598 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
good reason, they have been generally adopted. In reality, these 
theories are of two kinds. Just as the engine stops, eithér from 
lack of fuel or from an accumulation of ashes, so the organism may 
plunge into sleep either because the materials necessary for brain 
activity are exhausted, or because the waste material, generally toxic, 
is in too great a quantity. 
Among the substances necessary to the working of the nerve cen- 
ters there are two whose values are now well known, oxygen and chro- 
matophile. The nerve centers in action consume considerable quan- 
tities of oxygen, and we may therefore suppose that sleep is due to a 
lack of sufficient oxygen in the brain, and that it collects in reserve a 
supply of gas necessary for the coming awakening. This is the theory 
held by Sommer and Pfluger, relying upon the researches on respi- 
ratory interchanges made by Pettenkoffer and Voit. The other sub- 
stance necessary to the building up of the nerve cells, which accumu- 
lates during sleep, disappearing after prolonged activity, is that which 
Nissl discovered in nearly all the nerve cells and which is named 
chromatophile on account of its ability to give coloveasily. We might 
therefore explain sleep by the deterioration of chromatophile. That 
explanation was made by Daddi after having noted the disappearance 
of chromatophile during a case of prolonged insomnia. 
But though the nerve centers contain substances indispensable to 
their working, they also produce during their activity certain waste 
substances, just as the stove ready to be lighted is full of fuel, then 
when it is lighted produces ashes that accumulate and diminish the 
draft. What is the ‘‘ashes” of the nervous system? It is a product 
of disassimilation well known for a long time; it is that which leaves 
the lungs when oxygen enters, the chemical carbon dioxide, commonly 
called carbonic-acid gas. M. Raphaél Dubois has considered it the 
cause of sleep. ‘To tell the truth, he studied only the hibernal sleep 
of the marmot, and he concluded that a sleep through an entire winter 
is the same as daily sleep. We may ask whether this comparison is 
justifiable. Furthermore, according to M. Dubois, sleep is not a 
recuperation; we sleep because the carbon dioxide has accumulated 
in the blood, but, during sleep, the gas continues to accumulate until 
it is strong enough to excite the nerve centers to wake up. 
Besides the carbon dioxide, there are other less-known wastes from 
nerve action. These wastes, which have been called ‘“‘fatigue toxins” 
(which generate sleep) and which Moliére would certainly have named 
‘‘dormitive virtues,’ have also been considered as causes of sleep. 
Obersteiner thinks that these toxins consist principally of lactic acid; 
Preyer believes them easily oxidizable; Bing claims that they act 
chiefly in preventing oxidation; Errera and Bouchard believe them 
more or less analogous to the leucomaines; Lahuson thinks that they 
are autointoxicating narcotics. But these are merely hypotheses 
based on experiments as yet insufficient to prove their accuracy. 
