270 THE ANATOMY OF VERTEBKATED ANIMALS. 



dinal septum, as in Sjihargis among the Chelonia. The tra- 

 cheal tympanum is greatly enlarged in Gephalopterus, and in 

 many Ducks, Geese, and Divers ; and in these aquatic birds 

 the' enlargement is more marked in the males, and is usually 

 asymmetrical, the left side being generally the larger. 



" In the OpMdia, the bronchus opens at once into the lung ; 

 and the latter is an elongated sac, the walls of -which are pro- 

 duced into numerous septa, which render the cavity highly 

 cellular near the bronchus, while, at the opposite end, they 

 become smooth and but little vascular. In this latter region 

 the lung may receive its blood from the systemic and not 

 from the pulmonary circulation. The lungs are always un- 

 equal in size, and the left is usually the smaller. Very fre- 

 quently, especially among the poisonous snakes, one lung is 

 rudimentary or altogether absent ; and the posterior portion 

 of the trachea may take on the structure of a lung. 



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 manv Chamre- 

 leons, and in some Geckos, the posterior half of each lung is 

 produced into narrow diverticula, which lie among the abdomi- 

 nal viscera, and foreshadow the air-sacs of birds. 



In the Crocodilia each bronchus traverses its lung, and at 

 first retains, but soon loses, its cartilaginous rings. Lateral 

 apertures in the walls of the bronchus lead into sacculated 

 pouches, each of which resembles the lung of an ordinary La- 

 certilian. 



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

 going groups, the two lungs are free and invested on all sides 

 by the peritonaeum, in this they are fixed against the inner 

 periosteum of the carapace, and are covered by peritonajum on 

 their ventral face only. This resemblance to the arrange- 

 ment of the lungs in birds is increased by the presence of a 

 muscular diaphragm, the fibres of which spread over the ven- 

 tral faces of the lungs. 



In Aves the lungs are firmly fixed on each side of the ver- 

 tebral column, the dorsal surface of each lung being moulded 

 to the superjacent vertebrce and ribs. The muscular fibres of 

 the diaphragm arise from the ribs outside the margins of the 

 lungs, and from the vertebral column, and end in an aponeu- 

 rosis upon the ventral surface of the lungs. 



Each bronchus enters its lung nearer the centre than the 

 anterior edge ; and, immediately losing its cartilaginous oi 



