178 SECTIONAL ADDRESSES 



It has been generally assumed that the primary function of this control 

 of the vessels is to* maintain the arterial pressure at a constant level, and 

 this is quite reasonable, for a constant mean pressure is desirable to main- 

 tain a steady flow of fluid at rest from the capillaries to the tissues. It 

 can be shown that if these reflexes are put temporarily out of action, 

 considerable variation in pressure is liable to occur because of the 

 spontaneous contraction of certain vascular regions such as the spleen. 



More recently it has become evident that these reflexes may have another 

 and possibly more important function. Several facts led to this suggestion : 

 (i) that in physical exercise or mental stress, the blood pressure, like the 

 heart rate, does rise in spite of the reflexes ; (2) that the response of blood 

 pressure to posture may be normal if the reflexes are destroyed — a fact 

 which shows that the maintenance of mean pressure is not wholly 

 dependent on the reflexes ; (3) that as in the case of the vagus, different 

 animals, or even the same animals in slightly different circumstances, 

 show great variability in the activity of the reflexes — often it is possible 

 to throw the reflexes out of action without affecting the blood pressure 

 materially ; and (4) as might almost be anticipated, the conditions 

 which reduce the activity of the vagus also reduce the activity of the 

 depressor reflexes. Two procedures which produce most striking results 

 are the raising of venous pressure by the rapid injection of fluid and the 

 injection of adrenaline. It may be remembered that Bainbridge showed 

 that the rapid injection of fluid into the veins causes reflex cardiac accelera- 

 tion, partly by reducing the action of the vagus. This was the experiment 

 which led to the discovery of the right auricular reflex usually associated 

 with his name. 



Now since in exercise the venous pressure is increased and, if the stress 

 of the occasion is sufficient, adrenaline is secreted, we may consider what 

 happens to the circulation when the vasodepressor reflexes are thrown 

 out of action. As I have said, there is a rise of arterial pressure and a 

 generalised constriction of the vessels. It has become usual to consider 

 that this rise of arterial pressure is the result of an increased peripheral 

 resistance to the flow of blood from the arteries. Were this wholly true 

 we should expect to find that there is a reduced flow of blood to the veins. 

 If the animal, however, is in good condition, the reverse is the case : there 

 is an increased flow to the veins. It is as if there were at the periphery a 

 sponge-like reservoir which, when it is contracted, drives its store of blood 

 into the veins. In this connection it is interesting to remark that Bayliss 

 when investigating the aortic depressor nerve found that stimulation 

 caused not only a fall of arterial pressure, but a fall also of the venous 

 pressure — that is, an increased capacity of the circulation. He did not 

 consider the reverse possibility, since the then unknown function of the 

 carotid sinus prevented his discovery of tonic dilator impulses. 



When the depressor reflexes are cut off, the reverse, however, does not 

 necessarily occur experimentally. An increased flow into the veins does 

 not necessarily result in a rise of venous pressure, because at the same 

 time the heart is stimulated and the increased pressure is rapidly dealt 

 with. It can, as might be expected, be shown at the same time that there 

 is an increased output of the heart. It is not possible to measure the 

 output of the heart by the cardiometer method without there being some 



