Vomiting is a frequent symptom of cerebral disturbance. The fluctuations of pressure which the brain experiences as it rocks about on its “water-bed” within the skull is the cause of sea-sickness. Yet the motion of a ship may produce violent headache without nausea, the brain only, not the stomach, appearing to be troubled by the motion. Not that headache is a pain “inside the head.” Nor is it properly described as a pain in the scalp, although the messages which are felt in consciousness as headache originate in the endings of the nerves of the skin which covers the skull. The excessive sensitiveness of these nerves is due to vaso-motor conditions, usually the dilation, occasionally the constriction, of the bloodvessels of the scalp. But the vaso-motor condition is sympathetic with the disturbance of the brain; and the special urgency or efficiency of the messages from the skin results from their being delivered into excited brain-tissue. Nausea and headache are equally symptoms of the irritability of the brain caused by the motion of the ship. In one case messages from the stomach, in the other case messages from the scalp, acquire undue importance, owing to the agitated condition of the brain-tissue through which they pass. Not uncommonly the voyager, who wakes in the morning reconciled to the changes of pressure which he has experienced while recumbent, finds, when he stands upright, that the base of his brain is as sensitive as ever. Visual sensations also contribute to the brain-disturbance. So, too, do the movements of endolymph in the semicircular canals (_cf._ p. 335). It is, indeed, possible that this last factor is more important than the variations in pressure on the surface of the brain. Probably it accounts for the after-image of rolling which almost everyone experiences for at least a day after leaving the ship. Its cause being cerebral, the tendency to sea-sickness can be controlled by drugs which, like the bromides, chloral, alcohol, etc., deaden the brain.