SECT. II PHYSIOLOGY 177 



study. It has been seen, however, that it is not possible to under- 

 stand the function from the structure to the same degree in the case 

 of the plant as it is in that of a machine. This holds still more 

 strongly for the more specific vital phenomena. While it is true 

 that the phenomena of life can as yet not be thoroughly explained, 

 this does not negative the conviction that they only differ from the 

 processes in inorganic bodies by their much greater complexity ; 

 in principle a physico-chemical explanation of vital phenomena 

 can be attained. 



The most important phenomena of life are exhibited in the 

 following ways : — 



1. An organism, which appears to us as an individual, does not 

 consist of the same unchanged material, even when no further growth 

 in size is taking place. ^Yhile its external form remains constant, 

 progi'essive changes go on internally. New substances are taken 

 up from without, are transformed within the plant, and are again given 

 off from it. The organism has a metabolism. Inorganic nature 

 offers us no process analogous to this. 



2. As a rule, however, metabolism does not proceed so that the 

 absorption and giving-off of material are equal, but more is absorl»ed 

 than is given off. The mass of the organism is increased, it grows. 

 Growth is also known in the cases of chemical precipitates or deposits, 

 and of crystals. In these cases it tends to proceed in such a way 

 that no essential change of shape takes place (crystals), or that the 

 changes in shape are accidental and irregular (precipitates). The 

 organism, on the other hand, assumes by changes of its form quite 

 definite shapes, which follow in regular order. It passes through a 

 DEVELOPMENT which leads sooner or later to the production of new 

 organisms or daughter individuals ; reproduction takes place. 

 Growth, development, and reproduction are processes highly charac- 

 teristic of living beings. 



3. Lastly, organisms exhibit powers of movement ; they either 

 change their positions bodily, or they bring larger or smaller parts of 

 their bodies into other positions. Since inorganic bodies and dead 

 organisms may exhibit movements, it is only the kind of movement 

 and the means by which it is brought about that are characteristic 

 of living beings. 



In nature the three processes mentioned above, metabolism, 

 development, and movement, usually go on simultaneously. Meta- 

 bolism without movement of the substances concerned is impossible ; 

 development is liound up with metabolic changes and with movements ; 

 and, lastly, movements cannot occur without metabolism. Neverthe- 

 less, we may for descriptive purposes consider the three processes 

 separately, and thus divide Physiology into the following sections : 



1. The study of metabolism or chemical physiology, which may 

 also be termed the physiology of nutrition. 



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