tion or mental process and the change in the circulation. This is illustrated in the following curious experiment upon one of Mosso's friends, who was a student of literature: While his arm was in the apparatus, Mosso presented to him a few pages on which were pasted paragraphs in Greek and Italian indiscriminately. By watching the changes in the volume of the arm, Mosso was able to decide correctly when his friend was reading a Greek paragraph, because to the greater mental effort corresponded a greater contraction of the vessels.
But the amount of blood in the extremities varies not only with psychical but also with physiological activities—for instance, with the respiration. The size of the forearm was shown by the plethysmograph to diminish during a deep inspiration, to increase during a prolonged and powerful expiration; or, again, alterations may be called forth by irritation of the skin of the arm, or even of a distant part of the body, or by direct compression of such of the veins or arteries of the upper arm as do not lie too deep to be reached, or even by changing the position of a limb other than the one being experimented upon. In brief, an almost endless variety of circumstances affect the circulation of a given part, as shown by the changes of its volume; but, among these circumstances, the condition of the brain is especially influential.
Mosso, however, has not confined his investigations to the waking condition, but has extended them also to men asleep, thus discovering a very great increase in the volume of the forearm as a person gradually falls asleep. The large size of the forearm during sleep may be diminished by a dream, or by any cause that renders the sleep less profound. It was evident that persons hear in their sleep various sounds, which disturb their slumbers but do not wake them up. When his friend was asleep, Mosso saw him move as a dog near by barked, and at the same time the apparatus recorded a diminution in the size of the extremity. Observation in this case shows that the cerebral activity during sleep is much greater than is usually supposed, and that a person may dream, as is evident by his moving or making some sound, yet have no recollection of it upon awaking. The plethysmograph preserves a more accurate record, for the slightest movement or disturbance produces its effect upon the arm, diminishing its volume. As a person awakes naturally, the size of the forearm is gradually lessened, because the blood is withdrawn.
To summarize the result: whenever the brain acts in any way, blood is withdrawn from the arm and from all the extremities; when the brain is inactive, more blood circulates in the limbs, most during sleep.
By the same apparatus, Mosso also discovered that the circulation was changed by a dose of chloral very nearly, if not exactly, as in natural sleep; and that this drug, tested by these phenomena, produced a slumber very similar to normal sleep.
