94 The Organs of Respiration 



truth, although we should express the fact more exactly to say 

 that both air-bladder" and lung are developed from a primi- 

 tive cellular breathing-sac, originally a diverticulum from the 

 ventral walls of the oesophagus. 



The air-bladder varies in size as much as in form. In some 

 fishes it extends from the head to the tail, while in others it is 

 so minute as to be scarcely traceable. It often varies greatly 

 in closely related species. The common mackerel (Scomber 

 scombrus) has no air-bladder, while in the closely related colias 

 or chub mackerel (Scomber japonicus} the organ is very evident. 

 In other families, as the rock-fishes (Scorpcsnidce) , genera with 

 and those without the air-bladder are scarcely distinguishable 

 externally. In general, fishes which lie on the bottom, those 

 which inhabit great depths, and those which swim freely in the 

 open sea, as sharks and mackerel, lack the air-bladder. In the 

 sharks, rays, and chimaeras there is no trace of an air-bladder. 

 In the mackerel and other bony fishes without it, it is lost in 

 the process of development. 



The air-bladder is composed of two layers of membrane, the 

 outer one shining, silvery in color, with muscular fibres, the inner 

 well supplied by blood-vessels. The gas within the air-bladder 

 must be in most cases secreted from the blood-vessels. In river 

 fishes it is said to be nearly pure nitrogen. In marine fishes it 

 is mostly oxygen, with from 6 to 10 per cent of carbonic- 

 acid gas, while in the deep-sea fishes oxygen is greatly in excess. 

 In Lopholatilus, a deep-sea fish, Professor R. W. Tower finds 66 

 to 69 per cent of oxygen. In Trigla lyra Biot records 87 per 

 cent. In Dentex dentex, a shore fish of Europe, 40 per cent of 

 oxygen was found in the air-bladder. Fifty per cent is recorded 

 from the European porgy, Pagrus pagrus. In a fish dying from 

 suffocation the amount of carbonic-acid gas (CO 2 ) is greatly 

 increased, amounting, according to recent researches of Pro- 

 fessor Tower on the weak-fish, Cynoscion regalis, to 24 to 29 

 per cent. This shows conclusively that the air-bladder is to 

 some degree a reservoir of oxygen secreted from the blood, to 

 which channel it may return through a kind of respiration. 



The other functions of the air-bladder have been subject to 

 much question and are still far from understood. The follow- 

 ing summary of the various views in this regard we copy 



