BETWEEN THE BLOOD AND ALVEOLAR AIR 549 



According to the experiments of Krogh, the pulmonary respira- 

 tion of frogs is an active secretory process, but the cutaneous 

 respiration is due to diffusion alone ; it had been previously 

 shown by Waymouth Reid and Hambly that the transpiration 

 of carbon dioxide through the skin of the frog occurred equally 

 well in both directions. 



From the analogy of the secretory processes in glands, it would 

 be expected that the secretory activity of the lungs should be 

 influenced by the nervous system. There is evidence that such is 

 the case. Maar found that in the tortoise the gaseous exchange 

 depended upon the tonic influence of the vagus nerves. Section 

 of the vagus produced an increase in the absorption of oxygen by 

 the lung of the same side, and a decrease in the case of the 

 opposite lung ; the excretion of carbon dioxide showed a similar but 

 less marked relationship. Stimulation of the vagus produced the 

 opposite effects. These changes did not appear to be due to vaso- 

 motor changes, whereby the volume of blood flowing through the 

 lungs would be altered. The results of similar experiments upon 

 warm-blooded animals are much more uncertain, for it is difficult 

 to separate the effects produced by stimulation or section of the 

 vagus nerves and to decide whether the changes observed in the 

 respiratory exchange are due to changes in the activity of the 

 lungs or alterations in the flow of blood. The operative pro- 

 cedure for the collection of separate samples from the right and 

 left lung may easily introduce serious sources of error. According 

 to the researches of Maar and Henriques, the vagus does influence 

 directly the secretory activity of the lungs of warm-blooded animals. 



The influence of the nervous system upon the secretion of 

 oxygen by the swimming-bladder of the fish has been clearly 

 shown, and analogy is in favour of a similar action in the lungs. 



A still further piece of evidence in favour of the secretory theory 

 is the great capacity of the pulmonary tissue to reduce alizarin- 

 blue when, as in Ehrlich's experiments, it is injected into the living 

 body, and air is still passing in and out of the lungs. 



The body of evidence has thus been steadily increasing in 

 favour of the secretory theory, especially as regards the absorption 

 of oxygen. In the case of the excretion of carbon dioxide the 

 evidence is much more imperfect, and here there is even a want 

 of agreement among the supporters of the view that oxygen is 

 secreted. The witnesses on the same side give conflicting evidence. 



