SECTION II. 

 THE NERVOUS SYSTEM. 



CHAPTER I. 



GENERAL STRUCTURE AND FUNCTIONS OF THE 

 NERVOUS SYSTEM. 



THE nervous system is an apparatus of intercommunicating fibres and 

 cells, disseminated throughout the body, and standing in anatomical 

 connection with the various organs of the animal system. It has pro- 

 perties which are different from those of the other organized tissues, 

 and the effect of its operation is to bring the active phenomena of vari- 

 ous parts of the body into a definite relation with each other, and with 

 those of the outside world. It is therefore a medium of communica- 

 tion, by which the different animal functions are associated together in 

 harmonious action, and are stimulated or modified according to the 

 demands of the system itself or the varying influence of external condi- 

 tions. 



Each organ and tissue of the body possesses, independently of the 

 nervous system, certain characteristic properties or modes of activity, 

 which may be called into operation by any appropriate stimulus or 

 exciting cause. If the heart of a frog, after its removal from the body, 

 be touched with the point of a steel needle, it contracts and repeats 

 very nearly the movement of an ordinary pulsation. If the leg of the 

 same animal be separated from the thigh, the integument removed, and 

 the poles of a galvanic batter}^ applied to its exposed surface, a mus- 

 cular contraction takes place at the moment the electric circuit is com- 

 pleted. The application of heat, friction, or an irritating liquid to a 

 particular part of the integument brings on a local redness which again 

 subsides after the removal of the exciting cause ; and a solution of 

 belladonna dropped upon the cornea, when absorbed by the tissues and 

 brought in contact with the iris, produces a change in the condition of 

 its fibres and a dilatation of the pupil. In these instances, the organ 

 which performs the vital act is excited by the direct application of a 

 stimulus to its own tissues. 



But this is not the mode in which the natural functions of the animal 

 system are excited during life. The physiological stimulus which calls 



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