FUNCTIONS OF SWIMBLADDER OF FISHES. 8 1 



an hydrostatic function. He maintained that the fish possessed 

 a volitional control over the size of the swimbladder being 

 able to compress or distend the bladder at will. Delaroche 

 (1807) opposed the views of Borelli but advanced an hypothesis 

 similar to it in many respects. Moreau (1877), however, 

 maintained that he had disproved both these hypothesis. 

 Delaroche was among the first to show that an exchange of gas 

 probably occurred between the swimbladder and the blood. 

 He did not consider the former an organ of respiration however. 



Biot (1807), Provencal and Humboldt (1809) showed by 

 analysis of the air in the swimbladder that the mixture frequently 

 consisted almost entirely of oxygen, the percentage of oxygen 

 increasing in relation to the depth of the water in which fishes 

 lived. Moreau (1877) in his classical work of the function of 

 the swimbladder proved by ingenious experiments that many 

 of the ideas prevailing before his time were erroneous. He 

 showed that this organ serves to equilibrate the body of the 

 fish with the surrounding water at any level. He demonstrated 

 that such adjustment is not accomplished quickly, and that the 

 fish, therefore, does not use his muscles in regulating the volume 

 of the swimbladder. Moreau 's experiments also convinced 

 him that the gas is secreted into the swimbladder. About the 

 same time Johannes Miiller (i 842) took up the problem. Although 

 many of his views have later been shown untenable, they served 

 as an incentive to other investigators. He stated that the gas 

 entered the swimbladder by secretion from the blood. 



Several views have been held concerning the origin of gases 

 in the swimbladder. The different theories can be classified 

 into two distinct categories: (i) Those that hold that the gases 

 are derived directly from the atmosphere, attributed first to 

 Redi (1684); (2) and those that maintain that these gases are 

 derived more or less directly from the blood stream (a descendant 

 of Needham's secretory theory) . 



The first view has been given up by most modern investi- 

 gators, although Thilo (1906) has persistently contended that 

 the blood of fish is not sufficient in quantity to supply the amount 

 of gas found in the bladder. He asserts that in all cases the 

 bladder-gas must be procured by the fish directly from the 

 atmosphere. Objections are easily found to Thilo's hypothesis. 



