THE PHYSIOLOGY OF SLEEP. 



247 



Fig. 110. In these experiments the recorder attached to the 

 plethysmograph to register the changes in volume was of a differ- 

 ent kind (tambour) and the record reads in a reverse way to that 

 shown in Fig. 109, that is, a dilatation is recorded by a rise in 

 the curve and a constriction by a fall. The recorder being more 

 sensitive, the volume changes in the arm due to the heart beat are 

 clearly indicated. The legends attached to the illustration explain 

 the results of the experiments. 



Theories of Sleep. Many hypotheses have been advanced 

 to explain the nature and c'ausation of sleep. Confining ourselves 

 to the more recent hypotheses that attempt to explain the immediate 



,r 



Fig. 110. Sleep: A, effect of external impression (music box), insufficient to awaken 

 sleeper, a marked diminution in volume of the arm ; B, effect of external impression 

 (music box) sufficient to awaken sleeper; a stronger diminution in volume followed by 

 dilatation as the subject again fell asleep. 



cause of the production of the condition, the following brief de- 

 scription will suffice to show the nature of the theories proposed : 



1. The Accumulation of Acid Waste Products. Preyer* and 

 also Obersteiner have suggested that the accumulation of acid 

 waste products in the blood brings on a gradually increasing loss of 

 irritability or fatigue in the brain cells which results finally in a 

 depression of their activity sufficient to cause unconsciousness. It 

 is known that functional activity in the muscle is accompanied by 

 the formation of acid waste products, especially sarcolactic acid, 



* Preyer, " Centralblatt f. d. med. Wiss.," 13, 577, 1875; and Obersteiner, 

 "Allgemeine Zeitschrift f. Psychiatric," 29, 224, 1872-73. 



