CHANGES IN THE CIRCULATION. 309 



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 plethysmo- 

 graph 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 experi- 

 mented 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 gradu- 

 ally 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 sup- 

 posed, 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 plethy sinograph preserves a more accurate record, for the slight- 

 est movement or disturbance produces its effect upon the arm, dimin- 

 ishing its volume. As a person awakes naturally, the size of the fore- 

 arm 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, pro- 

 duced a slumber very similar to normal sleep. 



