CHAP, iv.] THE VASCULAR MECHANISM. 315 



pressure in the fluid of the closed chamber, which may be registered 

 and measured in the usual way. We shall have to speak again of a 

 modification of this instrument when we are dealing with the kidney. 



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 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 

 days after division, by which time commencing degeneration has 

 begun to interfere with the irritability of the nerve. For example 

 if the sciatic be divided, and some days afterwards, 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 con-/ 

 elude 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 i 

 the relative irritability of the two sets of fibres. The constrictor 

 fibres appear to predominate in these nerves, and hence constric- 

 tion 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 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, in contrast to ordinary motor nerves, ( 83), 

 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 stimulation moreover of even a freshly divided 

 nerve produces 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 augmeiitor (sym- 



