I.— PHYSIOLOGY 177 



This opening up of vessels previously closed necessitates the provision 

 of blood, and as there is only a limited amount of blood in the body, it 

 must be provided from other regions, otherwise the blood pressure 

 would fall and the circulation through the tissues be reduced. 



It is probable that practically all parts of the body, except possibly 

 the voluntary muscles, the heart muscle and the brain, provide the blood 

 necessary for the active muscles. It has been shown that any exercise, 

 actual or even contemplated, causes vasoconstriction. Constriction of the 

 spleen and of the intestine in animals has been observed. This was the 

 subject of a presidential address to this Section by Barcroft some years ago. 

 In man it has been shown that the vessels of the skin constrict under any 

 emotional stress or even anticipated activity. This, indeed, was one of 

 the first facts discovered by Mosso with his plethysmograph. 



In regard to the sympathetic constriction of the vessels, we are in the 

 same difficulty as we were in relation to the sympathetic acceleration of 

 the heart. We do not know how the actual nerve impulses which originate 

 the constriction arise. For convenience we say that they begin in the 

 higher centres of the brain. It is, however, probably preferable, it seems 

 to me, to consider that it is a sensory stimulation from the outside world, 

 which is the point in time which determines the psychical reaction which 

 results in motor movement. Certainly we know that stimulation of a 

 sensory nerve causes generalised vasoconstriction and commonly a rise of 

 blood pressure, and that similar changes but of lesser degree may be 

 recorded in a sleeping man. 



It has been usual to ascribe the shutting down of the blood vessels 

 solely to sympathetic activity, just as it was usual to ascribe cardiac 

 acceleration solely to such action. In the case of the heart we have clear 

 evidence that the reduction of the vagus restraint is just as important by 

 increasing the range of cardiac activity and creating a cardiac reserve. It 

 has now become evident that there probably exists an exactly parallel 

 mechanism which increases the range of vascular activity and similarly 

 enhances the reserve. 



The Maintenance of the Vascular Reserve. 



Just as we have the restraint of the heart by the vagus, which determines 

 the range of cardiac acceleration, so we have in relation to the blood vessels 

 a set of reflexes which determines the magnitude of the vasoconstriction of 

 the blood vessels. That is, they maintain the vessels of the body generally 

 in an actively dilated state. The afferent impulses which are concerned 

 in these reflexes have an exactly similar origin to those responsible for the 

 vagus restraint of the heart. They arise from the cardio-aortic region and 

 the carotid sinuses, and pass up the medulla by the aortic and carotid 

 depressor nerves. The evidence for this statement is essentially that if 

 the afferent impulses from these regions are cut off, there results a con- 

 striction of practically all the blood vessels in the body. It may be 

 remembered that for many years the existence of such tonic dilator control 

 of the vessels was denied, but the experiments on the carotid sinus by 

 Hering, Heymans and their co-workers have placed it beyond doubt. 

 Like the cardio-depressor reflexes the vascular-depressor reflexes are 

 operated by the intravascular pressure in these regions. 



