3S6 ZOOLOGY FOR MEDICAL STUDENTS chap. 



air-bladder but to its ventral side. Varied modifications of the form of 

 the air-bladder occur. It may be constricted into two or three portions 

 lying one behind the other : it may give rise to more or less tubular 

 outgrowths. In some cases a tubular outgrowth on each side in front 

 extends to the neighbourhood of the otocyst, its wall forming a portion 

 of the otherwise rigid boundary of the cavity enclosing the otocyst with 

 its perilymph. In many Teleosts the air-bladder retains throughout life 

 its communication with the alimentary canal (physostomatous condition 

 — Fig. 154, p. 367^ D) but more usually this communication becomes 

 obliterated and the air-bladder forms a completely closed cavity (physo- 

 elistie condition — Fig. 154, E). Apparently this closure has come 

 about in the course of evolution independently in several different cases 

 so it is no longer customary to regard fishes in which it has happened 

 as forming a natural group by themselves (" Physoclisti "). 



Functionally the air-bladder is of great interest. In some few physo- 

 stomatous Teleosts the lining of the organ is provided with a rich net- 

 work of blood-vessels/ air is swallowed into the air-bladder, and it fulfils 

 an important respiratory function. As will emerge later there is reason 

 to beheve that here we have to' do with a persistence of, or a reversion 

 to, the primitive function of the organ. Where there is a close relation 

 with the otocyst the air-bladder probably aids in the detection of 

 vibrations in the water. A wave of compression reaching the fish, and 

 causing a slight inward displacement of the wall of the air-bladder over 

 a great part of its surface, will tend to give an increased displacement 

 to the much smaller portion of wall in contact with the perilymph and 

 sheltered from the external pressure. 



The main function of the air-bladder in a typical teleost is neither 

 respiratory nor auditory : it is to act as a float. It used to be thought 

 that it helped the fish to sink or rise in the water : by muscular con- 

 traction of the body-wall the gaseous contents of the air-bladder were 

 compressed, the specific gravity of the animal's body was increased and 

 it sank. On the other hand relaxing the musculature of the body-wall • 

 allowed the gas to expand, the specific gravity was diminished and the 

 fish floated upwards. Modern investigation has shown that the mode in 

 which the air-bladder carries out its hydrostatic function is much more 

 complicated than it would be on this simple theory. In the first place 

 the air-bladder is an organ not for altering the specific gravity of the 

 fish but for keeping it constantly the same as that of the water in which 

 it is swimming, counteracting the changes that would otherwise be 



1 The air-bladder of teleosts receives its blood from various branches of 

 the dorsal aorta. 



