CH. XXII.] EXPERIMENTS ON VASO-MOTOR NERVES 307 



depressor nerve produces a lowering of blood-pressure for a different 

 reason, namely, a reflex relaxation of the splanchnic arterioles. 



Experiments on Vaso-motor nerves. The experiments on the 

 vaso-motor nerves are similar to those performed on other nerves 

 when one wishes to ascertain their functions. They consist of 

 section and excitation. 



Section of a vaso-constrictor nerve, such as the splanchnic, causes 

 a loss of normal arterial tone, and consequently the part supplied by 

 the nerve becomes flushed with blood. Stimulation of the peripheral 

 end causes the vessels to contract and the part to become compara- 

 tively pale and bloodless. This can be very readily demonstrated 

 on the ear of an anaesthetised rabbit. This is a classical experiment 

 associated with the name of Claude Bernard. Division of the cervical 

 sympathetic produces an increased redness of the side of the head, 

 and looking at the ear, the transparency of which enables one to follow 

 the phenomena easily, the central artery with its branches is seen to 

 become larger, and many small branches not previously visible come 

 into view. The ear feels hotter, though this effect soon passes off as 

 the exposure of a large quantity of blood to the air causes a rapid 

 loss of heat. On stimulating the peripheral end of the cut nerve, 

 the ear resumes its normal condition, and then becomes paler than 

 usual owing to excessive constriction of the vessels. 



Section of a vaso-dilator nerve, such as the chorda tympani, pro- 

 duces no effect on the vessels, but stimulation of its peripheral end 

 causes great enlargement of all the arterioles, so that the submaxillary 

 gland and the neighbouring parts supplied by the nerve become red 

 and gorged with blood, and the pulse is propagated through to the 

 veins ; the circulation through the capillaries may be so rapid that 

 the blood is arterial in colour in the veins. Another effect, free 

 secretion of saliva, we shall study in connection with that subject. 



Other examples of vaso-dilator nerves are the nervi erigentes to 

 the erectile tissue of the penis, etc., and of the lingual nerve to the 

 vessels of the tongue. 



It is, however, probable that all the vessels of the body receive 

 both constrictor and dilator nerves. But the presence of the latter 

 is difficult to determine unless they are present in excess ; if they 

 are not, stimulation affects the constrictors most. The effect of 

 section is also inconclusive ; for if a mixed nerve is cut, the only effect 

 observed is a dilatation due to removal of the tonic constrictor influence. 



To solve this difficult problem, three methods are in use : 



1. The method of degeneration. If the sciatic nerve is cut, the 

 vessels of the limb dilate. This passes off in a day or two. If the 

 peripheral end of the nerve is then stimulated, the vessels are dilated, 

 as the constrictor fibres degenerate earliest, and so one gets a result 

 due to the stimulation of the still intact dilator fibres, 



