With reference to the brain itself, I account to myself for the necessity of sleep more fully through an hypothesis which appears to have been first set up in Neumann's book, "Von den Krankheiten des Menschen," 1834, vol. 4, § 216. It is this, that the nutrition of the brain, thus the renewal of its substance from the blood, cannot go on while we are awake, because the very eminent organic function of knowing and thinking would be disturbed or put an end to by the low and material function of nutrition. This explains the fact that sleep is not a purely negative condition, a, mere pausing of the activity of the brain, but also shows a positive character. This makes itself known through the circumstance that between sleep and waking there is no mere difference of degree, but a fixed boundary, which, as soon as sleep intervenes, declares itself in dreams which are completely different from our immediately preceding thoughts. A further proof of this is that when we have dreams which frighten us we try in vain to cry out, or to ward off attacks, or to shake off sleep; so that it is as if the connecting-link between the brain and the motor nerves, or between the cerebrum and the cerebellum (as the regulator of movements) were abolished; for the brain remains in its isolation and sleep holds us fast as with brazen claws. Finally, the positive character of sleep can be seen in the fact that a certain degree of strength is required for sleeping. Therefore too great fatigue or natural weakness prevent us from seizing it, capere somnum. This may be explained from the fact that the process of nutrition must be introduced if sleep is to ensue: the brain must, as it were, begin to feed. Moreover, the increased flow of blood into the brain during sleep is explicable from the nutritive process; and also the position of the arms laid together above the head, which is instinctively assumed because it furthers this process: also why children, so long as their brain is still growing, require a great deal of sleep, while in old age, on the other hand, when a certain atrophy of
