CHAP, iv.] THE VASCULAR MECHANISM. 269 



nerve caused contraction of the muscular fibres, constriction of 

 the small arteries ; here stimulation of the nerve causes a widen- 

 ing of the arteries, which widening is undoubtedly due to relaxa- 

 tion of the muscular fibres. Hence we must distinguish between 

 two kinds of vaso-motor fibres, fibres the stimulation of which 

 produces constriction, vaso-constrictor fibres, and fibres the stimu- 

 lation of which causes the arteries to dilate, vaso-dilator fibres, 

 the one kind being the antagonist of the other. 



146. In the chorda tympani, the vaso-motor fibres are 

 exclusively vaso-dilator fibres, and this is true both of the part 

 of the nerve ending in the submaxillary and sublingual glands, 

 and the rest of the ending of the nerve in the tongue. Stimula- 

 tion of the chorda tympani (so far as the vaso-motor functions of 

 the nerve are concerned, for it has, as we shall see, other func- 

 tions), at any part of its course from its leaving the facial nerve 

 to its endings in the gland or tongue, produces only vaso-dilator 

 effects, never vaso-constrictor effects. The cervical sympathetic 

 on the other hand is not exclusively vaso-constrictor. It con- 

 tains as we have seen vaso-constrictor fibres for the ear. It also 

 contains vaso-constrictor fibres for other regions of the head and 

 face. For instance the branches of the cervical sympathetic 

 going to the submaxillary gland of which we just spoke (Fig. 73 

 n. sym. sm.), contain vaso-constrictor fibres for the vessels of the 

 gland ; stimulation of these fibres produces, on the vessels of the 

 gland, an effect exactly the opposite of that produced by stimula- 

 tion of the chorda tympani; to this point we shall have to return 

 when we deal with the gland in connection with digestion. And 

 we might give other instances ; in fact the dominant effect on 

 the blood vessels of stimulating the cervical sympathetic is a 

 vaso-constrictor effect. There are however certain cases in which 

 the opposite effect, a vaso-dilator effect, in certain regions has 

 been observed as the result of stimulating the cervical sympa- 

 thetic. And w r e may now turn to other nerves in which such a 

 double effect, now a vaso-constrictor, now a vaso-dilator effect, 

 may be more readily obtained. 



In the frog as we have seen, division of the nerves of the leg 

 leads to a widening of the arteries of the web of the foot of the 

 same side, and stimulation of the peripheral end of the nerve 

 causes a constriction of the vessels, which, if the stimulation ^ be 

 strong, may be so great that the web appears for the time being 

 to be devoid of blood. Also in a mammal division of the sciatic 

 nerve causes a similar widening of the small arteries of the skin 

 of the leg. Where the condition of the circulation can be readily 

 examined, as for instance in the hairless balls of the toes, espe- 

 cially when these are not pigmented, the vessels are seen to be 

 dilated and injected ; and a thermometer placed between the toes 

 shews a rise of temperature amounting, it may be, to several 

 degrees. If moreover the peripheral end of the divided nerve be 



