Page:The Harveian Oration,1902.djvu/43

 selves. In either case the circulation in the brain fails, and loss of consciousness ensues. The brain tissue, however, as such can carry on its functions at any pressure from zero up to 50 millimetres of Hg. or more. Thus, in a patient who had been trephined, and the brain, therefore, exposed to atmospheric pressure, Hill found that the intracranial pressure fell slightly below zero when he stood upright; and in a case of strychnine convulsions the pressure was estimated at 50 millimetres Hg., and yet both patients retained their mental faculties unimpaired. Up to the present no satisfactory experimental evidence has been furnished as to the existence of vaso-constrictors or vaso-dilators of the cerebral blood-vessels. Morison, however, and Gulland have both, within the last few years, demonstrated the presence of nerve plexuses on the vessels of the pia mater in all respects like those of vaso-motor nerves elsewhere. Yet no active change in the calibre of the cerebral blood-vessels can be produced by stimulation of the cervical sympathetic or stellate ganglion