o8 The Organs of Respiration 



enough, met with Uttle favor. Sorensen (1895) held that there 

 is but little evidence for attributing to the air-bladder the func- 

 tion of a lung. It is to be remembered, however, that, accord- 

 ing to Sorenscn's criterion no matter what exchange of gases 

 takes place between blood and air-bladder, it cannot be con- 

 sidered an organ of respiration, 'unless its air is renewed by 

 mechanical respiration.' 



"Sorensen also refutes, from anatomical and experimental 

 grounds, the many objections to Weber's theory of the function 

 of the ossicles. He would thus attribute to the air-bladder the 

 function of hearing; indeed in certain species the only reason 

 for the survival of the air-bladder is that ' the organ is still of 

 acoustic importance; that it acts as a resonator.' This idea, 

 Sorensen states, is borne out by the anatomical structure found 

 in Misgiiniiis and Clilarias, which resembles the celebrated 

 'CoUadon resonator.' This author attributes to the air-bladder 

 with its ' elastic spring' and various muscular mechanisms the 

 production of sound as its chief function." 



Origin of the Air-bladder. — In the more primitive forms, and 

 proliably in the embryos of all species, the air-bladder is joined 

 to the oesophagus by an air-duct. This duct is lost entirely in 

 the adult of all or nearly all of the thoracic and jugular fishes/ 

 and in some of the abdominal forms. The lancelets, lampreys, 

 sharks, rays, and chimosras have no air-bladder, but in the 

 most primitive forms of true fishes (Dipnoans and Crossoptery- 

 gians), having the air-bladder cellular or lung-like, the duct is^ 

 well developed, freely admitting the external air which the fisly 

 may rise to the surface to swallow. In most fishes the duct^ 

 opens into the oesophagus from the dorsal side, but in the more 

 primitive forms it enters from the ventral side, like the wind- 

 pipe of the higher vertebrates. In some of the Dipnoans the 

 air-bladder divides into two parts, in further resemblance to the 

 true lungs. 



The Origin of the Lungs. — The following account of the func- 

 tion of the air-bladder and of its development and decline is con- 

 densed from an article by \h. Charles Morris:* 



"If now we seek to discover the original purpose of this 



* The Origin of Ltmgs: A Chapter in Evolution. American Naturalist 

 December, iSq2. 



