RESPIRATION 



213 



per cent of an atmosphere, as it is in sea water ; so whenever the 

 oxygen percentage is sufficiently reduced by diffusion to make 

 the nitrogen pressure in the swim bladder more than 79 per cent 

 of an atmosphere, the nitrogen will follow the oxygen out through 

 the "oval" ; as will the CO 2 , and from a similar cause. But Jager 

 found also that the "oval" can be opened or closed by the relaxa- 



Figure 62. 



(X 330) Folds of the swim bladder epithelium of Gobius niger. 

 C.R.M., capillaries of the rete mirabile. I.C.C., intracellular capil- 

 lary (Woodland). 



tion or contraction of a ring of unstriped muscle surrounding its 

 periphery. When this ring is contracted the "oval" is covered up 

 by a layer of the ordinary lining membrane of the swim bladder. 

 Thus not only secretion, but also absorption of gas from the swim 

 bladder, is under complete physiological control. 



