Page:The Harveian Oration,1902.djvu/47

39 the state of rest the neuromuscular apparatus contains on the average only about 36 per cent, of the blood, while in the state of activity the percentage is nearly doubled. The rapidity of the circulation in the brain in a state of activity is greater than in the state of quiescence or sleep. This is brought about by the constriction of the splanchnic and cutaneous blood-vessels through the agency of the vaso-motor centre. In illustration of this an ingenious apparatus has been devised by Mosso. When a person lies flat on a couch swinging on a horizontal axis, with the head and feet accurately counterpoised, the head goes down when the brain is at work; on the other hand, the feet sink when the individual falls asleep, indicating that the blood has returned to the extremities. In tranquil sleep a sound or any kind of sensory stimulus suffices to reverse the position of the head and feet, and that, too, when the stimulus has not been sufficient to cause the sleeper to awake or to retain any knowledge of the event. Every sensory stimulus of moderate intensity has the effect of raising the blood pressure by constriction of the cutaneous and splanchnic areas. We may