252 COMPARATIVE ANATOMY. 



external air, and it is probable that the contained gas is given off 

 from the walls of the air-bladder itself. 



The air-bladder always lies above the peritoneum on the dorsal 

 side of the body-cavity, between the vertebral column, aorta, and 

 kidneys on the one hand, and the alimentary canal on the other : 

 it is invested by the peritoneum on the ventral side only. It is 

 more or less sac-shaped in form, and is only exceptionally paired 

 (Polypterus) ; it usually extends along the whole length of the 

 body-cavity, and has walls composed of connective, elastic, and 

 muscular tissue. In some Teleostei it is transversely constricted 

 so as to form several successive divisions ; in other cases it may 

 give rise to a more or less numerous series of caeca! processes. 

 Its internal surface may be either smooth or spongy, owing to 

 the formation of a meshwork of trabeculse, the structure of which 

 reminds one of the lungs of Dipnoi and Amphibia. 



Attention has already been directed to the relations which often 

 exist between the air-bladder and the auditory organ (see p. 207). 



2. THE LUNGS. 



The further development of the primitive lung- sacs is essen- 

 tially similar to that of a branched gland. They gradually increase 

 in size, and the part which connects them with the oesophagus 

 becomes drawn out into a tube, the windpipe or trachea; this 

 bifurcates to form two bronchi, one of which goes to either lung 



PD 





FIG. 204. A, B, C, DIAGRAMS SHOWING THE MODE OF DEVELOPMENT OF THE 



LTTNGS. 



PD, primitive alimentary tube ; S, S*, the lung-sacs, which are at first unpaired ; 

 t, trachea ; b, bronchus. 



(Fig. 204, S, S 1 , t, b). In their further growth, the bronchi branch 

 out into finer and finer tubes, and finally end in small vesicles or 

 infundibula, which are made up of a number of alveoli, and 

 are surrounded by blood capillaries, through the thin walls of which 

 the interchange of respiratory gases takes place (comp. Fig. 205). 



In the course of further development, annular cartilages become 

 developed in this system of tubes : the most anterior of these, that 



