278 



COMPARATIVE MOPRHOLOGY OF VERTEBRATES 



bronchi which is on the medial side (fig. 302, B). 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 tlie 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 



Fig. 301. Fig. 302. 



Fig. 301. — Lungs of Sphenodon, after Gegenbaur; the left lung opened to show the 

 alveoli. 



Fig. 302. — A , left lung of Iguana; B, right lung of Varanus, after Meckel, b, bron- 

 chus; c, connexion between dorsal and ventral chambers; cb, main bronchus; d, dorsal 

 chamber; lb, lateral bronchi; s, septa; sb, secondary bronchus; v, ventral chamber. 



internal structure shows considerable variation. The simplest con- 

 ditions are found in the snakes and in Sphenodon (fig. 301), where the 

 lungs consist of a single sac lined with inf undibula in the basal portion 

 (snakes) or throughout (Sphenodon). In the lizards (fig. 302) one 

 or more vertical partitions or septa extend from the distal wall of the 

 lung nearly to the entrance of the bronchus, thus dividing the lung 

 into chambers lined with alveoli; while a part of the bronchus may 

 extend (main bronchus, fig. 302, 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 



