612 SLEEP. 



granted, it must be viewed, not as the cause, but as a circum- 

 stance, or, in fact, a consequence, of ordinary sleep. Increase 

 the activity of an organ, you increase its circulation ; diminish 

 its activity, you diminish its circulation. The alteration of cir- 

 culation is usually not the cause but the consequence ; necessary, 

 indeed, to the continuance of the altered degree of activity in 

 the organ, but not the cause. The degree of activity of any 

 part, and the degree of its circulation, are exactly and 

 ably correspondent. If the circulation through a part 

 chanically increased or diminished, the sensibility and activity of 

 the part will, doubtless, be proportionally increased or diminished. 

 This example occurs in hemorrhage. Frequently both are af- 

 fected simultaneously, when diarrhoea renders the surface pale 

 and cold, both the blood is sent more sparingly to it, and the 

 energy of its vessels is diminished by the increase of energy in 

 those of the intestines. But, in ordinary sleep, the diminished 

 circulation appears only the consequence, for activity is always 

 followed by inactivity. Stimulate a muscle separated from the 

 body, it contracts, but it soon refuses to do so ; after a little 

 rest, it again contracts upon the renewal of the stimulus. The 

 case of the brain is analogous ; and when, after its daily activity, 

 it falls asleep, the diminution of its circulation consequently 

 ensues. The influence of sleep upon the cerebral circulation is 

 shown by the headach and other marks of congestion which 

 follow too much sleep. Boerhaave mentions a student who took 

 a fancy that sleep was the natural state of man, and so slept 

 eighteen out of the twenty-four hours, till he died of apoplexy. 

 The horizontal posture will not explain these ill effects, because 

 persons with spinal disease will lie a year upon the back without 

 them. 



Sleep is an inactive state of the brain, resulting ordinarily 

 from mere fatigue of the organ through its activity : though 

 pressure, want of proper blood, narcotics, &c. or want of exciting 

 causes, may prevent activity, and thus induce sleep. The case 



The most silly discussions may be found in old authors and modern twad- 

 dlers (see, for instance, Isis revelata, a book soon to be quoted) about the state of 

 the soul in sleep. " There have been great disputes," says Gall, " upon the ques- 

 tion, whether the soul can ever exist, as in sleep, without feelings or ideas? If 

 we lay aside all vain metaphysical subtlety, the answer is very easy. In this life 

 the soul receives its feelings and ideas by means of cerebral organs ; when these 



