THE NERVOUS SYSTEM. 121 



gether beyond the control of the will. Where convulsions 

 arise from sympathy, as in teething, or irritation of the 

 bowels, the painful impression is transmitted to the brain, 

 and hence reflected back upon the muscles. Thus pricking 

 the finger with a needle has thrown a nervous woman into 

 spasms, by exciting the brain to action. 



32. If a nerve which supplies any voluntary muscle be 

 divided, that muscle will no longer contract. This proves 

 that the influence of the brain is necessary. ; In palsy, there 

 is generally some disease of the brain, as the rupture of a 

 small blood-vessel, or tumour growing in its substance. J Here 

 the power of the will, or the faculty of volition is not des- 

 troyed ; but the brain is unable to carry into execution the 

 command of the will, as when a person is very willing to 

 take a load upon his back, but he happens not to have suffi 

 cient strength to carry it, 



33. During sleep, all voluntary motion is suspended, be. 

 cause the brain ceases to act. In those cases where persons 

 walk and talk in their sleep, the brain is not entirely dormant. 

 Some of the senses seem to be awake at times, while the 

 others are asleep. That is, those portions of the brain which 

 are excited to action by certain nerves of sense, are awake 

 and active, while other portions are dormant and at rest. 

 i*The brain needs repose and relaxation, as much as the 

 muscles. Periodical cessation of effort is necessary to both, 

 in order to a vigorous performance of their respective func- 

 tions. 



Questions. What functions belong to the nervous system ? What 

 does it embrace ? How many departments ? . What offices does each 

 perform ? What marks characterize each ? When their functions are 

 interrupted, what happens ? From what portion of the spinal mar- 

 row do the nerves of motion arise ? the nerves of sensation ? Des. 

 cribe the brain ; its divisions ; its membranes ; dura mater ; pice 

 mater ; What are the convolutions of the brain ? How do the cine, 

 ritious and medullary portions differ from each other ? Which is ex 

 terior ? How many cavities in the brain ? What are they called '? 



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