THE PHYSIOLOGY OF SLEEP. 261 



mograph to register the changes in volume was of a different 

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

 shown in Fig. 116, — 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 causation of sleep.* Confining ourselves 

 to the more recent hypotheses that attempt to explain the immedi- 

 ate 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. — Preyerf 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, 

 and that if not removed as quickly as formed these products cause 

 a diminution and finally a loss of irritability. The central nerve 

 tissues in activity show also an acid reaction. Moreover, if lactic 

 acid or its sodium salt is injected into the blood it brings on a con- 

 dition of fatigue and finally a state of unconsciousness. The theory, 

 therefore, supposes that during the waking hours the constant 

 activity of the muscles and nervous system results in a gradual 

 accumulation of these waste products, since their oxidation and 

 removal does not keep pace with their production. The end- 

 result is a diminishing irritability of the central nervous system, 

 especially perhaps of the cortex, which results finally in invol- 

 untary sleep, although normally the accumulation is not carried 

 to this extreme, since it is our habit to induce sleep, when the 

 sensations of sleepiness become apparent, by withdrawing ourselves 

 from excitations, mental or sensory. No experimental confirma- 

 tion of this theory has been furnished. 



2. Consumption of the Intramolecular Oxygen. — Pfliiger J suggests 

 that the cause of sleep lies essentially in the fact that the brain cells 

 during the waking hours use up their store of oxygen more rapidly 

 than it can be replaced by the absorption of oxygen from the blood. 

 The result is a gradual reduction in irritability; so that when 

 external stimuli are withdrawn the oxidations in the cells sink 



* For a very complete statement of the theories of sleep and for the 

 literature in general, see the excellent book by Pieron, "Le probleme physio- 

 logique du sommeil," Paris, 1913. 



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

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



I Pfliiger, "Archiv f. d. gesammte Physiologic," 10, 468, 1875. 



