FUNCTIONS OF SWIMBLADDER OF FISHES. 113 



weight for the normal glands of perch was 0.267 grams as com- 

 pared with 0.290 grams for glands of perch subjected to pressure 

 stimuli. A considerable variation occurs in the cutting of the 

 glands free from the swimbladder, and these figures are not 

 absolute quantitative comparisons; nevertheless they may be 

 taken as indicative. 



It is probable that afferent nerves perhaps located in the walls 

 of the swimbladder that are sensitive to changes in pressure 

 aid in the regulation of the specific gravity of the fish. Bohr 

 (1893) showed that the vagus exerts a control over the gas 

 secretion in the swimbladder. Its control is very likely a matter 

 of stimulating the swimbladder gland to secrete. For example 

 when a perch sinks into deeper water the pressure outside 

 increases. In order to maintain its equilibrium, the perch must 

 increase the amount of gas in the swimbladder. It does this 

 by taking oxygen from the blood. When the fish rises in the 

 water oxygen must be re-absorbed by the blood. When the 

 fish is not changing its depth appreciably the gas content of the 

 swimbladder is probably maintained by simple physical diffusion 

 of the gases from the blood, but when a greater difference of pres- 

 sure is met with the secretory process comes into play. 



The mechanism by which those fishes which have a duct 

 leading from the swimbladder maintain their equilibrium in the 

 water has not been studied in detail. Whether the process of 

 gas diffusion is fundamentally different in such fishes can not be 

 stated. Further investigation will be made using such fish as 

 carp, dogfish, and trout. In all events some modifications may 

 be expected for no such highly specialized organ as the rete 

 mirabile exists in these forms. 



The writer's hypothesis of the mechanism by which the swim- 

 bladder is able to regulate the specific gravity of the fish, is not 

 intended as a complete explanation of all the processes involved 

 in gas secretion. It does however offer a chemical and physical 

 explanation of a very controversial matter. The swimbladder 

 of fishes offers unusual opportunities for the study of physiological 

 processes accompanying gas secretion and no doubt new facts 

 may be learned from them of fundamental biological importance. 



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