714 FUNCTIONS OF THE CEREBRO -SPINAL NERVOUS SYSTEM. 



watch over his head, the knowledge possessed during the waking state, that 

 this suspension is either an act of negligence which requires notice, or indi- 

 cates some unusual occurrence, doubtless augments the effect which the dis- 

 continuance of the sound would of itself produce. 



580. It is not requisite, however, that the sound should be one habitually 

 attended to during the hours of watchfulness ; for it is sufficient if it be 

 one on which the attention has been fixed as that at which the slumberer is 

 to arouse himself. Thus the medical man, even in his first profound sleep 

 after a fatiguing day's work, is aroused by the first stroke of the clapper of 

 his night-bell ; and to those who are accustomed to rise every morning at 

 the sound of an alarm clock, the frequency and regularity of the occurrence 

 do not diminish, but rather increase, the readiness with which it produces 

 its effect, provided that the warning be promptly obeyed. On this usually 

 depends the efficiency of the awakening sound ; if it be regarded as a thing 

 to which there is no occasion to give heed, it very soon ceases to produce 

 any effect, the entire peal not being sufficient to awake the sleeper ; whilst, 

 on the other hand, the first stroke is enough to break the repose of him who 

 is impressed with the effectual desire of profiting by the warning. And thus 

 it may happen that, of two persons in the same room, either shall be at once 

 aroused by a sound which produces no disturbance in the slumbers of the 

 other. To this influence of previous impressions, whether habitual, or but once 

 forcibly made, we are also to refer the spontaneous termination of the state 

 of sleep at particular times, without any sensorial excitement from external 

 impressions. Thus, many persons who are accustomed to rise at a particular 

 hour, wake regularly at that hour, whether they have gone to rest early or 

 late ; so that the act of spontaneously awakening is no proof that the desir- 

 able amount of repose has been obtained. But what is more remarkable is, 

 that many individuals have the power of determining, at the time of going 

 to rest, the hour at which they shall rise, so as to awake from a profound 

 sleep at the precise time fixed upon. In others, however, the desire to rise 

 at a particular hour only induces a state of restlessness throughout the night, 

 destroying the soundness of the slumbers; the individual awakes many 

 times in the night, with the belief that the hour is past, and very possibly 

 oversleeps it after all, the system being worn out by the need of repose. 



581. The Amount of Sleep required by Man is affected by so many con- 

 ditions, especially nt/e, tr>nperinn<')tt, habit, and previous exhaustion, that no 

 general rule can be laid down on the subject. The condition of the f<t-fu# 

 in utero maybe regarded as one of continual slumber; the apparatus of 

 Animal life being completely secluded from all stimuli which could arouse 

 it into activity, whilst the energy of the Organic functions is entirely di- 

 rected to the building up of the fabric. On its first entrance into the world, 

 the infant continues to pass the greater part of its time in slumber; and this 

 is particularly to be noticed in cases of premature birth, the seven months' 

 child seeming to awake only for the purpose of receiving food, and giving 

 but little heed to external objects, whilst even the eight months' child is 

 considerably less alive to sensory impressions than one born at the full time. 

 The excess of activity of the constructive over the destructive operations, 

 which characterizes the whole period of infancy, childhood, and adolescence 

 (chap, xx ), requires that a larger proportion of the diurnal cycle shall be 

 passed in sleep (during which the former may be carried on without hin- 

 drance), than is requisite when adult age has been attained, the two sets of 

 changes being then balanced ; and the amount of sleep to which the system 

 shows itself disposed, gradually diminishes from three-fourths to one-half, 

 and from one-half to one-third, or even to one-quarter, of the twenty-four 

 hours. It is to be noticed that the sleep of children or young persons is not 



