432 LECTURE XVIII. 



metabolism. They estimate that about one-third of the total metabolism 

 takes place in this organ. We can understand this active metabolism, by 

 assuming that it is capable of performing a particularly intense kind of 

 work, and it is indeed possible that it is here that the work of secretion 

 comes into play. 



It must be admitted that the views of Bohr have been vigorously chal- 

 lenged. His results have been attributed to an insufficient equilibrium 

 being established between the blood and the alveolar air. Bohr himself 

 would not admit this; and now that fifteen years have passed since his first 

 important results were published, without the appearance of any data 

 which conclusively disputes it, we are compelled to place his opinions in 

 the foreground in our discussion of the gas-exchange which takes place in 

 the lungs; although the older assumption that the laws of gas diffusion are 

 sufficient to explain this phenomenon is, on account of its greater sim- 

 plicity, very attractive. It is, of course, not impossible, but on the other 

 hand extremely probable, that diffusion does in part account for some of 

 this exchange of carbon dioxide between the blood and the alveolar air. 

 Other factors are probably active at the same time, so that there is a very 

 active penetration of oxygen into the blood and removal of carbonic acid 

 from the latter. 



Bohr calls attention to the following observations in support of his 

 assumption. In the Amphibia it is well known that, besides the lungs, 

 the skin serves as an important organ of respiration. In the case of frogs, 

 simultaneous determinations of the gas-exchange of skin and lungs showed 

 that the taking up of oxygen by the skin is independent of the total extent 

 of the metabolism. It is almost constant, and amounts to 43 to 60 cubic 

 centimeters per kilogram of body weight in an hour. The carbon dioxide 

 elimination on the other hand showed variations of from 92 to 179 cubic 

 centimeters per kilogram in an hour. The gas-exchange taking place in 

 the lungs is different. Much more oxygen is absorbed through the lungs 

 than by the skin, and the variations are much greater (51 to 390 cubic 

 centimeters per kilogram in an hour) . The elimination of carbon dioxide 

 may, in winter when there is a considerable absorption of oxygen, sink 

 nearly to zero. Krogh, 1 who first mentioned these facts, found further 

 that a carbon dioxide tension of but a few per cent, in the atmosphere 

 surrounding the skin, caused a considerable increase in the amount of 

 oxygen taken up by the lungs alone, while at the same time the amount 

 taken up by the skin might be diminished. This action of the lungs does 

 not take place, however, if the cutaneous branch of the vagus nerve is cut. 

 The respiration by the skin is, on the other hand, apparently indifferent 

 to the nervous system. Evidently the gas-exchange by the skin takes place 

 by diffusion, while pulmonary respiration is more in the nature of a secretion. 



1 Skand. Arch. Physiol. 16, 378 (1904). 



