RESPIRATION 219 



parts. The taking up of oxygen, for instance, depends on a host 

 of conditions in the environment, such as the concentrations, or, 

 more correctly, the diffusion pressures, of ions of different sorts, 

 and of various other substances which are, or may be, passing 

 into and out of the cell. A minute trace of pilocarpine, for instance, 

 will set the oxygen-secreting cell violently taking up oxygen on 

 one side, and giving it off on the other; and probably we could 

 paralyze the oxygen secretion at once by reducing the concentra- 

 tion of calcium ions in the cell environment. 



In a secreting cell the rate of secretion, other conditions being 

 favorable, depends on the concentration of the dissolved material 

 to be secreted. This we can see with the utmost clearness in the 

 case of the kidney or intestinal epithelium. The rate of secretion 

 also depends on the concentration of the dissolved material on 

 the excretory side, as we can also see in the case of the kidney. 

 Clear evidence on. this point is summarized by Ambard in his 

 book La physiologie des reins, Paris, 1920. We are thus led 

 to the conclusion that the stability of the oxygen combination 

 on one side of the oxygen-secreting cell depends, other things 

 being equal, on the stability of the oxygen combination at the 

 other side, and that in proportion as the oxygen combination 

 at one surface becomes increased, the oxygen combination at the 

 opposite surface becomes more ready to release oxygen towards 

 the cell environment. It also seems probable that as we proceed 

 from the absorbing to the secreting side of the cell, the tendency 

 to give off oxygen becomes greater and greater. A cell of sub- 

 stantial thickness is therefore required to produce a large differ- 

 ence in oxygen pressure. The combination which dissociates itself 

 on the excretory surface will, if the concentration of oxygen at 

 that surface is not so high as to stop the dissociation, be constantly 

 resatu rating itself in part from the combination lying deeper in 

 the cell. Thus oxygen will travel from the absorbing to the se- 

 creting side of the gland cell, just as urea, or sodium, or phosphoric 

 acid, will travel from the absorbing to the secreting side of other 

 kinds of secreting cells. We can also imagine how, in the course 

 of their passage, chemical transformations may occur in the 

 transported material, so that, for instance, an intestinal cell which 

 takes up fatty acid may deliver fat on the other side, or a cell 

 which takes up sugar may transform it into fat, or amino acids 

 into proteins, or oxygen into CO 2 and water, or may perform any 

 of the numerous other syntheses or disintegrations with which 

 physiologists are familiar. 



