RESPIRATION 217 



Before going further let us try to form some sort of conception 

 as to what is occurring in a gland cell during the secretion of 

 oxygen. On the side of the cell next the lumen of the duct we have 

 a pressure of oxygen which may be 1,000 times as great as on the 

 side next the capillaries ; and yet oxygen may be passing inwards 

 from the capillaries towards the duct. The cell is permeable to 

 oxygen : for oxygen is passing through it. Yet the oxygen cannot 

 be free to dissolve in the ordinary way in the "protoplasm" of 

 the cell : for if this were the case the oxygen would run backwards 

 through the cell like water through a sieve. At a pressure of 1 1 5 

 atmospheres, to go back to our concrete example, 100 volumes of 

 water at ioC would take up 430 volumes of oxygen (measured 

 at o and 760 mm.) ; and if the oxygen were as freely soluble in 

 the cell water as in ordinary water the swim bladder would leak 

 outwards at a quite hopeless rate. If we start by looking upon 

 "living protoplasm" as a mere solution and suspension of colloid 

 and other material, we may as well give up the attempt to get any 

 insight whatever into even the most rudimentary physiological 

 processes. 



When we take a broad general view of the phenomena of life, 

 one of the most fundamental facts that appears is that the com- 

 position of each organism or part of an organism is distinctly 

 specific. The percentage and nature of each of the substances 

 which we can recover on disintegrating the living tissue are spe- 

 cific ; and the more we learn about the nature of these substances 

 the more clearly does this specific character emerge. It is evidently 

 no mere accident that muscle yields so much potassium, so much 

 phosphoric acid, so much water, and so much of various proteins. 

 These substances must be present in some kind of combination in 

 the living "substance" ; and if so the living substance cannot be 

 regarded as a mere solution of free molecules. The molecules are 

 in some sense bound, as they are in a solid ; and in so far as this is 

 the case the living substance must in certain respects behave as a 

 solid, impervious to the free passage of material by diffusion. 

 The layer of thin flattened epithelium lining appears to be gas- 

 tight (to oxygen at least) except where it covers the oval. At this 

 point the layer allows gas to pass freely. 



From this point of view we can understand why the living cells 

 of the oxygen-secreting gland should be gas-tight, or nearly so, 

 against diffusion backwards, but we have not yet considered how 

 the gas passes forward through them during secretion; and if 



