274 HUMAN PHYSIOLOGY 



is always to be borne in mind that efferent nerves carry 

 out from the brain and cord impulses which owe their 

 origin to the previous arrival of other impulses over 

 sensory paths. There is therefore an afferent as well 

 as an efferent side to this mechanism. 



A series of light blows rapidly delivered upon the 

 abdomen of a frog may slow or even stop the heart. 

 This is a clear case of reflex inhibition. Impulses run 

 to the medulla and cause the return of others which 

 find their way to the heart along the vagus fibers. In 

 like manner, afferent impulses may reflexly affect the 

 tone of the blood-vessels, either slackening or tightening 

 the arteries. A reflex change in which arterial tone is 

 lowered is called a depressor reaction; the contrasted 

 reflex, with heightening of tone, is a pressor response. 

 The adjustment made to support the fixation of the 

 attention and demonstrated with the plethysmograph 

 is, accordingly, a pressor reflex. 



The average aortic pressure is kept sufficient to insure 

 an adequate blood-supply wherever a channel may be 

 opened by vasodilation. It is important that this 

 pressure shall not greatly fall, and it is in the interests 

 of economy and safety that it shall not greatly rise. 

 An elaborate reflex mechanism exists to keep it within 

 reasonable limits. If the pressure tends to mount up, 

 of course one of the direct results will be a stretching 

 of the aorta. But there are receptors in the wall of 

 this great vessel which are stimulated when the high 

 tension is established. Impulses are set up in nerve 

 fibers which lead to the medulla. Reflex responses follow 

 which abate the rising pressure. 



The reflexes just referred to are of two kinds. Some 

 degree of inhibition is imposed upon the heart and there 

 is simultaneously a lowering of tone in the vessels of 

 the digestive tract. Thus the high arterial tension is 

 relieved both by easing the heart, and so reducing its 

 output, and by providing for the freer escape of the 



