58 NERVOUS SYSTEM. 



However, physiology goes no farther; it has but little 

 means of knowing what is the internal and intimate nature 

 of the mechanism of the seat of thoughts or ideas / as, for 

 example, we know that softening of the brain, characterized 

 sometimes by gay and sometimes by sad thoughts, has its 

 seat in the gray cortical substance ; little doubt can thus be 

 had that the seat of thought is in a general way located in 

 this substance ; as, moreover, a large number of vivisections 

 would seem to prove. 



The central phenomenon of volition is equally beyond our 

 study, at least when it forms no part of association of ideas. 

 Still, we know at least that injuries in the brain destroy those 

 manifestations called voluntary, and paralyze the voluntary 

 movements of the opposite side ; viz., movements of the right 

 side of the body are abolished by a lesion havi?ig its seat in 

 the left hemisphere of the brain, and vice versa. The centrif- 

 ugal conducting nerves of volition decussate on leaving the 

 brain. However, this decussation must not be localized only 

 at the lower extremity of the pyramids ; it extends through- 

 out a larger space from this point, to the most anterior portion 

 of the protuberance. A lesion which may be seated in any 

 part of this extent may then affect at the same time fibres 

 which have already crossed (decussated) and those which 

 have not ; thus there may ensue those peculiar alternating 

 paralyses, which for example maybe located in the right side 

 of the face and on the left side of the remainder of the body 

 (see physiology of the spinal cord, pp. 43 and 45). 



We find equally in the case of phenomena of motility as in 

 the volitional phenomena associations analogous to those 

 which we have explained in regard to sensation or sensi- 

 bility. Thus a centre becoming the seat of a lively action 

 can do so to such a degree that its activity may extend even 

 to the neighboring centres. 



This is the mechanism of all the little convulsive move- 

 ments and also of involuntary associated movements. This 

 also explains why it is that during a very intense and general 

 muscular exertion, as for instance when lifting a heavy weight, 

 a person involuntarily contracts the frontal muscles ; as, also 

 when sneezing, the eyes are involuntarily closed, etc. 



Thus we might as a general rule state that all our volun- 

 tary movements are associate d movements, because we cannot 



without the intervention of the brain. The cerebral memory is 

 simply in a higher degree a sort of medullary memory. 



