PROTECTIVE MECHANISMS OF THE NERVOUS SYSTEM 723 



between the first and second hours after its beginning. It then 

 decreases and remains rather light during the third and fourth hours. 

 A second but much slighter increase takes place during the fifth hour. 

 From here on it again decreases up to the hour of awakening. 



Theories of Sleep. The theories pertaining to the causation of sleep, 

 may be conveniently arranged as follows: 



(a) Anemia Theory (Coppie, 1854). It is held that the diminution and loss 

 of the irritability of the cerebrum, and especially of its cortical zone, is dependent 

 upon a decrease in its blood-supply. This view is based chiefly upon the experi- 

 ments of Mosso 1 which have shown that the volume of the arms and legs increases 

 during sleep, presumably on account of a transfer of blood from the cerebral into 

 the cutaneous circulatory channels. This transfer may be effected in two ways, 

 namely, by a constriction of the cerebral blood-vessels 2 or by a relaxation of the 

 blood-vessels in other parts of the body. The extracerebral circuits which could 

 be concerned in producing this diminution in the vascularity of the brain, are those 

 of the integument and portal organs. 



These facts have been made use of by Hill 3 in formulating a theory which has 

 as its chief element the fatigue of the vasomotor center. It is assumed that sleep 

 results in consequence of the loss of tonus of this center brought on by the con- 

 tinued activity during the working hours. This loss of the vascular tonicity in 

 turn leads to lower pressures and diminishes the blood flow throughout the body, 

 but more particularly that through the brain. The blood withdrawn from the 

 cerebral circuit, is said by Hill to be accommodated in the blood-vessels of the portal 

 organs, while Howell believes that it is transferred into those of the skin and sub- 

 cutaneous tissues. Brodmann 4 and Shepard 5 however, claim that the volume of 

 the brain is increased, during sleep, as is also that of the limbs. This contradiction 

 of the results of the investigators just cited, necessitates a certain modification of 

 the anemia theory, because it places the transfer of blood from the brain into other 

 blood-vessels into question. At best, therefore, we can go no further than to state 

 that the vascular relaxation and depression resulting during sleep, is general and 

 does not produce an actual anemic condition of the brain. 



(6) Inhibition theory, advocated by Brown-Sequard, 6 refers sleep to an inhibition 

 of cortical function, such as may be produced by passing an induction current of 

 low tension through the cranium (Leduc). This view has few points in its favor, 

 because it does not attempt to explain the mechanism by means of which this 

 inhibition is brought about. 



(c) The mechanical block theory refers sleep to an interruption of the conduction 

 paths caused by a retraction of the terminal fibers of the synapses. 7 This view 

 must also be said to contain a decided element of speculation, because it has never 

 been demonstrated that a retraction of this kind actually takes place. 



(d) The chemical theories of Preyer, 8 Pieron 9 and Pfliiger 10 refer the loss of the 

 irritability of the brain to a fatigue brought on by chemical means. Thus, it has 



1 Ueber den Kreislauf des Blutes im menschl. Gehirn, Leipzig, 1881; also: 

 Brush and Fayerweather, Am. Jour, of Physiol., v, 1901, 199. 



2 Jenson, Pfliiger's Archiv, ciii, 1903, 171. 



3 Physiol. and Pathol. of the Cerebral Circulation, London, 1896, and Howell, 

 Jour, of Exp. Med., ii, 1897, 313. 



4 Jour, fur Psych, und Neurologie, i, 1902, 10. 

 6 Am. Jour, of Physiol., xxiii, 1909. 



6 Archiv de physiol., 1889, 333. 



7 Duval, Compt. rend, de la Soc. biol., 1895; Cajal, Archiv fur Anat. und Phy- 

 siol., 1895, and Nicard, Le sommeil normal, Lyon, 1904. 



8 Centralb. f. d. med. Wissensch., xiii, 1875. 



9 Le probleme physiol. du sommeil, Paris, 1913. 

 10 Pfliiger's Archiv, x, 1875, 468. 



