1893.] on the Interchange of the Respiratory Gases. 43 



and of the carbonic acid of the blood into the air of the pulmonary- 

 cavity also through the pulmonary tissue. The word " interchange " 

 might be thought to mean that a certain volume of oxygen is ex- 

 changed for an equal volume of C0 2 , but such is not the case, as the 

 volume of oxygen taken up by the blood, and said to be consumed, is 

 larger than the corresponding volume of C0 2 emitted, the difference 

 being due to the absorbed, which may be considered as employed 

 in the phenomena of " tissue-change." 



During ordinary respiration in a perfect state of repose, and under 

 similar circumstances relating to temperature and food, the formation 

 of C0 2 and absorption of in the same person alters within certain 

 limits. Together with these changes there is a marked tendency for 

 the oxygen consumed (CO 2 produced and O absorbed) to assume a 

 constant figure in the same person, or it may be said that there is a 

 marked tendency to a decrease of C0 2 expired being accompanied by 

 an increase of oxygen absorbed, and vice versa. Four different 

 persons were experimented upon, and this tendency is very clearly 

 shown on three in the accompanying table, in which every figure is 

 the mean of the two readings most alike. 



In the fourth case, that of my present assistant, Mr. B. F. Davis, 

 the tendency is of a different kind. 



On a close consideration, the phenomenon observed on the three 

 first persons experimented upon admits, I think, of the following 

 -explanation : 



If the carbonic acid expired should suddenly increase, it does so at 

 the expense of the oxygen absorbed, and there is less of it left for 

 the purposes of tissue-change ; hence it is observed to diminish at 

 the same time as the C0 2 is increased. 



Table showing the tendency of the Oxygen consumed to remain 

 constant under similar circumstances. 



The Author. 





