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Physiological Theories of Sleep

Sleep is due to changes in the cerebral cirilation. A lack of blood in the brain causes what is known as cerebral anæmia. This cerebral anaemia may be due to a dilatation of the blood-vessels of the skin, which causes a fall of blood pressure in the brain. Sleep naturally results, in the same way that a lack of blood in the brain causes that transitory loss of consciousness known as fainting. According to these circulatory theories of sleep, cerebral congestion, or an increased amount of blood in the brain, produces insomnia. Observations on exposed brains after the skull has been trephined for injuries, seem to bear out these circulatory theories on sleep. Mosso’s famous observations in cases of this type, showed a cerebral anaemia during sleep. Yet strong pressure on the carotid arteries in the neck, for a short time, thus interfering with the passage of blood to the brain, causes a state of consciousness analogous to fainting, rather than genuine sleep. Tarchanoff showed that in puppies the brain becomes pale when the animals are asleep and that at the same time, it reacts less readily to electrical stimulation. Salmon has recently formulated an ingenious though unsatisfactory theory of sleep, based upon the functions of the pituitary body, a secretory gland which lies at the base of the brain. He points out the very marked relation between somnolence and pituitary tumors and therefore claims that sleep is due to a hypersecretion of the pituitary body and insomnia to a diminished secretion. There exists an analogy between the winter sleep of animals and our daily sleep, as this winter sleep is also due to the diminished secretory activity of the tissues.

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