MAURY ON SLEEP AND DREAMS. 389 



in any situation where, from necessity or decorum, a 

 struggle has to be made against sleep, we obtain an 

 easy estimate, sufficient to know how rapid are the 

 fluctuations which thus affect the most important organ 

 of our frame. A sudden drop of the head awakens to 

 consciousness, which is often lost again in a few 

 seconds of time ; and such alternations, as is well 

 known, are repeated over and over again. Anyone 

 who has passed a dozen or twenty hours on horseback 

 (we speak from frequent experience) must well recol- 

 lect the effects of this hurried repetition the loss of 

 balance from momentary slumber, the sudden awaking 

 in the effort to retrieve it, and the distressing efforts to 

 prevent relapse into sleep. Without pretending to 

 exactness in a matter thus vague and fluctuating even 

 in the terms applied, we venture to say on observation 

 that three or four distinct alternations of sleep and 

 waking that is, of consciousness lost and restored 

 may, and do, occur within a single minute of time. 

 Strange and sudden as these changes in our sensorial 

 existence may seem to be, they are yet compatible 

 with that continuity by gradation, already indicated as 

 the sole method of rightly interpreting the phenomena. 

 Connected with this subject is the curious clirono- 

 metry so often impressed upon sleep, testified by the 

 power of awaking invariably at some one determinate 

 hour. The explanation of this fact must be sought for 

 in what may be called the general chronometry of life ; 

 in the tendency, more or less, of all vital functions to 

 assume a periodical character, either from original 

 constitution, or from engendered habits acquiring the 



