CHAPTER 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 todo. These 
different parts are called organs of the body, and the things 
they do are called their functions. The nostrils, trachee, 
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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