XXV. THE NERVOUS SYSTEM. 

 Structure. 



The study of the preceding pages has taught us the part 

 played by the various organs and structures in our body 

 in the operation of tearing down and building up the 

 human machine, and the manner in which these operations 

 keep it in running shape. We have also seen that all 

 these systems are interrelated, the work of one being 

 directly dependent upon the activity of another, while in 

 our life we are conscious of the fact that these many parts 

 are all acting harmoniously and with comparatively little 

 attention on our part. For example, in so simple an 

 operation as walking, every system in our body is con- 

 cerned. The muscles contract and expand, and in so doing 

 produce motion, consume oxygen and nutriment, and give 

 off wastes and carbon dioxide. The necessity for food 

 must be met by a quickened flow of blood, while the in- 

 crease in appetite warns us to supply the digestive system 

 with more nourishment. The lungs must expand and con- 

 tract to furnish the blood its oxygen, and the excretory 

 system must increase its activity a little to dispose of the 

 wastes. Even in our sleep these systems are all more or 

 less active, and the needs of one part of the body can be 

 met only by special effort on the part of all the others. 



Manifestly such a harmonious action would be utterly 

 impossible unless the organs had some means of com- 

 munication by means of which the needs of one might be 



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