Now it is essential to the perfect function of the brain, not only that it shall have a due supply of blood, but that this blood shall be of that quality we term _oxygenated_. If there be a simple _deficiency_ of this scarlet blood, a state of sound undisturbed sleep will ensue (slightly analogous to the condition of _syncope_, or fainting). This may be the consequence of any _indirect_ impression, or the natural indication of that _direct_ debility, which we witness in early infancy, and in the “second childishness and mere oblivion” of old age. But this deficiency of arterial blood may be depending on a more positive cause, _venous congestion_, impeding its flow; for in sleep, the breathing being slower, the blood becomes essentially darker. Even arterial blood itself will become to a certain degree _carbonized_, by _lentor_, or stagnation. Venous congestion and diminution of arterial circulation are not incompatible; indeed, Dr. Abercrombie reasons very ably on their relative nature, implying the necessity of some _remora_ of venous circulation to supply that want or vacuum which the brain would otherwise experience from the deficiency of the current in the arterial system. Thus will the languid arterial circulation of the brain, which causes sleep in the first instance, produce, secondarily, that congestion of blood in the veins and sinuses, which shall reduce it to disturbed slumber, and excite the dream. May we not account, on this principle, for the difficulty which many persons experience in falling into a _second_ slumber, when they have been disturbed in the _first_? IDA. Combe, I believe, observed, through a hole in a fractured skull, that the brain was elevated during an apparent dream. EV. This is a matter of frequent observation with us. There was, in 1821, at Montpelier, a woman who had lost part of the skull, and the brain and its membranes lay bare. When she was in deep sleep, the brain lay in the skull almost _motionless_; when she was dreaming, it became elevated; and when her dreams (proved by her relating them when awake) were on vivid or animating subjects, but especially when she was awake, the brain was _protruded through the cranial aperture_. Blumenbach states that he, himself, witnessed in one person a sinking of the brain, whenever he was asleep, and a swelling with blood when he awoke. David Hartley, therefore, may be half right and half wrong when he imputes dreams to an impediment to the flow of blood, a collapse of the _ventricles_, and a diminished quantity of their contained _serum_.