Page:The Harveian Oration,1902.djvu/48

 see in this a protective mechanism whereby impressions on the organs of sense not only awaken perception but provide the centres and apparatus of thought and volition with the means of energising in accordance with the needs of the occasion. The sensory nerves and vaso-motor centre therefore act, in the figurative language of Mosso, as "sentinels on the defensive, watching continuously, and sounding the alarm when danger is nigh."

The circulation in the brain is largely dependent on the tone maintained by the vasomotor centre. Conditions which materially lower the vascular tone, such as prostrating diseases, certain toxic agents, and the like, tend to induce cerebral anaemia and impairment of cerebral energy, especially in the upright posture. For though under normal conditions the influence of gravity is not appreciably felt, it becomes very apparent when the vascular tone is defective. Hence in such conditions syncope readily occurs when the individual is suddenly raised from the horizontal to the vertical position, or obviated when the positions are reversed. In a less degree the