258 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



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 the toads and 

 aglossa the alveoli are more extensively developed in correlation with 

 the more terrestrial habits. 



It has recently been shown that a number of terrestrial urodeles are lungless in 

 all stages of development, and that no traces of larynx or trachea occur, even after 

 the gills are absorbed. In these species there is a great development of capillaries 

 in the skin and in the walls of the mouth and pharynx, the respiratory functions 

 being transferred to these parts. In the frogs the skin is also respiratory and it is 

 largely supplied by the cutaneous arteries which arise from the same arch as the 

 pulmonary arteries. 



In the amphibia the air ducts enter the anterior end of the lungs, 

 but in the amniotes the lungs extend anteriorly to the entrance of the 

 bronchi which is 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 forcing the bronchi toward the centre of the lungs. In the 

 amniotes, also, the ducts are characterized by the presence of cartilage 

 in their walls, so that they are true bronchi. These bronchi may also 

 extend inside of the lungs, often dividing into secondary and tertiary 

 bronchi inside them. 



REPTILES. In many reptiles (snakes, amphisbaenans, many 

 skinks) the lungs are asymmetrical (left usually larger in snakes, right in 

 lizards) and exceptionally one may be absent in snakes. The internal 

 structure shows considerable variation. The simplest conditions are 

 found in the snakes and mSphenodon (fig. 267), where the lungs consist 

 of a single sac lined with infundibula in the basal portion (snakes) or 

 throughout (Sphenodori). In the lizards (fig. 268) one or more par- 

 titions or septa extend from the distal wall of the lung nearly to the en- 

 trance of the bronchus, thus dividing the lung into chambers lined with 

 alveoli; while a part of the bronchus may extend (main bronchus, 

 fig. 268, B) to the extremity of the lung. In the chameleons the septa 

 do not reach the distal wall so that the chambers communicate here as 

 well as at the proximal side, the result being that the bronchus enters a 

 cavity, the atrium, which connects with the chambers separated by the 

 septa, and these in turn open into a terminal vesicle, a condition recall- 

 ing the parabronchi of the birds, soon to be described. This resem- 

 blance is heightened by the development in these same lizards of long, 

 thin-walled sacs from the posterior part of the lung which extend among 



