1 86 



THE CIRCULATION OF THE BLOOD AND LYMPH 



vessels of the submaxillary gland even on the opposite side, whether 

 the sympathetic has been divided or not, so long as the chorda 

 tympani is intact, and this dilatation is not accompanied by a flow 

 of saliva. Stimulation of 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 depres- 

 sor of the rabbit. 



Many of the peripheral 

 ' nerves contain fibres 

 whose stimulation is fol- 

 lowed by dilatation of the 



Fig. 81. Diagram of Depressor 

 in Rabbit. X, vagus ; SL, 

 superior laryngeal ; D, de- 

 pressor. Arrows show course 

 of impulses. 



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

 end of depressor stimulated at i ; stimulation 

 stopped at 2. Time- trace, seconds. 



bloodvessels in special regions, usually the areas to which they are 

 themselves 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 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. 



Sometirres 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 



