572 



Zoology. — "Till' Phi/sio!o,/i/ of //«^ Air-hlii<l<li'r of Fis/ws." (From 

 llie Pliysiological Lal)oralor_y of the Amslerdaiii Universily.j 

 By Dr. K. Kuipek Jr. (Cominmiicated by Prof. Max Wkber). 



(Communicated in the meeting of May 29, 1915.) 



III. J'/ie Ductus pneumaticu.'i of the Plnj.so^touii. 



Boyle (1H70) already demonslrated cxperiiiieiilally tliat a tencli 

 when exposed to a lower pressure llian the one under which it 

 lives, can allow air-bubbles to escape from the air-bladder by means 

 of the ductus pneuniaticus. For a long time if was supposed that 

 this channel also served to lead into the air-bladder gases which 

 the fish had imbibed at the surface of the water. Evidently this 

 would only be possible if the tension of the gas in the bladder is 

 less than that of the atmos^ihere. The former being in a great 

 majority of instances greater than the latter, this mode of filling 

 the air-bladder is precluded. Besides a iisli rising to the surface has 

 to leave its "plan des moindres efforts", the plane where its S.G.=1. 

 At the surface the tension is less, the air-bladder expands, the fish 

 grows specifically lighter; it floats. To reach its static plane again 

 it has to perform muscular labour in a direction opposite to the 

 upward pi'essure. If it admitted air at the surface, which would 

 l(jwer its static plane in the water, the exertion in going down 

 again would have to be greater still. This view of the function 

 of the duel us has been relinquished by almost every one. 



An annular nuiscle shuts off the entrance of the ductus into the 

 esophagus. Structuie and action of this muscle were first closely 

 studied by Gl'yenot. He proved that this muscle has a tonic tension. 

 Wheri a pliysostomus is exposed to a decreased pressure, air-bubbles 

 do not immediately escape from the mouth. Oidy when the decrease 

 amounts to about 5 centimetres of mercury the gas leaves the 

 bladder. At the death of the animal this tonus disappears. The 

 resistance which the sphincter offers to the air in the bladder is 

 reduced to about two fifths. The opening of the sphincter is brought 

 about under the influence of the central nerve-system. The muscles 

 get nerve-branches from the Eatnus intestinalis Vagi. This appears 

 distinctly froui the microscopic preparations (microtomic sections) 

 which I made of the sphincter and its surroundings. The gas-bubbles 

 ave not emitled continually bat intermittently. This suggests the 

 probaliility thai the tonus of the sphincter is relaxed every now 



