i8 4 



THE CIRCULATION OF THE BLOOD AND LYMPH 



the depressor produces its usual result after section of the vagi. It 

 has been suggested that 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 stimulation 

 is followed by dilatation of the bloodvessels in special regions, 



Fig. 81. Diagram of De- 

 pressor Nerve in Rabbit. 

 X, vagus; SL, superior 

 laryngeal branch of vagus; 

 D, depressor fibres. The 

 arrows show the course of 

 the impulses that affect 

 the blood-pressure. 



Pig. 82. Blood -Pressure Tracing: Rabbit. Central 

 end' of depressor stimulated at i ; stimulation 

 stopped at 2. Time-trace, seconds. 



usually the areas to which they are themselves distributed, accom- 

 panied by constriction of distant and, it may 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 com- 

 pensates 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. 



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 splanchnic area; but 



