138 THE HUMAN BODY. 



the encephalon which preside over motion. As for the seat 

 of the intellectual faculties, we cannot doubt that it is 

 situated in the encephalon, but science possesses no exact 

 data regarding the part played by the different organs con- 

 tained in the cranial cavity in the elaboration of thought, 

 and the soul cannot perceive the mysterious tie which unites 

 it with these organs. 



The nervous system, which gives motion and sensibility to 

 every part of the body, is itself absolutely dependent on the 

 circulation. It determines ' and regulates its progress by 

 exciting the action of the heart, but it must itself be excited 

 by the afflux of blood which the arteries bring to it; and just 

 as the heart slackens or even stops its action under the in- 

 fluence of certain impressions, the functions of the brain, 

 spinal cord, and nerves are inevitably suspended when the 

 blood does not come to awaken the nervous energy. Any 

 impediment to circulation of the blood induces paralysis, 

 more or less complete, in the parts beyond the obstacle, and 

 no sooner does the nourishing fluid stop its motion, or even 

 slacken, in. its course toward the brain, than syncope or 

 fainting supervenes; that is to say, the functions of the 

 encephalon are weakened or cease altogether. 



In giving a summary description of the nervous system, 

 we have proceeded from centre to circumference, but in 

 describing the nervous functions the reverse course seems 

 preferable. 



Functions of the sensitive and motor spinal nerves. Sensa- 

 tion may be destroyed in a portion of the body while it still 

 possesses the power of motion, and conversely a limb may 

 lose the power of motion and still remain sensitive to ex- 

 ternal impressions. This independence of motion and of 

 sensation revealed to the physiologists of antiquity the exist- 

 ence of two orders of nerves, one sensitive the other motor. 

 Boerhaave and other modern anatomists accepted these 

 doctrines, on which Lamarck based and promulgated theories 

 nearly approaching the truth; but Sir Charles Bell was the 

 first to distinguish, by actual experiment, the nerves of 

 sensation from those of motion, and to show that they sprang 

 from two distinct portions of the spinal cord. 



