266 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



below the level necessary to arouse consciousness. During sleep 

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

 ically combined by anabolic processes to form the irritable living 

 matter is again replenished. The fundamental conception on 

 which this theory rests, the idea, namely, of a storage of intra- 

 molecular oxygen, has been practically abandoned. 



3. Toxin Theories. Quite a number of writers have assumed 

 that some special toxin, which might be called a hypnotoxin, 

 is formed during the waking hours, and finally accumulates in 

 sufficient quantity to inhibit the activity of the cortical cells. A 

 specific view of this kind has been proposed by Pieron, and to a 

 certain extent has been supported by experiments. It has been 

 shown that very young dogs, when deprived completely of sleep, 

 will die within four to six days. Pieron, making use of the method 

 of experimental insomnia, states that dogs kept awake for periods 

 of thirty to three hundred hours show evidence of an intoxication, 

 not only in the extreme somnolence manifested, but, microscopi- 

 cally, in that the cortical cells in the frontal region are distinctly 

 altered. When the blood or cerebrospinal liquid of such animals 

 is injected into the brain- ventricles (fourth ventricle) of another 

 animal, the latter is thrown into a condition of somnolence, and 

 exhibits changes in his cerebral cortex similar to those in the dog 

 suffering from insomnia. This assumed hypnotoxin is precipitated 

 by alcohol and is destroyed by a temperature of 65 C. In his 

 complete theory the author supposes that the toxin acts directly 

 to arouse somnolence, but that the sudden advent of deep sleep 

 is due to the fact that the toxin sets into action some unknown 

 nervous inhibitory mechanism. 



4. The Neuron Theory. Duval,* 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. 



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

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



