644 MANUAL OF PHYSIOLOGY. 



coming sensory impressions ; others say it gives rise to motor 

 paralysis. 



Human pathology helps us but little, for it is impossible to say 

 whether a given lesion simply abolishes the function of the part 

 or acts as an irritant to it, or in some degree produces both these 

 effects. Local lesions of the optic thalami have been met with, 

 iu some of which sensory, and in others, both sensory and motor 

 defects have been observed in the patients. 



We must, then, remember that the occurrence of motion as the 

 result of stimulation, or the absence of muscular power as the 

 result of destruction of the optic thalami must not be accepted as 

 conclusive evidence of the motor function of the active elements 

 the nerve-cells of this part, because these results may depend 

 on the indirect influence of the sensory impulses coming from 

 these cells. 



CEREBRAL HEMISPHERES. 



It is now universally regarded as a recognized fact that in man 

 the hemispheres of the brain are the seat of the mental faculties 

 perception, memory, thought, and volition. The cerebral cortex 

 is the part of the nervous system in which the subjective percep- 

 tion of the various sensory impulses takes place, and in which 

 impulses are converted into impressions or mental operations. It 

 is in the cortical nerve-cells the so-called voluntary impulses, 

 causing movement of the skeletal muscles, have their origin. It 

 is thus a sensory and a motor organ. But it has a far wider range 

 of function than is expressed by saying it is both sensory and 

 motor ; indeed, in this it would be no better than the other nerve- 

 centres in the spinal cord, etc. The cells of the cortex of the 

 brain seem to differ from those of the lower nerve-centres (which 

 can also receive, and at once send out, corresponding impulses), 

 iu this : when an impulse arrives at the cerebral cells, it there 

 excites a change, which, besides producing an immediate effect, 

 leaves a more or less permanent impression ; the impression per- 

 sists, and if the cell be well supplied with chemical energy in 

 the shape of nutriment, the impression may be reproduced at a 

 subsequent period. This revival of impressions, the effects of 



