38 THE HUMAN BODY 



The Motor System. In all members of the animal kingdom 

 with the exception of certain parasites adaptation to the environ- 

 ment is secured mainly through movement. Both for obtaining 

 food and for escaping danger movements either of the whole body 

 or of parts of it are constantly being resorted to. 



In all higher animals the motor mechanism is made up of skeletal 

 muscles, which by their action upon the movable bones of the 

 jointed skeleton bring about the various bodily movements. 



The Receptor System. It is obvious that the Body cannot 

 execute movements adapting it to its surroundings unless it knows 

 what its surroundings are. A blind man, be he never so agile, can- 

 not escape the onward rush of the approaching car while he is 

 ignorant of its coming. He will starve in the midst of abundant 

 food if he does not know where it is to be found. 



The Body obtains knowledge of its environment by means of a 

 set of structures known as the sense-organs. In these the property 

 of irritability is developed to a high degree, and so long as they all 

 function properly not much that is important for the organism 

 to know about need escape its knowledge. 



The Conductive System. Organs for making movements and 

 organs for receiving impressions from the surroundings are not of 

 themselves adequate to the maintenance of adaptation. It is 

 necessary that the information gained by the sense-organs be trans- 

 mitted to the muscles so that their movements may correspond to 

 the requirements of the situation. This function is performed by 

 the nervous system. The conduction of stimuli from sense-organs 

 to muscles is not, however, a simple matter. Impressions are con- 

 tinually coming into the body by way of a number of different 

 channels. Movements must be made not in obedience to any one 

 of these impressions by itself but for the advantage of the whole 

 Body as indicated by all of them taken together. To this end a 

 certain part of the nervous system is adapted for receiving all 

 sorts of incoming-stimuli and before passing them on to the motor- 

 organs combining and modifying them to produce the best results. 



The systems which are not immediately concerned in the adapta- 

 tion of the Body to its environment but which serve rather to keep 

 it in proper condition for activity may next be considered. Ac- 

 tivity in the Body involves the manifestation of energy, but in its 

 energy relations the Body is on exactly the same plane as any 



