A MANUAL OF PHYSIOLOGY 



the function of the nerve is to act as an automatic check upon 

 the blood-pressure in the interest both of the heart and the vessels, 

 its terminations in the aorta or the ventricular wall being 

 mechanically stimulated when the pressure tends to rise towards 

 the danger limit. In rare cases, efferent inhibitory fibres for the 

 heart have been found in the depressor of the rabbit. 



Many of the peripheral nerves contain fibres whose stimula- 

 tion is followed by dilatation of the bloodvessels in special 

 regions, usually the areas to which they are themselves dis- 

 tributed, accompanied by constriction of distant and, it may 



FIG. 69. DIAGRAM OF DE- 

 PRESSOR NERVE IN RAB- 

 BIT. 



X, vagus ; SL, superior 

 laryngeal branch of vagus ; 

 D, depressor fibres. The 

 arrows show the course of 

 the impulses that affect the 

 blood -pressure. 



FIG. 70. BLOOD-PRESSURE TRACING : RABBIT 

 (MERCURY MANOMETER), g 



Central end of depressor stimulated at i ; 

 stimulation stopped at 2. Time-trace, seconds. 



be, more extensive vascular tracts. Thus, the usual local effect 

 of stimulating the afferent fibres of the lowest three thoracic 

 nerves, in whose anterior roots run the vaso-motor fibres for 

 the kidney, is a dilatation of the renal vessels (Bradford), and 

 the usual local effect of stimulating the infra-orbital or supra- 

 orbital nerve a dilatation of the external maxillary artery. 

 But the general effect in both cases is vaso-constriction in other 

 regions of the body, which more than compensates the local dila- 

 tation, so that the arterial blood-pressure rises. It is not difficult 

 to see that both of these changes render it easier for the part to 

 obtain an increased supply of blood. 



Sometimes the reciprocal relation between vaso-dilatation in one 

 part of the body and vaso-constriction in another is only apparent. 

 For example, stimulation of the cut end of the sciatic causes, as we 

 have already seen, extensive vaso-constriction and a notable rise in 

 the blood-pressure. The constriction certainly involves the splanch- 



