VASOMOTOR ACTIONS. 217 



stricter fibres ; if we put aside as exceptional and doubtful the result of 

 certain observers who obtained vaso-dilator effects in the mouth and face, 

 we may say that in no region to which the fibres of the cervical sympathetic 

 are distributed can any vaso-dilator action be observed as the result of 

 stimulation of the nerve at any part of its course. In the chorda tympani, 

 on the other hand, the vasomotbr 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. 

 Stimulation of the chorda tympani (as far as the vasomotor functions of 

 the nerve are concerned, for it has, as we shall see, other functions) at any 

 part of its course, from its leaving the facial nerve to its endings in the 

 tongue or gland, produces only vaso-dilator effects, never vaso- constrictor 

 effects. 



With many other nerves of the body the case is different. In the frog, 

 division of the sciatic nerve 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, especially when these are not pigmented, the vessels are 

 seen to be dilated and injected, and a thermometer placed between the 

 toes shows a rise of temperature amounting, it may be, to several degrees. 

 If, moreover, the peripheral end of the divided nerve be stimulated, the 

 vessels of the skin become constricted, the skin grows pale, and the temper- 

 ature of the foot falls. And very similar results are obtained in the fore- 

 limb by division and subsequent stimulation of the nerves of the brachial 

 plexus. 



The quantity of blood present in the bloodvessels of the mammal, though it 

 may sometimes be observed directly, has frequently to be determined indirectly. 

 The temperature of passive structures subject to cooling influences, such as the 

 skin, is largely dependent on the supply of blood ; the more abundant the supply 

 the warmer the part. Hence, in these parts variations in the quantity of blood 

 may be inferred from variations of temperature ; but in dealing with more active 

 structures there are obviously sources of error in the possibility of the treatment 

 adopted, such as the stimulation of a nerve giving rise to an increase of temper- 

 ature due to increased metabolism, independent of variations in blood-supply. 



The quantity of blood may also be determined by the pkthy smog raph. In this 

 instrument a part of the body, such as the arm, is introduced into a closed chamber 

 filled with fluid, ex. gr., a large glass tube, the opening by which the arm is intro- 

 duced being secured with a stout caoutchouc membrane. An increase or decrease 

 of blood sent into the arm will lead to an increase or decrease of the volume of the 

 arm, and this will make itself felt by an increase or diminution of 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 some- 

 times 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 irrita- 



