THE PHYSIOLOGY OF SLEEP. 251 



the viscera and the consequent diminution in the blood-flow through 

 the brain; but the sleep that occurs at the end of the day is un- 

 doubtedly connected with a dilatation of the blood-vessels of the 

 skin of the trunk and extremities. What the condition in the vis- 

 ceral organs may be at such times we have at present no means 

 of knowing. 



Hypnotic Sleep. The sleep that can be produced by so-called 

 suggestion, the sleep of hypnotism, has been studied by means 

 of the plethysmographic rnethod.* The result, so far as the 

 volume of the arm and hand is concerned, shows that in this con- 

 dition, unlike normal sleep, there is a marked diminution in volume, 

 and, therefore, we may believe, an increased constriction of the 

 blood-vessels of the skin. This observation accords with the 

 blanched appearance of the skin of the extremities, and with the 

 statement that in deep hypnotic sleep the skin does not bleed 

 readily when pricked with a needle. In view of our limited knowl- 

 edge, however, it would be hazardous to base any comparison 

 between normal and hypnotic sleep upon this single fact. 



* Walden, "American Journal of Physiology," 4, 124, 1900-1901. 



