316 THE ANATOMY OF VEETEBEATED ANIMALS. 



frequently, especially among the poisonous snakes, one lung 

 is T-udinientary or altogether absent; and the posterior 

 portion of the trachea may take on the structure of a Itmg. 



The lungs of Lizards much resemble those of the OpMdia, 

 and they are elongated and unequal in size in the snake- 

 like Lacertilia. In the ordinary lizards they are more 

 rounded and the trachea and bronchi are shorter. In many 

 Chamseleons, and in some Geckos, the posterior half of 

 each lung is prodiiced into nan-ow diverticula, which lie 

 among the abdominal viscera, and foreshadow the aii- sacs 

 of bii'ds. 



In the Crocodilia each bronchus traverses its lung, and 

 at fii'st retains, but soon loses, its cartilaginous rings. 

 Lateral apertm-es in the walls of the bronchus lead into 

 sacculated pouches, each of which resembles the liing of an 

 oi'dinary Lacertilian. 



The Chelonia have similar lungs ; but while, in the fore- 

 going gi'oups, the two lungs are free and invested on all 

 sides by the peritoneum, in this they are fixed against the 

 inner periosteum of the carapace, and are covered by peri- 

 toneiim on their ventral face only. This resemblance to 

 the arrangement of the lungs in birds is increased by the 

 presence of a muscular diaphragm, the fibres of which 

 spread over the ventral faces of the lungs. 



In Aves the lungs are fii-mly fixed on each side of the 

 vertebral column, the dorsal surface of each lung being 

 moulded to the supeijacent vertebrte and ribs. The muscular 

 fibres of the diaphragm arise from the ribs outside the 

 mai-gins of the lungs, and from the vertebral column, and 

 end in an aponeurosis upon the ventral surface of the 

 lungs. 



Each bronchus enters its lung nearer the centre than 

 the anterior edge ; and, immediately losing its cartilaginous 

 or bony rings, dilates, and then traverses the lung, gradually 

 naiTowing, to the posterior edge of that viscus, where it 

 terminates by opening into the posterior air sac, which 

 generally lies in the abdomen. From the inner side of the 

 bronchus caaials are given off, one near its distal end, and 



