716 FUNCTIONS OF THE CEREBRO -SPINAL NERVOUS SYSTEM. 



powers. Thus Mania, when prolonged or frequently recurring, subsides 

 into Dementia ; and if it continue for any length of time, is sure to be fol- 

 lowed by a great sense of wretchedness and prostration, frequently accom- 

 panied by continual restlessness. Such effects, too, in a less aggravated 

 degree, result from habitual deficiency of sleep; whether this be due to 

 emotional excitement which keeps repose at bay, or to a voluntary determin- 

 ation to keep the intellect in activity. This is a very common occurrence 

 among industrious students, who, with a laudable desire for distinction, 

 allow themselves less than the needed quantum of repose. Headache, ten- 

 sion, heat, throbbing, and various other unpleasant sensations in the head, 

 give warning that the brain is being overtasked ; and if this warning be not 

 taken, sleep, which it was at first difficult to resist, becomes even more diffi- 

 cult to obtain; a state of general restlessness and feverish excitement is 

 induced ; and if, in spite of this, the effort be continued, serious consequences 

 in the form of cerebral inflammation, apoplexy, paralysis, fever, insanity, 

 or loss of mental power, more or less complete, are nearly certain to be in- 

 duced. Some individuals can sustain such an effort much longer than others, 

 but it is a great mistake to suppose that they are not equally injured by it; 

 in fact, being possessed with the belief that they are not suffering from the 

 exertion, they frequently protract it, until a sudden and complete prostra- 

 tion gives a fearful demonstration of the cumulative effects of the injurious 

 course in which they have been persevering. Those, consequently, who are 

 earlier forced to give way, are frequently capable of accomplishing more in 

 the end. In regard to the degree of protraction of sleep which is consistent 

 with a healthy state of the system in other respects, it is difficult to speak 

 with certainty. Of the numerous well-authenticated instances on record, 1 

 in Avhich sleep has been continuously prolonged for many days or even 

 weeks, it is enough here to state that they cannot be regarded as examples 

 of natural sleep ; the state of such persons being more closely allied to hys- 

 teric coma. An unusual tendency to ordinary sleep generally indicates a 

 congested state of the brain, tending to apoplexy ; and it has been stated 

 that apoplexy has been actually induced by the experimental attempt to 

 ascertain how large a proportion of the diurnal cycle might be spent in 

 sleep. Thus, on either side, inattention to the dictates of Nature, in respect 

 to the amount of sleep required for the renovation of the system, becomes a 

 source of disease, and should therefore be carefully avoided. 



583. Dreaming. We have hitherto spoken of Sleep in its most complete 

 or profound form; that is, the state of complete unconsciousness. But with 

 the absence of consciousness of external tilings, there may be a state of men- 

 tal activity of which we are more or less distinctly cognizant at the time, 

 and of which our subsequent remembrance in the waking state varies greatly 

 in completeness. The chief peculiarity of this state of dreaming appears to 

 be, that there is an entire suspension of Volitional control over the current 

 of thought, which flows on automatically, sometimes in a uniform coherent 

 order, but more commonly in a strangely incongruous sequence. The former 

 is most likely to occur when the mind simply takes up the train of thought 

 on which it had been engaged during the waking hours, not long previously; 

 and it may even happen that, in consequence of the freedom from distraction 

 resulting from the suspension of external influences, the Reasoning processes 

 may thus be carried on during sleep with unusual vigor and success, and 

 the Imagination may develop new and harmonious forms of beauty. 2 The 



1 Such, for example, as that of Samuel Chilton (Phil. Trans., 1694), and that of 

 Mary Lyll (Trans, of Koy. Soc. of Edinb., 1818). 



1 Thu-i, Comloivi't saw in his droams tho final steps of a difficult calculation which 

 had puzzled him during the day; and Condillac tells us that, when engaged in his 



