SLEEP AND ITS COUNTERFEITS. 607 



monotonous impressions from without. The same may be said of an 

 order of stimuli that has hitherto not received its due share of atten- 

 tion. I mean the afflux of those confused, mostly unfelt, impres- 

 sions from the viscera and tissues generally. Under certain condi- 

 tions after a meal, for instance these may set up, in the cerebral 

 centers to which they converge, an excitation that leads to an inhibi- 

 tion of the higher brain-regions, and so to a state of sleep. 



Similar considerations will assist us in explaining the effect of the 

 usual methods of hypnotization. The stimulation of one of the cere- 

 bral sensory centers by repeated gentle and monotonous sounds or 

 touches, or, in the case of the visual organs, by the convergence of the 

 eyes and persistent gazing at a small object, so interferes with the 

 activity of the higher centers as to lead to various perverted motor 

 aud mental manifestations. Certain " nervous " individuals, but above 

 all hysterical subjects, are more amenable to these effects than are 

 others. Repetition in all cases increases the liability to hypnotiza- 

 tion, and in extreme cases the recollection of the processes previously 

 used becomes sufficient to induce sleep. Finally, there are subjects, 

 such as " the Soho sleeper," in whom, owing to the extreme instability 

 of their cerebral equilibrium, a kind of spontaneous hypnotization may 

 be observed. 



It would be premature, in the actual state of our knowledge, to 

 speculate upon the nature of the changes in the nervous system upon 

 which the phenomenon of inhibition depends. It has been ingeniously 

 compared to that of the mutual interference of two rays of light or 

 two waves of sound. But this analogy does not account for all the 

 facts ; and in connection with this topic we may mention the views 

 recently propounded by Professor Brown-Sequard upon what he calls 

 nervous " dynamogeny." It is a well-known fact that under the influ- 

 ence of various sensorial or emotional stimuli, of moderate intensity or 

 pleasurable quality, our nervous energy, as measured by the muscular 

 effort we are enabled to put forth, is increased to a considerable ex- 

 tent. Recent researches by Dr. Fere have thrown additional light 

 upon these " dynamogenic " or " force-producing " processes, of which 

 the reviving effect of smelling-salts is a familiar illustration. In this 

 instance a diffusive wave invades the whole brain from the olfactory 

 centers, and produces such a change in its constituents as to restore 

 its functions. It has likewise been shown that every form of mental 

 activity is accompanied with increased nerve-power as directly meas- 

 ured by the squeeze of the hand on the dynamometer. It would thus 

 seem that nervous cell-matter is liable to undergo certain modifica- 

 tions under the influence of various impressions derived from other 

 nerve-regions, in virtue of which it becomes more powerful. But, with- 

 out even attempting to define more closely the " dynamogenic " 

 change, we may perhaps assume it to be the counterpart of what takes 

 place in inhibition, and describe the latter as a nervous process in 



