It is true that we do not yet understand precisely in what the phenomenon of sleep consists; we do not know fully what change in the operation of the brain occurs for its induction. It may be from deviated or lessened currents of blood in certain portions, or from the opposite condition. Both these theories have been advocated by men more or less eminent as physiologists; some maintaining that while in sleep the brain contains a larger amount of blood, that there exists a diminished action of the vaso-motor nerves which control the coats of the vessels of the brain, and that in consequence they become more fully distended than when the brain is in a conscious state of activity. On the other hand, others become equally positive, from observations made on portions of the brain which have become exposed through the effects of injuries to the skull, that these vessels contain less blood during sleep than when in other conditions. I think these observations are conclusive, and that there can be no doubt as to the fact that there exists a diminished quantity of blood in the vessels of the brain in sleep; but that this is the cause of the occurrence of that condition of the brain which constitutes sleep does not appear to be so certain. It is quite possible that this diminished quantity of blood is rather a _consequence_ than a _cause_. I am more inclined to think that sleep is primarily caused by a diminution, or cessation, of some of the electrical currents which constantly are passing through portions of the brain while in a state of consciousness, and which are probably necessary to a condition of consciousness, and that the anæmic condition of the brain which is observed during sleep is a result of such change in these currents. But from whatever primary cause it may occur, we know that it is only when there are frequent periods for this condition of the brain in the case of the child, that its nervous system develops and becomes strong in the largest measure. And on the other hand, when, for any cause, whether it be pain or artificial excitement, sleep is prevented, the whole system speedily becomes deranged, and manifests its sense of indignation by irregular or imperfect development and suffering. The necessity of sleep for the system might be illustrated by the presentation of many remarkable and curious facts, such as those of persons who are greatly exhausted sleeping during surgical operations; of physicians sleeping while walking to or from visits to their patients, or while sitting beside them when in conditions of great suffering. I have myself, when greatly fatigued from excessive professional labor, slept through a considerable portion of a disagreeable and somewhat painful dental operation. The torture resulting from the deprivation of sleep for long periods is said to be greater than that of hunger or thirst, or from the infliction of the severest bodily injury.