RESPIRATION 21 1 



about again quite unconcerned in the tank, their respective swim 

 bladders having compensated by secretion or absorption of gas 

 for the disturbance in equilibrium caused by the sinker or float. 



Such facts as these pointed to the conclusion that the gas secre- 

 tion is under the control of the nervous system ; but this was not 

 clearly demonstrated by Moreau. It was not till sixteen years 

 later that Bohr showed that the secretion after emptying the 

 swim bladder by puncture ceases after the branch of the vagus 

 supplying the swim bladder is cut.^ I well remember the interest 

 with which I saw this experiment when Bohr showed it while he 

 was staying with me in Oxford a few months before he published 

 his paper on the subject. Dreser^ had meanwhile already shown 

 that the secretion of oxygen, like that of saliva, sweat, etc., is 

 excited by the action of pilocarpine. 



It is clear that a fish may require to get rid of gas from its 

 swim bladder, as well as to secrete gas. If the duct is open, there 

 is of course no difficulty in getting rid of gas ; but it is different 



Vasculor/^emlr, 



rUrte 



Figure 59. 

 Diagram of arrangement of "oval." 



if the duct is closed. The oxygen might, conceivably, be secreted 

 backwards; but often there is a large percentage of nitrogen in 

 the gas, and there might be trouble about this. It was discovered 

 by Jager^ that in fishes with a closed swim bladder there is an 

 oval window-like area on the dorsal side of the swim bladder 

 (Figure 59). Over this area there is nothing but a thin layer of 

 flattened cells between the air of the swim bladder and an under- 

 lying layer containing a close network of capillaries. This thin 

 layer seems to permit free diffusion outwards of the gas in the 

 swim bladder. Assuming this to be the case, the oxygen will 

 freely diffuse into the blood capillaries, where, as already seen, 



Bohr, Journ. of Physiol., XV, p. 499, 1893. 

 "Dreser, Arch. f. Exper. Pathologic, XXX, p. 160. 

 ' Jager, Pfliiger's Archiv, XCIV, p. 65, 1903. 



