The acquisition of oxygen by the blood in the lung 183 



diffusion ; though Fredericq performed yet other aerotonometric 

 experiments which supported the diffusion theory, the fact remained 

 that those who studied tensions with aerotonometers were divided. 



Whilst Bohr's theory of pulmonary secretion was more compli- 

 cated than its rival, it had a certain charm about it which has 

 appealed to many minds. Its reasonableness was based upon morpho- 

 logical considerations which the diffusion theory entirely ignored. 

 It started with the swim-bladder of the fishes ; this organ in the 

 cod if emptied gradually refills, and the gas on analysis yields the 

 remarkable result that some 80 % f its volume is oxygen. What- 

 ever secretion may be, here is a case of it. Is it strange that the 

 swim-bladder of the cod should have this power ? It is an offshoot 

 of the alimentary canal; if the cell of the stomach can secrete 

 hydrochloric acid; if the cells of diverticula, such as the pancreas 

 and the liver, secrete sodium carbonate, is there anything strange 

 about the idea that another diverticulum of the great secre ting- 

 mass the alimentary tract should secrete oxygen ? Rather would 

 it be strange if it could not secrete. Moreover the unity in 

 function between the swim-bladder and other diverticula, such as the 

 glands of the stomach, is made more evident when we discern, as 

 Bohr did, that the secretory function of the swim-bladder is governed 

 by the vagns just as is the secretory function of the stomach. Cut 

 the vagi the swim-bladder will secrete oxygen as little as the stomach, 

 similarly crippled, will secrete pepsin. 



Passing from the fishes to the other vertebrate types, the alleged 

 secretion in the lung was studied very carefully by Maar in the 

 tortoise. It proved to be possible, and in fact simple, to study the 

 gaseous exchange in each lung of the tortoise separately. To quote 

 Krogh on the subject: "While the necessary operation is rather 

 difficult in the mammals, and may vitiate the results by injuring the 

 animal, the tortoise appears to be specially adapted for this kind of 

 experiment. The trachea divides just behind the head, and the main 

 bronchi run parallel to one another and loosely connected along the 

 whole length of the neck." 



Maar discovered that when the vagus was divided the gaseous 

 exchange of the lung was so altered as to make it clear that the 

 vagus has a very definite action on the pulmonary respiration of the 

 tortoise, but the action is not easy of interpretation. As explained 

 by Maar, however, there was a sort of tonic action, the vagus pro- 

 viding inhibitory nerves. The figures were regarded as being lifted 



