SLEEP. 



681 



citement of the feelings keeping up a. forced state 

 of mental activity, which no voluntary effort 

 can subdue. The state of suspense is in most 

 persons more difficult to bear with equanimity, 

 and is more opposed to the access of sleep, 

 by the continual perturbation which it induces, 

 than the greatest joy or the direst calamity 

 when certainty has been attained. 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 sentence 

 will be certainly carried into execution, they 

 usually sleep more soundly, and this even on 

 the very last night of their lives. That the 

 continued excitement of the feelings, whilst 

 producing an indisposition to sleep, really 

 occasions as great a demand for it in the 

 system as is produced by the most active ex- 

 ercise of the intellectual powers, is evident 

 from the very exhausting effects of its pro- 

 traction ; which necessitate a long period of 

 tranquillity for restoration to health. 



Among the most powerful of the predis- 

 posing causes to sleep, is the absence of 

 sensorial impressions : thus darkness and 

 silence usually conduce to repose ; and the 

 cessation of the sense of muscular effort, which 

 takes place when we assume a position that is 

 sustained without it, frequently acts as the 

 complement of all other influences. 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 complete preventive of sleep. Thus 

 it happens that persons living in the neigh- 

 bourhood of the noisiest mills or forges cannot 

 sleep elsewhere ; and when, to induce repose 

 in illness, the mill or the forge has been 

 stopped, the cessation of the sound only 

 occasions more obstinate wakefulness. Such 

 instances, perhaps, fall within the next 

 category of predisposing causes, namely 

 the monotonous repetition of sensorial impres- 

 sions. Every one knows how efficacious a 

 provocative of sleep is the droning voice of 

 a heavy reader, especially when his subject is 

 equally prosaic. The ripple of the calm 

 ocean upon the shore, the murmur of a 

 rivulet, the sound of a distant waterfall, the 

 rustling of foliage, the hum of bees, and similar 

 monotonous impressions upon the auditory 

 sense, are usually found to induce sleep ; and 

 Boerhaave relates, that being desirous of pro- 

 curing sleep for one of his patients troubled 

 with obstinate insomnia, he directed a brass 

 pan to be so placed as to receive a succession 

 of drops of water, the sound of which had the 

 desired effect. A lulling influence, however, 

 is not universally thus produced ; for we have 

 known a case in which sleep was altogether 

 kept away by the sound of dropping water, 

 which seems to have occasioned a state of 

 emotional excitement. Not only is the repe- 

 tition of auditory impressions provocative of 

 sleep ; uniform succession of gentle movements 

 has a similar effect upon the sensorium 

 through the sense of vision. The sleep thus 

 induced, however, is usually characterised by 



certain peculiarities which will be described 

 hereafter. The recurrence of impressions 

 received through the sense of touch has the 

 same effect. Thus Dr. Elliotson says*, 

 " I know a lady who often remains awake in 

 spite of every thing, till her husband very 

 gently rubs her foot ; and by asserting to a 

 patient my conviction that the secret of an 

 advertising hypnologist whom I allowed to try 

 his art upon the sleepless individual, and 

 which he did for a time successfully, was to 

 make him gently rub some part of his body 

 till he slept, he confessed this to be the fact." 

 The rocking of the infant's cradle, or the 

 gentle swaying of the body backwards and 

 forwards in the arms, are predisposing causes 

 of sleep well known to nurses. 



In these and similar cases, the influence of 

 the impressions would seem to be exerted 

 in withdrawing the mind from the conscious- 

 ness of its own operations, the loss of which, 

 as we shall presently point out, is the tran- 

 sition-step of the passage into complete un- 

 consciousness. The reading of a dull book 

 acts in the same mode. There is a monotony 

 of sensorial impressions, the eyes wandering 

 on from line to line and from page to page, 

 without any mental interest in the sensations 

 received ; and if the voluntary effort of atten- 

 tion be intermitted, the thoughts pass off 

 along their own spontaneous train, whilst the 

 sensorial centres are left free to the soporific 

 influence of monotony. 



The foregoing are the chief causes of sleep, 

 which operate directly through the sensorial 

 organs themselves. We have now to consider 

 those whose action is indirect, being exerted 

 primarily on the organic functions. Of these 

 the first in order of importance are those 

 which produce increased pressure of blood 

 within the vessels of the encephalon. Thus 

 the assumption of the recumbent position 

 operates in this method as a powerful predis- 

 ponent to sleep, as well as by rendering all 

 muscular effort unnecessary for the mainte- 

 nance of the position of the body. To this 

 cause again we are probably to attribute, in 

 great part at least, the drowsiness which suc- 

 ceeds a full meal, the pressure within the 

 encephalic vessels being increased by the pres- 

 sure of the distended stomach upon the ves- 

 sels of the abdomen ; but the circulation of 

 imperfectly assimilated matter in the blood 

 may possibly concur in the production of the 

 result. The influence of pressure is most 

 characteristically seen in cases of gradual 

 effusion of blood or of serum from the vessels 

 of the brain : this at first occasions a state of 

 sopor but little different from profound or- 

 dinary sleep ; but with the increase of the 

 effusion there is an increase in the depth of 

 the slumber; the patient can no longer be 

 aroused by sensorial impressions which were 

 at first sufficient to re-excite consciousness, 

 and at last complete coma comes on.-}- A 



* Physiology, p. 609. 



f Dr. Marshall Hall has advanced the hypothesis, 

 that ordinary sleep is the result of congestion of the 

 brain produced by compression of " certain veins," 



