THE CIRCULATION. 



331 



Irritation of the nerves of testes and middle ear usually lower 

 blood-pressure. As a rule, irritation of a sensory nerve of the skin is 

 followed by a contraction of the blood-vessels, and especially those 

 supplied by the splanchnics. The rise which is created depends upon 

 the strength and nature of the stimulus. During this condition there is 



Fig. 110. Fick's Plethysmograph. (From Tigerstedt's "Human 

 Physiology," copyright 1906, by D. Appleton and Company.) 



ae, Glass cylinder, m, rubber band to close the glass cylinder, r, s, Register- 

 ing manometer. 



vasoconstriction of the splanchnic vessels, while at the same time the 

 blood-vessels of the skin and muscles are more or less dilated. 



The nervous depressor by irritation of its central end lowers 

 blood-pressure. The condition of depression is due to a vasodilation 

 of the arterioles, especially of the vessels supplied by the splanchnics. 



L Hence, in the reflex relation of the afferent nerves to the vaso- 



