218 RESPIRATION 



"living material" behaved like an ordinary solid no such explana- 

 tion would be forthcoming. But evidently a living cell does not 

 behave like an ordinary solid : for it is constantly taking up and 

 giving off material, not merely during secretion, but at every 

 moment of its existence. This is evident from a general considera- 

 tion of the phenomena of nutrition, and becomes still more evident 

 if by altering the environment of a cell we disturb the labile 

 balance between living cells and their surrounding liquids. In the 

 secretion of oxygen and many other substances, such as urea, 

 sugar, salts, etc., the substance taken up on one side of the cell is 

 given off in the same form on the other side. In the processes of 

 ordinary nutrition, on the other hand, the taking up and giving 

 off may be on the same side of the cell, and the substance given off 

 may be in a different chemical form from that taken up. We have 

 no reason to believe, however, that there is any fundamental dis- 

 tinction between the taking up and giving off during ordinary 

 nutrition and during secretion. Nearly a century ago Johannes 

 Miiller, at the end of his famous memoir on secreting glands, 10 

 after pointing out that his observations negatived the mechanical 

 theories of secretion then current, suggested that secretion must 

 be regarded as a process akin to growth, the only difference being 

 that whereas in ordinary growth the material deposited tends to 

 remain where it is, in secretion it is always being carried away 

 and replaced. Johannes Miiller's theory was bound up with his 

 vitalistic physiology, and the clue which he was grasping at was 

 swept from the hands of physiologists by the wave of mechanistic 

 speculation which passed over physiology about the middle of 

 last century. But now that we know from nearly a century of 

 painful experimental investigation what to the genius of a great 

 biologist like Miiller was evident enough, that mechanical theories 

 of secretion are impossible, we can take up the clue again. 



When oxygen (or indeed any other substance entering into 

 cell metabolism) is taken up on one side of the cell, we are led by 

 the experimental facts to assume that the oxygen enters into 

 easily dissociable chemical combination. Were this combination 

 not easily dissociable we could not understand why a cell should 

 be so enormously sensitive, as we shall see later that it is, to 

 changes in the concentration of oxygen and other substances in 

 its immediate environment. Now all we know about cell metab- 

 olism points to the conclusion that the balance of stability at any 

 one part of the cell depends on the balance of stability at other 



"Johannes Miiller, De Glandularum Secernentium Structura Penitiori, 1830. 



