THE PHYSIOLOGY OF SLEEP. 245 



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. 



2. Consumption of the Intramolecular Oxygen. Pfliiger* 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 external stimuli are withdrawn the oxidations in the cells sink 

 below the level necessary to arouse consciousness. During sleep 

 the store of intramolecular oxygen that is, the oxygen syntheti- 

 cally combined by anabolic processes to form the irritable living 

 matter is again replenished. 



3. The Neuron Theory. Duval, f Cajal, and others have applied 

 the neuron doctrine to explain the occurrence of sleep. According 

 to the neuron conception, the connection between the cells in the 

 cortex and the incoming impulses along the afferent paths is made by 

 the contact of the terminal arborizations of the* afferent fibers with 

 the dendrites of the cell. Assuming that these latter processes are 

 contractile, Duval supposes that sleep is caused mechanically by 

 their retraction, which results in breaking the connections and 

 thus withdrawing the brain cells from the possibility of external 

 stimulation. Conductivity is re-established upon awaking by the 

 elongation and intermingling of the processes again re-establishing 

 physiological connections. The numerous efforts made to demon- 

 strate the fact of a retraction of the dendritic processes by histo- 

 logical examinations of brains during sleep or narcosis have, how- 

 ever, not been successful. 



4. Anemia Theories of Sleep. Numerous facts in physiology 

 make it practically certain that during sleep there is a diminished 



* Pfliiger, "Archiv f. d. gesammte Physiologie," 10, 468, 1875. 

 t Duval, " Comptes rendus de la soc. de biol.," February, 1895; and Cajal, 

 "Archiv f. Anat. (u. Physiol.)/' 375, 1895. 



