THE RESPIBATION OF AN INLAND LAKE 341 



The absorption and distribution of oxygen constitute one of the 

 fundamental problems of life for any large and active organism. The 

 difficulty of solving the problem is increased by the fact that no large 

 reserve stock of oxygen can be maintained. In the case of a human 

 being there may a food supply in the tissues sufficient to sustain life 

 for weeks, even though no new supply is taken in. There is water 

 enough in the body to maintain life for days; but if the supply of 

 oxygen is shut off, life can be continued only for a very few minutes on 

 the stock of oxygen contained in the body. So narrow is the space 

 between abundance of oxygen and death from oxygen starvation. In a 

 cold-blooded animal — with which the lake ought to be compared — 

 processes of respiration are slower, but the relative situation is not 

 materially different. The result of these conditions is that in any large 

 animal enormous surfaces must be provided for the absorption of 

 oxygen and there must be a very perfect mechanism for its distribution. 

 Such respiratory systems exist in a great variety of forms, many of 

 which are extremely complex and efficient. In the case of man the ab- 

 sorbing surface of the lungs is said to amount to about two thousand 

 square feet — an area as great as that of floor, ceiling and walls of a 

 room 20 feet square and 15 feet high. The necessity for arrange- 

 ments for a large absorbing surface increases with the size of the 

 animal, since in a large organism the area of the general surface is far 

 smaller in proportion to its mass than in a small organism of the 

 same shape. In a lake, whose size is enormous as compared with that 

 of any living being, the absorbing surface is very small as compared 

 with its mass ; being only the upper surface of the water. The lake is, 

 therefore, at a great disadvantage in the matter of absorbing oxygen 

 as compared with the animal. Still further, all higher animals, both 

 cold-blooded and warm-blooded, contain in their blood some chemical 

 substance which has a special affinity for oxygen and which can rapidly 

 pick up large quantities of it. Such a substance is wholly lacking in 

 the water of the lake, whose respiratory power is correspondingly 

 small as regards both the rapidity with which oxygen can be taken 

 up and the amount which can be absorbed. It is indeed true that 

 water will absorb, according to the general laws of the absorption of 

 gases, about twice as much oxygen as nitrogen under similar conditions. 

 This fact allows the lake to take in a larger stock of oxygen than would 

 otherwise be possible, and that part of the atmosphere which is dis- 

 solved in the lake contains about one third oxygen instead of one 

 fifth, as is the case outside. But even this amount is very little in 

 comparison with the enormous volumes which a substance like hemo- 

 globin can take up. It is also true that the mass of the water of the 

 lake, in comparison with the mass of the organisms which draw their 

 oxygen from it, is relatively far greater than the mass of the blood 



