CHAPTEE IV 



FUNCTION AND STRUCTURE 



37. Organs and functions. An animal does certain things 

 which are necessary to life. It eats and digests food, it 

 breathes in air and takes oxygen from it and breathes out 

 carbonic-acid gas ; it feels and has other sensations ; it pro- 

 duces offspring, thus reproducing itself. These things are 

 done by the simplest animals as well as by the complex 

 animals. But while with the simplest animals the whole 

 body (which is but a single cell) takes part in doing each 

 of these things, among the complex animals only a part 

 of the body is concerned with any one of these things. 

 Only a part of the body has to do with the taking in of 

 oxygen. Another part has to do with the digestion of 

 food, and another with the business of locomotion. These 

 parts of the body, as we know, differ from each other, and 

 they differ because they have different things to do. These 

 different parts are called organs of the body, and the things 

 they do are called their functions. The nostrils, tracheae, 

 and lungs are the organs which have for function the pro- 

 cess of respiration. The legs of a cat are the organs which 

 perform for it the function of locomotion. The structure 

 of one of the higher animals is complex because the body 

 is made up of many distinct organs having distinct func- 

 tions. The things done by one of the complex animals are 

 many ; around each of the principal functions or necessary 

 processes, as a center, are grouped many minor accessory 

 functions, all helping to make more successful the accom- 



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