Page:The Harveian Oration,1902.djvu/37

 medulla itself. For, as Franck has shown, when the brain is isolated from the rest of the body, with the exception of one vagus, injection of blood into the carotid causes the heart to beat faster or slower according to the degree of pressure exerted. And it has been argued by Roy and Adami that in this we have a protective against cerebral congestion, inasmuch as the slowing of the heart from increased intracranial tension is brought about by action on the medullary centres and not from within the heart itself. The protective mechanism of cardiac acceleration is exemplified in the increased rapidity of the heart, which is excited by conditions which tend to lower the pressure in the bulbar centres—such as change from the horizontal to the vertical posture, and under all conditions which lead to dilatation of any large vascular area.

All organs in a state of activity are able, either by local action on the blood-vessels or by reflex vaso-dilatation, to secure an increased flow of blood. If the vascular area thus dilated is at all extensive the lowering of blood pres-