nerve may produce dilation. 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 shew the presence of the latter. 



In the splanchnic nerves which supply fibres to the blood 



