METABOLISM 191 



stance, or at least proceed with extreme slowness in its absence. 

 Such, for example, is the action of chlorin on metallic copper 

 or iron, or of oxygen upon many of the elements even at high 

 temperatures. 



Aside from these considerations, however, the importance of 

 the part played by water in the animal economy is sufficiently 

 obvious, while it constitutes in most cases more than half of 

 the weight of the body (97) and therefore may be regarded 

 as having structural importance. 



