573 



and then. A more accurate idea of tlie mechanism of this spiiincter- 

 orifice was tiie object of this [lart of my iiixestigatioiis. 



At the outset it must be observed that the pressure-decrease to 

 whicii a fish is submitted has to surpass a certain minimum before 

 the aniuial lels a]i air-bubble escape. Already at a smaller difference 

 in tension than that at which air-bubbles are sent forth, the fish 

 shows by the restless uiotion of its fins that it responds to this 

 difference. In the various S[)ecies and also in the various individuals 

 of the same species the difierence between the pressure-decrease at 

 which tin-reactions and at which air-bubbles appear is higldy variable. 

 Besides the miuinium change at which fm-reactioiis are ol)served 

 fluctuates strongly. Hence we shall have to experiment on as great 

 a numlier of animals as possible in order to obtain reliable results. 



If a tish adapts itself to a modified pressure, this does not take 

 place at once. The relaxation of the sphincter seems to last but a 

 short time and may repeat itself at intervals as long as the fish 

 has not entirely adapted itself. The first air-bubble will be followed 

 after a shorter or longer lime by others. Generally speaking the 

 interval between two air-bubl)les will gradually become greater and 

 this is quite natural for after each air-bubble the fish becomes more 

 adapted to the new pressure. This lengthening of the intervals is, 

 however, by no means regular. 



If for instance a fish is exposed for a long time at a stretch, to 

 a pressure-decrease which does not immediately cause air-bubbles 

 to escape, then the long action of this weak stimulus has the same 

 effect as the short action of the stronger one. It could not be de- 

 monstrated that the product of time and degree of stimulus was a 

 constant one, but it was very evident that below a certain minimum 

 of pressure-difference no bubbles were emitted, and that above it, 

 at the increase of the pressure-difi'erence, the periods before the 

 emission of the first air-bubble grew smaller and smaller. 



If a fish is narcotized then the sphincter-reflex, as was shown 

 by GuvENOT, is retarded. The opening of the sphincter is the result 

 of the removal of the tonus in the muscle. It is an inhibitory reflex 

 removed by narcosis. I can confirm the results of Gcyénot's experiments. 

 It is remarkable that this reflex-retardation remains a long time 

 after the narcosis. First the respiratory rhythm grows normal, then 

 the equilibrium is restored, afterwards defensive reflexes, caused by 

 fright or decreased pressure manifest themselves. Onl/ much later 

 the tonus-reflex of the sphincter becomes active again. It seems that 

 the centre whence the efferent part of this reflex proceeds remains 



