In the following pages will be found the substance of three Lectures which I delivered as Hunterian Professor at the Royal College of Surgeons in February last.<br><br>I have largely been enabled to carry out the researches of the past five years therein detailed, through the munificence of the Worshipful Company of Grocers. This volume contains a report of the work done by me as their Research Scholar. In writinof some of the sections of this work I have drawn on papers previously published by me in the Proceedings of the Royal Society, the Journal of Physiology, and the British Medical Journal; other sections, such as those on the Cerebral Pulsation, the Cerebro-spinal Fluid, Cerebral Anaemia, and Cerebral Compression are new, and contain an account of experiments hitherto unpublished.<br><br>I have been partly supplied with the means of research by grants from the Royal Society Government Grant, and the British Medical Association; and I owe a debt of gratitude to Professor Burdon Sanderson, Professor Schäfer and the Authorities of the London Hospital Medical College, whose laboratories and laboratory appliances have been placed at my disposal.