THE CIRCULATION OF THE BLOOD AND LYMPH 161 



interest of the heart, its terminations in the ventricular wall 

 being mechanically stimulated when the pressure tends to 

 rise towards the danger limit. In rare cases, efferent inhi- 

 bitory fibres for the heart have been found in the depressor 

 of the rabbit. 



Many of the peripheral nerves contain fibres whose 

 stimulation is followed by dilatation of the bloodvessels in 

 special regions, usually the areas to which they are them- 

 selves distributed, accompanied by constriction of distant 

 and, it may be, more extensive vascular tracts. Thus, the 

 usual local effect of stimulating the afferent fibres of the 





FIG. 54. DIAGRAM OF DE- 

 PRESSOR NERVE IN RABBIT. 



X, vagus ; SL, superior laryn- FlQ SS ._BLOOD-PRESSURE TRACING (RABBIT), 



geal branch of vagus ; D de- (MERCURY MANOMETER.) 

 pressor fibres. The arrows show 



the course of the impulses that Central end of depressor stimulated at i ; stimula- 



affect the blood-pressure. tion stopped at 2. Time trace seconds. 



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 dilatation, 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. 



II 



