SECRETION AND ABSORPTION OF GAS IN 

 THE SWIMMING-BLADDER AND LUNGS. 



PART II.— LUNGS. 



IN the previous paper, I gave an account of the evidence 

 which proves that free oxygen, and in some cases at 

 least nitrogen, is actively secreted by the epithelium of the 

 swimming-bladder. The present paper will deal with the 

 much more difficult question whether the exchange of gases 

 between the blood and the air present in the alveoli of the 

 lungs is also brought about in whole or part by active 

 secretion or absorption. The fact that morphologically the 

 swimming-bladder is so closely related to the lungs in- 

 creases, perhaps, the probability in favour of the existence 

 of active secretion or absorption by the lungs. On the 

 other hand, it must be remembered that the stream of 

 oxygen is outwards from the blood in the swimming-bladder, 

 while it is inwards to the blood in the lungs. 



Between the air in the alveoli and the blood passing 

 through the capillaries of the lung there is interposed prac- 

 tically nothing but the cells forming the capillary walls and 

 the layer of extremely delicate flattened epithelial cells con- 

 stituting the lining membrane of the alveoli. As protoplasm 

 is semifluid in consistency, and as gases pass with the 

 utmost readiness through water by diffusion, the most 

 natural supposition from a purely physical standpoint is that 

 the exchange of gases is simply due to diffusion, the 

 epithelium playing merely a passive part in the process. 

 Venous blood, when brought into contact with air outside 

 the body, takes up oxygen and gives off carbonic acid, 

 undergoing what appears to be just the same change as that 

 brought about in blood passing through the lungs. The 

 fact that the change occurs so much more rapidly in the 

 lungs than in blood shaken with air outside the body is 

 accounted for by the enormous surface presented by the 

 alveolar walls, and the minuteness of the subdivision of the 

 blood-stream in the lungs. The diameter of a capillary 



