TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 37 



to consider first the systems which are particularly concerned in 

 adapting the Body to its environment. 



Adaptive Systems. The Motor System. In all members of the 

 animal kingdom with the exception of certain parasites adaptation 

 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. 



There are many types of movement in animals which are not 

 concerned immediately with adaptation to the environment. The 

 movements of breathing, for example, the beat of the heart, and 

 the activities of the stomach. These have to do with maintenance. 

 As emphasized in the last paragraph, however, the classification 

 of systems as adaptive or maintenance is for convenience, and with 

 reference to their most conspicuous functions, and is not to be 

 taken as excluding the systems in one group from having important 

 activities in the other. 



The Supporting System. In all but the very simplest animal 

 forms movements are made effective by the action of the muscles 

 upon certain of the supporting tissues. These tissues play, there- 

 fore, a very real, although passive part in adaptation. By includ- 

 ing the supporting system among the adaptive systems of the Body 

 we emphasize the importance of the supporting structures in mak- 

 ing muscular action effective, although here again we must bear in 

 mind that they are also intimately associated with other systems 

 whose chief function is maintenance. 



The Receptor System. It is obvious that the Body cannot 

 ( 'xocute 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. 



