592 THE HUMAN BODY. 



doubtful. Stimulation of many cerebral regions is fol- 

 lowed by no results; and that of others by movements the 

 power of voluntarily executing which is not, even tempo- 

 rarily, lost when that brain-part is removed; these cerebral 

 areas have been supposed to be concerned with mental 

 faculties other than volition, the stimulation exciting sen- 

 sations, perceptions, or emotions; but this is still very 

 doubtful. Localized disease of regions of the human brain 

 has, so far, given better results than physiological experi- 

 ment on the lower animals. The power of using words to 

 express ideas seems intimately connected with a small area 

 on the fore part of the left cerebral hemisphere, and to be lost 

 (producing the condition known as aphasia) when that 

 part is diseased; and many cases have been recorded in 

 which a wound of the skull has been followed exactly by 

 loss of the power of voluntarily moving those muscle groups 

 which (accepting the results of electrical stimulation in 

 the lower animals) might be supposed to be normally excited, 

 through the will, from the cerebral area injured. Absence of 

 recovery unless the brain injury is cured seems, moreover, 

 to be the rule in man; while as we have seen this is not the 

 casein lower animals: this may, perhaps, indicate a more 

 precise division of physiological labor in the human brain; 

 which is a priori probable, considering its great superiority 

 as a mental apparatus. 



What the use of two cerebral hemispheres is, cannot at 

 present be said. Injury of one produces its main effects, 

 so far as sensation and motion are concerned, on the oppo- 

 site side of the Body; but other faculties, as that of using 

 (Speech, seem located on one side only; and in the brains of 

 the higher human races the surface markings on the two 

 sides are not perfectly symmetrical, which may indicate 

 some difference in function. It has been suggested that in 

 many cases we only learn to use one side of the brain, and 

 that the other is in reserve in case of injury or disease; but 

 the evidence is inconclusive: a good deal may, however, be 

 said for the view that a good deal of brain in every one's 

 skull is never used. It is there untaught but ready to be 

 educated. 



