180 THK XKKVOUS SYSTEM AND ITS CONSKHVATIOX 



sleep and its depart ure have been demonstrated in the 

 normal human being l>y nn indirect, but sensitive, method. 

 This is the use of the instrument known as the plethysmo- 

 irraph ( Fig. 24), wliieh, in its usual form, is a glass cylinder 

 designed to accommodate 1 the hand and forearm of the 

 subject. A tight but comfortable closure at the end 

 toward the elbow is secured and the whole is filled with 

 water. A delicate gage is connected with the interior 

 and any displacement or drawing back of water is made 

 evident. A displacement must mean an increase in the 

 volume of the arm and hand, a withdrawal must indicate 

 a shrinkage. Such changes in volume, when they occur 

 at all rapidly, may be ascribed to changes in the amount 

 of blood in the part. More gradual changes might be due 

 to variations in the quantity of the lymph. 



When a person schools himself to go to sleep with his 

 arm enclosed in a plethysmograph it is found that a 

 marked dilation of the member occurs at about the time 

 when the threshold is crossed. This teslilies to a diminu- 

 tion of the tonic activity of the vasoconstrictor center. 

 It has already been pointed out that such a slackening 

 will result in a more sluggish cranial circulation. Hence 

 it is inferred that when the record from the plethysmo- 

 graph shows a widening of the vessels in the arm there is 

 a simultaneous flagging of the blood-supply to the brain. 

 \Yhen waking occurs there is a sharp contraction of the 

 arm. It is very significant that external stimuli applied 

 during sleep, but not sufficing to rouse the sleeper, never- 

 theless cause vasomotor responses. In the experiments 

 directed by Ilowell, he himself being the subject, the 

 watcher at the bedside saw that occasional noises and 

 vibrations interrupting the quiet of the night registered 

 themselves through the distinct narrowing of the vessels 

 of the arm which they called forth. The reaction might 

 be described as a threat of waking. 



< hi the side of hygiene there is much to lie said of sleep. 

 Questions frequently asked and not at all easy to answer 

 are: llou much sleep is best and how far do individuals 



