THE NERVOUS SYSTEM. 



81 



of the white matter is essentially the same as in the 

 brain, but the proportion varies in different parts of 

 the cord, the white predominating in the cervical re- 

 gion and the gray being much better developed in the 

 lumbar region, where the nerve cells for control of the 

 lower extremities occur. The gray is least well devel- 

 oped in the dorsal region. Through the center of the 

 cord runs a small hole or canal filled with cerebro-spinal 

 fluid, the central canal of the cord. 



FIG. 29. Functional areas of the cerebral cortex, left hemisphere. 

 (A. A. Stevens.) 



The brain is the seat of intelligence and will, the cen- 

 ter of all voluntary action. Molecular change in some 

 part of the cerebral substance is the indispensable 

 accompaniment of every phenomenon of conscious- 

 ness. Indeed, -the brain is never in a state of complete 

 repose, there being dreams even during sleep. The 

 brain is not sensitive to injury in the sense of pain. It 

 can be lacerated without much pain. 



Various centers exist in the brain, of which the most 

 important perhaps is the motor center. The visual 

 center is in the occipital lobe, the auditory center in the 



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