THE BRAIN 137 



in mastication, -swallowing, coughing, and many other 

 acts. They are also contracted when simple animal 

 outcries are made. The power of speech is much more 

 than the power to command these muscles; it is the power 

 to make them express thought. When the alleged 

 center suffers from local impairment through disease 

 the disorder which results is not a paralysis but a loss 

 of the ability to make thought vocal. This is what 

 we mean by motor aphasia. 



Of course all the evidence that can be gathered for 

 the existence of a speech center is based upon post- 

 mortem discoveries of abnormal conditions at this 

 place when disorders of language have developed during 

 life. The correlation of disease with loss of coherent 

 speech has indicated that in left-handed persons the 

 control of speech is dependent on the right half of the 

 brain, that is, on the side which also presides over 

 the skilled hand. So in the average individual who is 

 right-handed the mechanisms for speech and for the 

 more competent hand are again associated but on the 

 left side. Claims for the existence of centers for read- 

 ing and writing and for the interpretation of words heard 

 have been strongly urged. Further detail would be out 

 of place in a book of this size. 



Summary. The cerebrum, like other parts of the 

 brain, is a place where impulses are received and so 

 applied as to cause other impulses to be sent out. But, 

 far beyond any other division, it is influenced by the 

 passage of impulses through it. It is not so much 

 organized by inheritance as by experience. Its capacities 

 at birth are potential; the manner of their realization 

 cannot be foretold. In old age it has registered the 

 story of a life; it is the material basis of memory and 

 so of every individual attainment. The suggestions 

 relating to personal hygiene will be given a place in 

 another chapter. 



