762 COMPARATIVE ANATOMY 



the summer for food. This can be understood the better when it is re- 

 membered that there is no O, at all at the bottoms of lakes in summer. 

 The swim bladder is supposed to make it possible for its possessor 

 to regulate its equilibrium while in its watery medium. This supposition 

 has the following- facts upon which to rest its validity : ground-feeding 

 teleosts do not have it, but those who must adjust their position in such 

 a way as to obtain the requisite food do have it, while in many of these 

 there is a diverticulum from it to various portions of the ear. 



LUNGS AND AIR DUCTS 



In all the higher forms of animals and in some few fishes dipnoids 

 the lungs arise as an outpushing from the ventral side of the pharynx 

 immediately behind the last gill pouch. This outpushing divides almost 

 immediately into a right and left half, and just as the outgrowing from 

 the digestive tract carried the covering of that tract before it, so, too, a 

 peritoneal covering is carried before the respiratory organs. 



As development goes on, the growing part protrudes into the coelom 

 so that the parts lying therein have an entodermal lining which was 

 derived from the epithelium of the pharynx, while the outer layer of 

 peritoneum is serous mesenchyme carrying blood and lymph vessels, 

 nerve and smooth-muscle fibers between the two. That portion of the 

 respiratory system from the pharynx to the lungs consists of trachea, 

 bronchi, and their accessories. These together constitute what are com- 

 monly called air ducts. The lungs are treated as distinct from these. 



On the ventral side of the trachea, in air-breathing animals, there 

 is a separation which forms the larynx (Fig. 442), the beginning of which 

 can be studied in amphibia, in the lower forms of which, a simple pair 

 of cartilages are developed on the sides of the glottis (the glottis simply 

 being an elongated slit connecting the pharynx with the air ducts). These 

 cartilages develop in the position of a reduced visceral arch. In other 

 forms, such as the Urodeles, the more cephalic ends of the lateral car- 

 tilages separate from the rest and form an arytenoid which is the first 

 of the laryngeal cartilages, and is imbedded in the walls of the glottis. 

 The balance of the lateral cartilages may remain as it is, or divide into 

 any number of pieces. However, the more cephalic pair of these pieces 

 often fuse in the mid ventral line to form the cricoid, which is the sec- 

 ond element of the laryngeal framework. There are various antagonistic 

 muscles attached to these cartilages which make it possible to open and 

 close the opening. 



The vocal cords are formed by a pair of folds of the laryngeal lining 

 which extend parallel to the margins of the glottis. Sound is produced 

 by the vibrations caused by the air passing over these cords as they are 

 relaxed or tightened in different degrees. The larynx is quite rudimen- 

 tary in reptiles and birds. In the latter the syrinx, shortly to be de- 

 scribed, takes the place of the larynx. 



