710 FUNCTIONS OF THE CEREBRO -SPIN AL NERVOUS SYSTEM. 



remain awake is forced to give way to Sleep, when this is required by the 

 exhaustion of nervous power, must be withiu the experience of every one ; 

 and the only way in which the Will can even retard its access, is by de- 

 termiuately fixing the consciousness upon some definite object, and resisting 

 every tendency in the thoughts to wander from this. It does not appear to 

 be of any consequence, whether this exhaustion be produced by the active 

 exercise of volition, reflexion, emotion, or simple sensation ; still we find that 

 the volitional direction of the thoughts in a course different from that in which 

 they tend spontaneously to flow, is productive of far more exhaustion than 

 the automatic activity of the mind ; whilst, on the other hand, the excess of 

 automatic activity, whether as regards the intellectual operations or emotional 

 excitement, tends to prevent sleep. This is particularly the case when the 

 feelings are deeply interested ; thus the strong desire to work out a result, or 

 to complete the survey of a subject, is often sufficient to keep up the intel- 

 lectual activity as long as may be requisite (a state of restlessness indeed 

 being often induced, which prevents the access of sleep for some time longer) ; 

 so, again, anxiety or distress is a most frequent cause of wakefulness ; and it is 

 generally to be observed that the state of suspense is more opposed to the 

 access of sleep, than the greatest joy or the direst calamity when certainty 

 has been attained. 1 But although an excess of automatic activity is opposed, 

 so long as it continues, to the access of sleep, yet it cannot be long protracted 

 without occasioning an extreme exhaustion of nervous power, which neces- 

 sitates a long period of tranquillity for its complete restoration. 



574. Whilst, however, the necessity for Sleep arises out of the state of the 

 nervous system itself, there are certain external conditions which favor its 

 access ; and these, in common parlance, are termed its predisposing causes. 

 Among the most powerful of these, is the absence of sensorial impressions; 

 thus darkness and silence usually promote repose; and the cessation of the 

 sense of muscular effort, which takes place when we assume a position that 

 is sustained without it, is no less conducive to slumber. There are cases, 

 however, in which the continuance of an accustomed sound is necessary, in- 

 stead of positive silence, the cessation of the sound being a complete preven- 

 tive of sleep; thus it happens that persons living in the neighborhood of the 

 noisiest mills or forges cannot readily sleep elsewhere. Such cases are ref- 

 erable either to the influence of habit, which causes the attention of the in- 

 dividual to be more attracted by the suspension of the sound than by its con- 

 tinuance ; or to the fact that the monotonous rrpftition of sensorial impres- 

 sions is often more favorable to sleep than their complete absence. Thus it 

 is withiu the experience of every one, that the droning voice of a heavy 

 reader on a dull subject is often a most effectual hypnotic; in like manner, 

 the ripple of the calm ocean on the shore, the sound of a distant waterfall, 

 the rustling of foliage, the hum of bees, and similar impressions upon the 

 auditory sense, are usually favorable to sleep; and the muscular and tactile 

 senses may be in like manner affected by a uniform succession of gentle 

 movements, as we see in the mode in which nurses " hush off" infants, or in 



lea-t, remission of agony, and will slumber until the fire is applied to awaken him ; 

 and the Medical Practitioner has frequent illustrations of the same fact. That the 

 continued demand for muscular activity is not incompatible with the access of sleep, 

 is obvious from what has been already said of the persistence of (lie automatic move- 

 ment.- in that condition ; it is well known that, previously to the shortening of ibe 

 hours of work, factory children frequently fell asleep while attending to their ma- 

 chines, although well aware that they should incur severe punishment bv doing so. 



Thus it i> a common observation, that criminals under sentence of death sleep 

 badly so long as they entertain any hopes of ;i reprieve; but when once the\- are 

 satisfied that their death is inevitable, they usually sleep more soundly, and (his even 

 on the very last night of their lives. 



