SECTION III. 



THE NERVOUS SYSTEM. 



CHAPTER I. 



GENERAL STRUCTURE AND FUNCTIONS OF 

 THE NERVOUS SYSTEM. 



THE nervous system is an apparatus of communication, by which 

 the various parts of the body are brought into relation- with each 

 other, and different organs excited to harmonious or alternating action. 

 Its effects are produced by an influence transmitted from one region to 

 another, stimulating or modifying the animal functions according to 

 the requirements of the system at large. It differs in its properties 

 and mode of action from the other anatomical structures of the body, 

 to which it is superadded for their regulation and control. 



The specific physiological properties or modes of activity, belonging 

 to a bodily organ, may often be called into operation by a direct 

 stimulus or exciting cause. The poles of a galvanic battery, applied 

 to the muscles of a frog's amputated leg, produce contraction and move- 

 ment ; a solution of atr opine dropped on the cornea of a living animal, 

 when absorbed and brought in contact with the iris, causes a change 

 in the condition of its fibres and a dilatation of the pupil ; and if the 

 heart of a frog, after removal from the body, be touched with the point 

 of a needle, it repeats the movement of an ordinary pulsation. In 

 these instances, the physiological act is in response to a stimulus oper- 

 ating directly on the tissues of the organ. 



But this is not the mode in which the animal functions are excited 

 during life. The stimulus which calls into action the living organs 

 is not direct, but indirect, in its operation. In the normal condition, 

 the muscles are never made to contract by an external stimulus applied 

 to their own fibres, but by one which operates on some other organ, 

 adjacent or remote. The functional activity of the glands is increased 

 or diminished by causes acting on other parts ; as where a flow of 

 saliva from the parotid is produced by food introduced into the mouth, 

 or where the cutaneous perspiration is modified by mental conditions. 

 The various organs are thus connected with each other by a mutual 

 sympathy which regulates their physiological action ; and this connec- 

 tion is established by means of the nervous system. 



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