VASO-MOTOR REFLEXES 89 



of the body. The blood is therefore held up in the arteries, 

 and if the output of the heart is unaltered a rise in the 

 arterial blood-pressure must result. This is called a pressor 

 effect. On the other hand, a pre-existing normal degree 

 of tonus may be reduced. Blood rushes out more quickly 

 from the arteries, and the heart continuing unaltered, the 

 blood-pressure must fall. This is known as a depressor 

 effect. 



It is to be expected that since the majority of arterioles 

 receive a double nerve supply, constrictor and inhibitor, a 

 pressor effect will involve both an excitation of the vaso- 

 constrictors and an inhibition of the vaso-dilators, and 

 similarly that a depressor effect will involve both an 

 excitation of the dilators and inhibition of the vaso-con- 

 strictors. In other words, when the medulla is stimulated 

 the changes in the cahbre of the arterioles are produced 

 by reciprocal innervation, in exactly the same way as 

 movement at a joint. There is evidence that this is the 

 case, but under normal conditions the vaso-constrictors 

 are much more active than their antagonists, the latter 

 appearing to play a minor role. Vaso-constriction may 

 therefore be said to be produced principally by stimulation of 

 the centre, vaso-dilatation by inhibition of the same centre. 



The factors influencing the vaso-motor centre are of 

 two kinds — nervous and chemical. Of the former it may 

 be said that stimulation of any sensory nerve causes 

 universal vaso-constriction. If a posterior root is stimu- 

 lated an efferent antidromic impulse causes vaso-dilatation 

 in the part supphed by the nerve, and an afferent impulse 

 causes reflex vaso-constriction throughout the rest of the 

 body, with consequent rise of arterial blood-pressure. 

 In these two ways the part innervated by the nerve 

 receives an increased blood-supply at the expense of the 

 remainder of the body. 



Among the nervous influences affecting the centre are 

 the psychical. As is well known, vaso-constriction is one 

 of the physiological expressions of emotional states. 



