1044 PHYSIOLOGY 



afferent nerves may be classified, according as they affect the general blood 

 pressure or the circulation through restricted areas of the body, as general 

 and local. 



The afferent impulses affecting' the general blood pressure are distin- 

 guished as pressor and depressor, and these names are sometimes applied 

 to the nerves which carry the impulses. A pressor reflex is one which 

 induces a rise of general blood pressure by constriction of the blood vessels, 

 especially in the splanchnic area (Fig. 477). Effects of this kind are pro- 

 duced by stimulation of nearly 

 all the sensory nerves of the 

 P. skin. Practically all impulses, 

 which if consciousness were 

 present would be attended with 

 pain, cause also a rise of general 

 blood pressure. A rise of pres- 

 sure may be produced by the 

 . stimulation of such nerves as the 

 | Spleen fifth, the central end of the 

 splanchnic nerves, or of the 

 nerves distributed to the surface 

 of the body. This rise occurs 



FIG. 478. Simultaneous tracing of arterial in all animals under morphia 

 blood pressure and splenic volume from -i plirarp T n f i, p Tfl Khit whpn 



a rabbit, showing the marked swelling of and curare. Jn tne rabbit, wn< 



the spleen associated with fall of general anaesthesia is induced by means 



blood pressure on stimulation of the cen- t , , , ^-i^^t^ 



tral end of the depressor nerve. The nerve of cmoral or chloroform, sfcmu- 



was excited between a and 6. (BAYLISS.) lation 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 produced by 

 stimulation 1 of this nerve is effected chiefly by dilatation of the splanchnic 

 area (Fig. 478), 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 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. 



