clavicular air sac 



cervical air sac 



dorsal bronchi 



ventral bronchi 

 clavicular air sac 



anterior thoracic air sac 



ventral bronchi' 



lateral bronchi dorsal bronch 



posterior thoracic air sac 

 abdominal air sac 



dorsal <- 



-> ventral 



recurrent bronchi 



anterior thoracic air sac 

 lateral bronchi 



Figure 9-30. Semidiagrommotic lungs and air sacs of the bird. A, dorsolateral view of right lung 

 of 9-day-old chick foetus; B, adult system in ventral view. (A after Witschi, 1956) 



freely into the body cavity. Among anurans the body sur- 

 face has an important accessory respiratory function. 



Embryological development in reptiles and amphibians The 



lungs arise from the floor of the pharynx as in the higher 

 forms. First a laryngotracheal groove appears in the floor of 

 the pharynx, and from this paired outgrowths arise. The 

 medial groove is later converted into the trachea, and the 

 bilateral outgrowths become the lungs. 



The alligator or bird parallels the mammal in the course 

 of the development of the lung buds, which branch succes- 

 sively in the formation of the organ (Figure 9-31). In the 

 bird the dorsal and ventral bronchi branch repeatedly to 

 form the parabronchi; those of the dorsal and ventral 

 bronchi meet midway between these two series along an 

 oblique plane and fuse. In this way, what were blind-end- 

 ing tubules come to form loops. In most reptiles and amphi- 

 bians the lung buds merely expand, with a few knob-like 

 diverticula, to form the definitive lung. 



In the reptile a variety of membranes form an association 

 with the lungs, and occasionally the lungs are nearly cut off 

 from connection with the abdominal cavity, as is the case in 

 Tupinambis. However, a diaphragm as such is never formed. 

 In the amphibian there is no attempt to isolate the lungs 

 from the remainder of the body cavity. 



From their position and paired origin, it is generally as- 

 sumed that the lungs represent a posterior set of gill pouches. 

 In the case oi Hypogeophis, there are eight pairs of pharyngeal 

 pouches; the most posterior, eighth pair, gives rise to the 

 lungs. In some salamanders seven pairs of gill pouches are 

 presumed to occur; the lungs form from the seventh pair. 



Swim bladder 



In fishes respiration is usually carried on through the 

 agency of gills. A hydrostatic organ, the swim bladder, may 

 be used as a supplementary and emergency respiratory organ. 



282 • THE VISCERA 



