xr AIR-BLADDER 309 
(g) and (hk) 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.’ | 
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 
eases to 87 per cent., 
in deep-sea species as 
compared with their 
shallow water con- 
geners. <A trace of 
carbondioxide is also Fie, 185.—Vertical section through a “red gland” 
usually present. The (diagrammatic). ae Capillary blood - vessels ; g, 
a tubular glands. (From Vincent and Barnes.) 
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.” 
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. 
4 > 
