130 HAND-BOOK OF PHYSIOLOGY. 



deranged, while all the rest are performed in a natural manner. 4. The 

 same opinion is supported by the fact that the several mental faculties 

 .are developed to their greatest strength at different periods of life, some 

 being exercised with great energy in childhood, others only in 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 pre- 

 sumed, 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 ha^ve 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 occupations, 

 when suddenly, hours or even a day or two afterward, 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 be- 

 coming 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 afterward, by some chance combination 

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

 ory 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 instantaneous. 



Aphasia. From the apparently greater frequency of interference 

 with the faculty of speech in disease of the left than of the right half of 

 the cerebrum, it has been thought that the nerve-centre for language, in- 

 cluding in this term all articulate expression of ideas, is situated in the 

 left cerebral hemisphere. A large number of cases are on record in 

 which aphasia, or the loss of power of expressing ideas in words, has been 

 associated with disease of the posterior part of the lower or third frontal 

 convolution on the left side. This condition is usually associated witli 

 paralysis of the right side (right hemiplegia). The only conclusion, how- 



