RESPIRATION 251 



composition, the rest being allowed to pass. In this process the 

 glomerular epithelium will of course be doing work; but every 

 living tissue seems to be always doing work, even when it is "rest- 

 ing." During a glomerular diuresis there may be no extra work for 

 the epithelium to do, and it will simply act as a filter, just as the 

 lung epithelium during rest under normal conditions acts like a 

 nonliving membrane. Barcroft and Straub have shown that during 

 certain kinds of diuresis there is no increased consumption of 

 oxygen by the kidney, and therefore presumably no work done by 

 the kidney in the process of separation of the extra urine formed. 36 

 It is probable that under normal conditions a pure filtration 

 diuresis of this type never occurs at all; but the possibility of 

 producing it experimentally throws much light on the mode of 

 action of the glomeruli and also of the lung epithelium. Possibly 

 the substances carried to the lungs during anoxaemia act in the 

 same way as a diuretic drug acts on the kidneys. 



In concluding this long chapter I must make some reference 

 to criticisms which have been made on our experiments. Part of 

 these criticisms are the evident outcome of a natural conservative 

 desire to save some remnant of the old mechanistic theory of 

 glandular secretion. The lungs and the kidney glomeruli were 

 the last remaining strongholds that there seemed much hope of 

 defending, and I can admire the spirit which has animated the 

 defenders. It is different, however, with the criticisms made by 

 my friend Mr. Barcroft in his recent book, 37 as he fully ac- 

 knowledges the difficulties of the diffusion theory and the inherent 

 probability of secretory activity in the lungs. 



He bases these criticisms on the work of his pupil, Mr. Hart- 

 ridge. The latter devised a new and thoroughly sound method of 

 determining the percentage saturation of the blood with CO by 

 delicate measurements of the shifting in position of the absorption 

 bands of oxy- and CO-haemoglobin; and he showed clearly that 

 his method, although it requires elaborate apparatus, is capable 

 of giving accurate results. Armed with this method he proceeded 

 to repeat, as he thought, some of the experiments (not yet pub- 

 lished except in a short abstract) of Douglas and myself on man. 

 Unfortunately he modified the method in essential respects, neither 

 taking precautions that the subject was breathing a constant per- 

 centage of oxygen, nor using whole blood in the saturator, nor 

 experimenting in a way calculated to elicit any evidence of active 



* Barcroft and Straub, Journ. of Phystol., XLI, p. 145, 1911. 

 " Barcroft, The Respiratory Function of the Blood,, p. 204. 



