514 HANDBOOK OF PHYSIOLOGY. 



adult age; and that, as their energy decreases in old age, there is not a 

 gradual and equal diminution of power in all of them at once, but, on 

 the contrary, a diminution in one or more, while others retain their full 

 strength, or even increase in power. 5. The plurality of cerebral organs 

 appears to be indicated by the phenomena of dreams, in which only a 

 part of the mental faculties are at rest or asleep, while the others are 

 awake, and, it is presumed, are exercised through the medium of the 

 parts of the brain appropriated to them. 



Unconscious Cerebration. In connection with the above, some 

 remarkable phenomena should be mentioned which have been described 

 as depending on an unconscious action of the brain. 



It must be within the experience of every one to have tried to recol- 

 lect some particular name or occurrence; and after trying in vain for 

 some time the attempt is given up and quite forgotten amid other occu- 

 pations, when suddenly, hours or even a day or two afterwards, the 

 desired name or occurrence unexpectedly flashes across the mind. Such 

 occurrences are supposed by many to be due to the requisite cerebral 

 processes going on unconsciously, and, when the result is reached, to our 

 all at once becoming conscious of it. 



That unconscious cerebration may sometimes occur, is likely enough; 

 and it is paralleled by the unconscious walking of a somnambulist. But 

 many cases of so-called unconscious cerebration are better explained by 

 the supposition that some missing link in the chain of reasoning cannot 

 at the moment be found; but is afterwards, by some chance combination 

 of events, suggested, and thus the mental process is at once, with the 

 memory of what has gone before, completed. 



Again, in the vain endeavor to solve a difficult, or it may be an easy 

 problem, the reasoner is frequently in the condition of a man whose 

 wearied muscles could never, before they have rested, overcome some 

 obstacles. In both cases, of brain and muscle, after renewal of their 

 textures by rest, the task is performed so rapidly as to seem instan- 

 taneous. 



Sleep. All parts of the body which are the seat of active change 

 require periods of rest. The alternation of work and rest is a necessary 

 condition of their maintenance, and of the healthy performance of their 

 functions. These alternating periods, however, differ much in duration 

 in different cases; but, for any individual instance, they preserve a gen- 

 eral and rather close uniformity. Thus, as before mentioned, the periods 

 of rest and work, in the case of the heart, occupy, each of them, about 

 half a second; in the case of the ordinary respiratory muscles the periods 

 are about four or five times as long. In many cases, again (as of the 

 voluntary muscles during violent exercise) while the periods during 

 active exertion alternate very frecuently, yet the expenditure goes far 

 ahead of the repair, and, to compensate for this, an after repose of some 

 hours become necessary; the rhythm being less perfect as to time, than 

 in the case of the muscles concerned in circulation and respiration. 



Obviously, it would be impossible that, in the case of the Brain, there 

 should be short periods of activity and repose, or in other words, of con- 

 sciousness and unconsciousness. The repose must occur at long inter- 

 vals; and it must therefore be proportionately long. Hence the necessity 



