THE PLACE OF BIOLOGY. 23 



which broke his instrument. He then knew that he 

 had made a most significant discovery, as the gas he 

 was dealing with must be nearly pure oxygen ; so he 

 got another eudiometer and with proper precautions 

 made a large number of analyses, which he published 

 in the Memoir es de la Societe oVArcueil of 1807. These 

 analyses showed that the greater the depth from which 

 a fish was taken the more nearly did the gas from the 

 swim-bladder approximate to pure oxygen. 



A long time elapsed before the significance of Biot's 

 discovery was realised by physiologists, and it was 

 seventy years later before the subject was again effec- 

 tively taken up in a very interesting monograph by 

 Moreau, 1 a French physiologist. He showed experi- 

 mentally what the real functions of the swim-bladder 

 are ; while later physiologists have added many further 

 contributions on : which I must not stop to dwell. 

 Perhaps the most noteworthy of these is the discovery 

 by Bohr of Copenhagen that the secretion of oxygen is 

 under the control of the nervous system, the secretory 

 nerve being a branch of the vagus. 



As nearly pure oxygen has been obtained from the 

 swim-bladders of fishes living at a depth of 4500 feet, 

 it follows that oxygen may be secreted into the swim- 

 bladder and retained in it in the gaseous form at a 

 pressure of over 120 atmospheres, whereas the partial 

 pressure of oxygen in the surrounding sea- water is only 

 about one-fifth of an atmosphere. It seems perfectly 

 clear, therefore, that the liberation of oxygen and its 

 retention by the semi-liquid wall of the swim-bladder 

 is the result of an active physiological process in the 

 1 Memoires de physiologie, Paris, 1877. 



