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IV. Phylogenesis of the states of sleep
The phylogenetic study of sleep allows us to dissociate (250),
during the course of evolution, the appearance of slow sleep from that
of PS. In reptiles (tortoise, for instance) only the state of slow sleep
was found and PS could not be observed (201).
In birds (chicks, hens, pigeons), a "slowsleep"state is quite
typical and electrically resembles that of mammals (275,
276, 421)
though there are no spindles and few slow waves, and PS is extremely short
(10 sec). However, PS is quite distinct it is accompanied by a short acceleration
of the electrical activity of the hyperstriatum, by an important reduction
but not by total disappearance of the neck muscle tone, by bursts of phasic
ocular movements, and by bradycardia. Until now, all mammals observed
[rat (75, 96,
313, 417),
mouse (433), rabbit (17,
148, 263,
435), opossum (407),
cat and dog (55, 405),
sheep (235), goat (391,
392), macacus (436),
and chimpanzee (5, 372)]
present the two states of sleep just studied. In certain species a few
special features are known, e.g. in the rabbit PS is hormonally dependent
and a long habituation is necessary in order to observe it. On the other
hand, in newborn ruminants, the two states of sleep may be easily observed;
PS tends to decrease considerably in adult animals (235,
392) and this may explain why
PS has not been described before (37).
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