LUNGS AND AIR DUCTS 



269 



the gases in the bladder. (This is possibly to be explained by the rapid dissocia- 

 tion of the oxyhaemoglobin of the blood on account of a reduction of pressure 

 resulting from a quick transfer from deep water to the surface.) In the physos- 

 tomous fishes the gases escape through the mouth. In the physoclists there is 

 usually a thinner region — the so-called oval — in the dorsal wall of the air bladder 

 where the gases may be absorbed, passing into the adjacent blood-vessels. 



While the pneumatic duct usually connects with the dorsal side of the 

 alimentary canal (fig. 290, A), it enters the left side in Erythrinus (C), and in 

 the mid-ventral line in Polypterus and Calamoichthys. In Polypterus there 

 are paired swim bladders, the right 

 being the longer. The blood in this 

 genus comes from the efferent bran- 

 chial arteries and consequently is 

 arterial. 



The swim bladder is supposed 

 to have hydrostatic functions, 

 aiding in the recognition of dif- 

 ferences of pressure due to changes 

 in depth. In the clupeids the air 

 bladder sends a diverticulum into 

 the head, there giving a branch to 

 each ear. In some physostomes 

 (siluroids, cyprinids, gymnonoti) 

 parts of the anterior vertebrae are 

 modified into a chain of bones 

 — the Weberian apparatus — 

 adapted to convey differences of 

 bladder pressure to the internal 



ears. One pair of bones is con- „ t,. r • ui jj • 



^ Fig. 290. — Diagrams of air bladder in 



nected with the dorsal wall of the fishes, after Dean, .4, Physostomous fishes; 



air bladder, a second with a ^; ^c'fJatdu"! ^""^ ^""''''' ^' ^'^'^'''''"^' 

 diverticulum (sinus impar) of 



the internal ear, while others are intercalated between these two (fig. 

 289). Changes in the distention of the bladder are thus conveyed to 

 the inner ear and probably affect the sense organs. 



LUNGS AND AIR DUCTS 



Lungs arise in all tetrapoda and in a few fishes (dipnoi) as a diver- 

 ticulum from the ventral side of the pharynx, immediately behind 

 the last gill pouch. The diverticulum divides almost as soon as out- 

 lined into right and left halves, each the anlage of the corresponding 



