ix PSYCHO-PHYSICAL PHENOMENA 465 



abnormal state, there are well-known instances of men sleeping on 

 horseback, and soldiers falling asleep on a long night-march. 

 Galen relates that in a night's journey he slept through an entire 

 stage. These curious instances are explicable on the theory of 

 partial sleep, which invades all centres except those on which the 

 automatic activity of the locomotor muscles depends. 



To sum up, therefore, and define the characteristic features of 

 sleep, apart from all the concomitant factors, such as diminution 

 of sensorial excitation due to the quiet of night, reduction of 

 mental and muscular activity, and the horizontal posture, it may 

 be said to consist specially in variations of the cardio- vascular, 

 the respiratory, and, above all, the sensori-motor activities. 



VII. The complex physiological phenomena of sleep, as set out 

 above, prove clearly that it is not confined to the brain, but 

 involves the entire organism more or less. Nevertheless, the 

 most characteristic feature of sleep is the resulting state of the 

 nervous system, which has led many to regard the brain as the 

 part most implicated. 



The principal hypotheses put forward to explain the genesis 

 and nature of sleep are based on the changes that can be observed 

 in the functional activity of the brain in waking and sleeping. 

 These must be examined not so much for their intrinsic value as 

 for the experimental researches made in connection with them. 



The earliest were the circulatory theories of sleep. Observa- 

 tions on the cerebral vessels during sleep are contradictory : some 

 authors described an ischaemia, others a cerebral hyperaemia. 

 Blumenthal (1795) was the first who stated that sleep is associated 

 with contraction of the cerebral vessels, from his observations on 

 the exposed brain of a boy with a cranial fracture. Bonders, 

 Kussmaul and Tenner, Durham, Hammond, Cl. Bernard, and 

 others held the same opinion, after trephining the skull in 

 animals. Salathe (IS 1 ??) gave the same interpretation to his 

 observations on the movements of the fontaiielles in infants. 



Mosso, Howell, and Lehmann made observations with the 

 graphic method on certain subjects with cranial defects, and 

 saw that changes in the cerebral circulation could be observed 

 even in the period of drowsiness that precedes slumber : the 

 sphyginograms were lower and more uniform in proportion as 

 sleep became more profound. While the vessels of the forearm 

 dilated during sleep, those of the brain contracted. Stimulation 

 of any peripheral sense-organs during the waking state sufficed to 

 produce a rise in the cerebral pulsations and a simultaneous fall 

 in those of the forearm. The same antagonistic effects were pro- 

 duced during sleep, even when the stimulation was not strong 

 enough to cause awakening. During gradual awakening dilatation 

 of the vessels and increase of brain volume was observed, while the 

 vessels of the forearm contracted ; in sudden awakening by strong 



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