62 ORGANISM AND ENVIRONMENT 



suddenly reminds us that it is alive : for it begins to do 

 something which at once recalls living things when it 

 delivers oxygen at a higher pressure than that at 

 which it receives it. The passage of oxygen molecules 

 is accelerated in the inward direction, and this accel- 

 eration applies to them alone, and not to other mole- 

 cules, so it is selective. It does not occur in a non- 

 living membrane, and its presence is evidently depend- 

 ent, firstly upon the peculiarities of the living mem- 

 brane, and secondly upon the presence of a special 

 stimulus acting on the membrane. We know, also, 

 that the specific peculiarities of living tissues depend 

 upon the maintenance of their external environment. 

 Hence we can say that the acceleration depends, not 

 only upon the factors just mentioned, but upon the 

 integrity of the general environment of the mem- 

 brane in more familiar words, upon its nutrition, 

 temperature, etc., and upon the regulated removal of 

 so-called waste products. 



Active secretion of oxygen is not a new phenomenon 

 in physiology. It is now over a century since the 

 famous physicist Biot made the discovery that the gas 

 in the swim bladder of deep sea fishes is nearly pure 

 oxygen. The pressure of oxygen in sea water is only 

 about a fifth of an atmosphere, and is doubtless less 

 than a tenth of an atmosphere in the blood circulating 

 outside the walls of the swim bladder. Yet inside the 

 swim bladder the oxygen pressure in the case of deep 

 sea fishes may be 100 atmospheres or more. It was 

 shown in 1877 by Moreau that fishes secrete just 

 sufficient oxygen into their swim bladders to bring 



