820 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



brum be induced by the protraction of its functional activity, Sleep will at last 

 supervene, from the absolute inability of the organ to sustain any futher demands 

 upon its energy, even in the midst of opposing influences of the most powerful 

 nature. 1 That the strongest voluntary determination to remain awake is forced 

 to give way to Sleep, when this is required by the exhaustion of nervous power, 

 must be within the experience of every one ; and the only way in which the 

 Will can even retard its access, is by determinately 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, reflection, 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 attended with 

 far more effort than the automatic activity of the mind ( 823); 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 strongly 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 intellectual activity as long as may be requisite (a state of restless- 

 ness, however, being often induced, which prevents the access of sleep for some 

 time longer); so, again, anxiety or distress is a most frequent cause of wakeful- 

 ness ; 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. 2 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 with- 

 out occasioning an extreme exhaustion of nervous power, which necessitates a 

 long period of tranquillity for its complete restoration. But whilst 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 re- 

 pose ; 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, instead of positive silence, the cessation of the sound being a com- 

 plete preventive of sleep ; thus it happens that persons living in the neighbor- 

 hood of the noisiest mills or forges cannot readily sleep elsewhere. Such cases 

 are referable, either to the influence of habit, which causes the attention of the 



1 Thus it is on record, that during the heat of the battle of the Nile, some of the over- 

 fatigued boys fell asleep upon the deck ; and during the recent attack upon Rangoon, the 

 Captain of one of the war-steamers most actively engaged, worn out by the excess of con- 

 tinued mental tension, fell asleep and remained perfectly unconscious for two hours, within 

 a yard of one of his largest guns, which was being worked energetically during the whole 

 period. So even the severest bodily pain yields before the imperative demand occasioned 

 by the continued exhaustion of the powers of the sensorial centres ; thus Damiens slept 

 upon the rack, during the intervals of his cruel sufferings ; the North American Indian at 

 the stake of torture will go to sleep on the least 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 

 the automatic movements in that condition (g 726) : it is well known that, previously to 

 the shortening of the hours of work, factory children frequently fell asleep whilst attend- 

 ing to their machines, although well aware that they should incur severe punishment by 

 doing so. 



2 Thus it is a common observation that criminals under sentence of death sleep badly 

 so long as they entertain any hopes of a reprieve ; but as soon as they are satisfied that 

 their death is inevitable, they usually sleep more soundly, and this even on the very last 

 night of their lives. 



