AIR-BLADDER 



309 



(g) and (7t) The structural modifications involved in the con- 

 nexion of the air-bladder with the auditory organ, and its 

 adaptation for sound-production, as well as its use in respiration, 

 are considered elsewhere. 1 



The Gases of the Air-Bladder. The gaseous contents of the 

 air-bladder consist of oxygen and nitrogen, but the relative 

 proportions of the two 

 gases differ in different 

 Fishes, and even in 

 the same Fish, under 

 different conditions. 

 Normally the propor- 

 tion of oxygen is con- 

 siderably less in fresh- 

 water than in marine 

 Fishes, and amongst 

 the latter the propor- 

 tion of oxygen is often 

 enormously greater, 

 amounting in some 

 cases to 87 per cent., 

 in deep-sea species as 

 compared with their 

 shallow water con- 

 geners. A trace of 

 carbondioxide is also 

 usually present. The 

 gases are derived from 

 the blood as the latter circulates through the capillaries in the 

 walls of the bladder, and it is highly probable that the "red 

 glands " take an important part in the process ; at all events, ex- 

 perimental research has shown that the " secretion " or diffusion 

 of gas into the air-bladder, as well as the absorption of gas 

 from the bladder into the blood, take place most rapidly in those 

 Fishes in which " red glands " or " red bodies " are present. 2 



The Functions of the Air-Bladder. Probably no single organ 

 in any group of Vertebrata is associated with the performance of 

 a greater variety of functions than the air-bladder of Fishes. 

 Originally evolved, it may be, as a glandular caecum in certain 



1 See Chaps. XIV. XIII. and X. * Moreau, Ann. d. Sci. Nat. (6) iv. 1876, Art. 



FIG. 185. Vertical section through a "red gland" 

 (diagrammatic). c, Capillary blood - vessels ; g, 

 tubular glands. (From Vincent and Barnes.) 



