CHAPTER V. 

 THE CENTEAL NERVOUS SYSTEM. 



SECTION I. 



THE essential characteristic of life is the power of reaction to a 

 stimulus, and in the higher animals this reaction is effected through 

 the central nervous system, and constitutes reflex action. The life of 

 the individual is to a large extent made up of a long series of reflex 

 acts, varying in complexity, and carried out in response to stimuli 

 arising in the outer world or within his own body. 



In response to external stimuli, the animal acts as a whole and 

 carries out movements directed to atta'ck' or defence, to the procuring 

 of food, and the like ; in response to internal stimuli, the activities 

 of the different organs of the body are co-ordinated in such a way 

 that the individual behaves as such and not as a group of independent 

 organs. 



Instances of this nervous control will be referred to in connection 

 with the work of the heart, the function of respiration, the production 

 of the digestive juices, and the other functions which are concerned 

 with animal life. 



But the activities of the central nervous system are not limited 

 to the management of these vital processes. In addition, the central 

 nervous system is the seat of those processes which are concerned with 

 conscious existence. Impressions are conveyed to the brain from the 

 outer world, and give rise to sensations of smell, taste, hearing, sight, 

 and touch, and these in their turn call forth emotions, such as pleasure 

 or pain. The activities of the central nervous system find expression 

 in muscular movement, resulting in locomotion, speech, writing, or 

 gesture. In the study of the various nervous functions, the science 

 of Physiology has to deal with the mechanism by means of which 

 afferent impressions are received and conducted to the nerve centres, 

 with that by which they are associated in these centres, arid with the 

 further mechanism by means of which efferent impulses are transmitted 



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