RESPIRATORY SYSTEM 765 



syrinx, thus making it possible to change the sounds uttered. 



In the mammals the cartilaginous rings of the trachea are dorsally 

 incomplete, this position being closed by membrane. A structure of this 

 kind permits the tube to remain open and yet also permits it to "give" 

 a little when food passes down behind it through the oesophagus. 



THE LUNGS 



In the lung fishes there is usually a single sac, although several 

 types of these animals have paired lungs. The pulmonary arteries spring 

 from the last efferent branchial artery of both sides. The blood supply, 

 therefore, under normal conditions, is arterial, and the lungs cannot act 

 as respiratory organs. "In times of drought (Protopterus), or of foul 

 water (Ceratodus), the gills no longer function, and the pulmonary 

 arteries bring venous blood to the lungs." 



In amphibia the two lungs are elongated. They are united at their 

 bases though true bronchi are absent. They may or may not have 

 alveoli. In the frog the two lungs are distinct, the walls being divided 

 into a series of sacs or infundibula lined with alveoli. The infundibula 

 open into a central chamber, which, since it is ciliated and has numerous 

 glands in its walls, may be compared to a bronchiole. 



In those terrestrial urodeles which are lungless in all stages of de- 

 velopment, no traces of larynx or trachea occur at all, even after the gills 

 are absorbed. In such species there is a considerable development of 

 capillaries in the skin as well as in the walls of the mouth and pharynx, 

 so that the respiratory functions are transferred to these parts. In the 

 frogs, as already shown, the skin is respiratory and largely supplied by 

 the cutaneous arteries arising from the same arch as the pulmonary 

 arteries. 



The air ducts enter the anterior end of the lungs in amphibia, while 

 in higher forms the lungs extend anteriorly to the entrance of the 

 bronchi on the medial side. This change is in part the result of the 

 transfer of the heart into the thorax, the position of the pulmonary 

 arteries, thus forcing the bronchi toward the center of the lungs. In am- 

 niotes, also, the ducts are characterized by the presence of cartilage in 

 their walls, so that they are true bronchi. The bronchi may extend in- 

 side of the lungs and divide into secondary and tertiary bronchi. 



In reptiles the lungs are often non-symmetrical, sometimes one even 

 being absent. In the snakes the lungs consist of a single sac lined with 

 infundibula either in part or throughout. In the lizards there are one 

 or more verticle septa dividing the lung into chambers lined with alveoli 

 while a part of the bronchus may extend to the extremity of the lungs. 

 In the chameleons the septa do not reach the distal wall, consequently 

 the chambers communicate so that the bronchus enters a cavity known 

 as the atrium. This connects with the various chambers separated by 

 the septa and these in turn open into a terminal vesicle. This whole 



