270 



A SSOCIA TION FIBERS. 



Writing 



Speech 



the most practical definition of sensation that we can give is 

 that it is the interpretation that the cells of the gray matter of 

 the brain give to the nerve impulses that come from without. 



This will apply to 

 ordinary sensa- 

 tions. But sensa- 

 tions may be sub- 

 jective; that 

 is, they may 

 exist without 

 any corresponding 

 external exciting 

 cause. For some 

 unexplained reason 

 the cells of the 

 brain are active ; 

 and their activity, 

 however caused, 

 constitutes what we 

 call a sensation. 

 Certain drugs, such 

 as hashish, may 

 excite an unusual 

 degree of cerebral 

 activity. Here the 



Fig. 85. Connection of Brain Centers by Association a C t i O H is TOUSed 



Fibers. (After Landois & Stirling.) 



(The dotted lines from the hand, mouth, and eye 

 represent afferent fibers from the skin, muscles, and 

 joints of the hand, lips, orbit, etc.) 



through afferent 

 nerves, but through 

 unusual channels ; 

 that is, the subject 



sees, but not through the nerves of sight. Many Hallucina- 

 tions are explainable to a certain degree ; others we may not 

 account for. 



