NERVOUS CONTROL OF THE BLOOD-VESSELS 1127 



anaesthesia is induced by means of chloral or chloroform, stimula- 

 tion of sensory nerves may cause a fall of blood pressure. 



The chief example of a depressor nerve we have already studied 

 in dealing with the reflexes from the heart. The fall of pressure pro- 

 duced by stimulation of this nerve is effected chiefly by dilatation of 

 the splanchnic area (Fig. 474), though, as Bayliss has shown, practically 

 all the vessels of the body partake in the relaxation. The lowering of 

 blood pressure produced by stimulation of this nerve differs from that 

 obtained on stimulating the sensory nerves of the rabbit under chloral, 

 in that its effect lasts as long as the stimulation is continued, whereas 



B,P, 



Splecu 



FIG. 474. Simultaneous tracing of arterial blood pressure and splenic volume 

 from a rabbit, showing the marked swelling of the spleen associated 

 with fall of general blood pressure on stimulation of the central end 

 of the depressor nerve. The'nerve was excited between a and 6. (BAYLISS.) 



in the latter case the effect shows signs of fatigue and disappears 

 before the excitation is shut off. 



So far as the general blood pressure is concerned the most 

 important impulses arriving at the centre are those from the vascular 

 system, especially from the heart itself, and those from the higher 

 parts of the brain. Whatever the condition of the heart the brain 

 always demands a normal arterial pressure, since on this depends 

 the supply of a proper quantum of blood to the master tissues of the 

 body. A failing heart therefore evokes indirectly constriction of the 

 blood-vessels, a fact which may lead to a vicious circle in cases where 

 the heart is unable to perform its normal functions and to empty itself 

 against the resistance of the blood-vessels. In this case the heart 

 dilates more and more, until the slightest increase in the demands 

 upon it, as by a slight muscular exertion, may suffice to stop its action 

 altogether. 



Under normal circumstances every part of the body receives 

 just so much blood as it needs for its metabolic requirements. Hence 



