78 BIOLOGY AND PHYSICAL SCIENCES 



physiological study that of nutrition. But 

 the more closely living activity in general is 

 examined, the more clear does it become that 

 all living activity is structural or metabolic 

 activity, either directly or indirectly. The 

 changes in the retina when light falls upon it 

 are metabolic or structural activities. The 

 same is true of the activities of nerve cells, 

 muscle cells, gland cells, or any other living 

 cells ; and the gross visible movements of 

 the body, no less than its gross visible struc 

 ture, are but the outer sign of metabolic 

 activity. 



The body can also be affected mechanically 

 or chemically by influences from without ; 

 but effects so produced are of not the 

 slightest interest to a biologist, except in so 

 far as they may be connected with living 

 activity. 



The living structure is evidently organised : 

 that is to say every part of it bears a definite 

 relation to every other part. As, however, 

 the structure is the outcome of metabolic 

 activity, it follows that the metabolic activity 

 of the living body is also organised, every 

 aspect of it bearing a definite relation to 



