The causes of central illusions and hallucinations are generally to he found in derangements of some kind in the blood circulating in the brain. These may either relate to its quantity or its quality.
Physical influences calculated to produce cerebral hyperæmia or congestion may give rise to illusions or hallucinations. Brierre de Boismont refers to a case, on the authority of Moreau, in which an individual was able to obtain hallucinations of sight by inclining his head a little forward. A similar case was not long since under my own care. A gentleman, while sitting at his table writing, happened to raise his eyes without moving his head, and saw before him the figure of an old woman with black cloak and hood. Throwing himself back in his chair in his amazement, he found that the image slowly disappeared; and, as often as he repeated these movements, a like series of phenomena occurred. On examining him, I found that he wore a very high, old-fashioned stock, which, as he sat at the table with his head bent forward, compressed the large veins of the neck, and prevented for a time the return of blood from the brain. On changing his neck-wear for other of more modern fashion, he was enabled to bend his head and raise his eyes without encountering the apparition.
A gentleman once consulted me who, for several weeks, had seen, just as he lay down, the figure of a very old man, who stood by the side of his bed grinning and beckoning to him. At first he was deceived, and started suddenly from his bed, whereupon his visitor disappeared. He marie several tests which satisfied him as to the real character of the phantom, and then, like a sensible man, tried to get to sleep, but in this attempt he succeeded badly.
The explanation of such cases is very simple. The recumbent posture facilitates the flow of blood to the brain, and at the same time tends, in a measure, to retard its exit. Hence the appearances were due to the resulting congestion. As soon as the individual rose in bed, or stood erect, the reverse conditions existed, the congestion disappeared, and the apparition went with it.
The influence of cerebral hyperæmia in causing hallucinations seems to be clearly established. Ferriar wrote a treatise with the special object of proving that this is the only cause. This is an extreme view, however, which can not be sustained, for that the very opposite condition, cerebral anæmia, is an immediate cause of hallucinations is seen in the facts that, during starvation and other conditions producing great bodily exhaustion, hallucinations are common occurrences.
A striking instance has recently come under my observation, which shows, undoubtedly, that a reduction in the amount of blood circulating within the cranium may give rise to hallucinations. A young woman affected with epilepsy had repeated seizures while in my consulting-room, and, with a view of arresting them, I exerted strong pressure on both carotid arteries. Her face instantly became pale,
