CH. XXI.] PRESSOR AND DEPRESSOR NERVES 305 



impulses to the vaso-motor centre may be divided into pressor and 

 depressor. 



Most sensory nerves are pressor nerves. The sciatic or the vagus 

 nerves may be taken as instances ; when they are divided and their 

 central ends stimulated, the result is a rise of blood-pressure due to 

 the stimulation of the vaso-motor centre, and a consequent constric- 

 tion of the arterioles all over the body, but especially in the 

 splanchnic area. Fig. 299 shows the result of such an experiment. 

 It is necessary in performing such an experiment to administer curare 

 as well as an anaesthetic to the animal, in order to obviate reflex 

 muscular struggles, which would themselves produce a rise in arterial 

 pressure. 



Many sensory nerves also contain depressor fibres which produce 

 the opposite effect. The most marked bundle of these is known as 

 the depressor nerve. In most animals this is bound up in the trunk 

 of the vagus ; but in some, like the rabbit, cat, and horse, the nerve 

 runs up as a separate branch from the heart (or, according to some 

 recent observations, from the commencement of the aorta), and joins 

 the vagus or its superior laryngeal branch, and ultimately reaches the 

 vaso-motor centre. When this nerve is stimulated (the vagi having 

 been previously divided to prevent reflex inhibition of the heart), a 

 marked fall of arterial blood-pressure is produced (see fig. 300). 

 Stimulation of this nerve affects the vaso-motor centre in such a way 

 that the normal constrictor impulses that pass down the vaso-con- 

 strictor nerves are inhibited. The fall of pressure is very slight after 

 section of the splanchnic nerves, showing that the splanchnic area is 

 the part of the body most affected. The normal function of this 

 nerve is to adapt the peripheral resistance to the heart's action : if 

 the constriction of the arterioles is too high for the heart to overcome, 

 an impulse by this nerve to the vaso-motor centre produces reflexly 

 a lessening of the peripheral resistance. 



N.B. The term depressor should be carefully distinguished from inhibitory; 

 stimulation of the peripheral end of the vagus produces a fall of blood-pressure due 

 to inhibition (slowing or stoppage) of the heart (see figs. 277 and 278) ; stimulation 

 of the central end of the 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 



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