330 PHYSIOLOGY. 



If an afferent nerve is cooled, or after section allowed to regenerate to 

 a certain extent, or the stimulus is weak, then when irritated, it may 

 produce a fall of blood-pressure. 



The vasomotor reflex ensues primarily in the same vascular loca- 

 tion where the afferent irritation is made. This explains the conges- 

 tion of the intestines upon opening the abdominal cavity and the 

 injection of the skin after friction or warmth. Generally the localized 

 reflexes cause a vasodilation, but it may be a vasoconstriction which 

 may spread to the opposite half of the body, or to parts at a considerable 

 distance, as the arterioles of the intestine innervated by the splanchnic 

 nerves. If one hand be plunged into ice-water, the blood-vessels of the 

 opposite hand also contract. 



Fig. 109. Elevation of Arterial Pressure by Vasoconstriction. A result 

 of irritation of the central end of sciatic in curarized dog. (HEDON.) 



P, Carotid pressure. S, Signal-magnet tracing. 



Vasomotor reflexes can take place through the vasodilators for 

 Bayliss has shown that after removing the influence of all the vasocon- 

 strictors of the posterior extremity, vasodilation ensued in the same 

 region by stimulating the vagus. 



In the chorda tympani and nervi erigentes are nerves which cause 

 a vasodilation from afferent impulses, presumably from an action on a 

 vasodilator center. 



Fear blanches the face by a psychic action on the vasomotor 

 centers, whilst blushing is a result of a psychic effect on the same 

 centers. 



As the blood-vessels are often dilated at one place and contracted 

 at another in the body from the same sensory stimulation, the blood- 

 pressure may be increased or decreased in these regions. 



