Page:The Harveian Oration,1902.djvu/36

28 tions in the differential pressure between the arteries and veins are obviated or compensated; and that each organ obtains its due measure of blood in accordance with the calls made upon it. We cannot doubt that the cardiac and vascular reflexes are essentially of a protective nature, like reflex actions in general. But though the teleological significance of many of these is obvious, the usefulness of others is still involved in obscurity. We can readily understand why, as Marey has beautifully demonstrated, the heart should be inhibited when the resistance in front is too great for it to contend with safely; and why, under similar conditions, impulses should be conveyed by the depressor nerve, which diminish the resistance by dilating the great vascular area of the abdomen. In this we have an instance of the protective action of the vascular and cardiac reflexes on the heart itself. On the other hand, the vagus may be called into play with the view of protecting other organs, especially the brain, from undue pressure and congestion. Great rise of intracranial pressure causes slowing of the heart. This is brought about by direct action on the