THE FISHES. 185 



familiar, the fish sinks in proportion to the degree of pres- 

 sure to which the contained air is subjected, and as the 

 compressed air is again permitted to expand, the creature, 

 becoming more buoyant, rises towards the surface. 



In many fish, the perch, &c., the air-bladder is closed 

 and there is no escape for the confined air ; and in those 

 fish with this form of bladder which live at great depths, 

 the very bringing them up to the surface, the air or gas no 

 longer being compressed by the weight of the water, bursts 

 the swimming-bladder. This is often seen in fishing for 

 cod, &c. 



Muller says : " The air of the swimming-bladder is not 

 derived from without. It is secreted by the inner surface 

 of the sac. The proportion which the oxygen bears to the 

 nitrogen is sometimes greater and sometimes less in the 

 air of the swimming-bladder than it is in the atmosphere. 

 In the air of the swimming-bladder of fishes in inland 

 lakes Ermann found but a small proportion of oxygen. 

 From the air of the air-bladder of fishes which live in the 

 depths of the ocean, Biot obtained from 69 to 87 per cent, 

 of oxygen, while the air with which the water of the ocean 

 at a considerable depth was impregnated contained only 

 29 parts of oxygen and 71 parts of nitrogen in 100 parts. 

 The composition of the air varies, however, even in the 

 same species of fish. In spring and summer the air is 

 said to contain less oxygen than in autumn ; and some- 

 times it contains no oxygen. According to the mean 

 result obtained from a great number of experiments, the 

 air of the swimming-bladder of the carp would consist of 

 71 parts oxygen, 52 parts of carbonic acid, and 377 parts 

 of nitrogen in 1000 parts. In some fishes, as in the carp, 

 the air-bladder communicates with the pharynx. In many 

 fishes this communication does not exist at all, and in them 

 there is usually a red vascular and peculiar tissue in the 

 walls of the bladder destined for the secretion of the air, 

 and even when the air-bladder does communicate with the 

 pharynx, it is most probable that the air is secreted." 



It is hardly necessary to say that fish breathe by means 

 of their gills ; but fish absorb oxgyen and exhale carbonic 



