314 VASO-MOTOR NERVES OF THE LIMBS. [BOOK i. 



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

 sympathetic are distributed can any vaso-dilator action be ob- 

 served as the result of stimulation of the nerve at any part 

 of its course. In the chorda tympani, on the other hand, 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. Stimulation of the chorda tympani, (as far as the vaso- 

 motor 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 shews 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 temperature of the foot 

 falls. And very similar results are obtained in the forelimb by 

 division and subsequent stimulation of the nerves of the brachial 

 plexus. 



The quantity of blood present in the blood vessels 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 temperature due to increased metabolism, inde- 

 pendent of variations in blood supply. 



The quantity of blood may also be determined by the plethysmo- 

 graph. 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 introduced 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 



