LUNGS 



291 



give rise to a tubular and sponge- 

 like meshwork (comp. Fig. 236). 



The lung of Snakes exhibits an in- 

 termediate form, for in spite of the 

 finely-meshed tissue arising from the 

 periphery, it still retains a narrow central 

 cavity. The right lung only is as a rule 

 fully developed in Snakes and Amphis- 

 boenians, owing to the elongated form of 

 the body, while the left remains in a 

 rudimentary condition, or even disap- 

 pears entirely. 



In the Chameleon (Fig. 236) the an- 

 terior portion of the lungs is much more 

 compact and spongy than the posterior, 

 which grows out into numerous sac-like 

 processes, some of which reach as far 

 back as the pelvic region ; their form 

 is very variable, being spindle-shaped, 

 club-shaped, or lobulated, and their 

 walls are very thin ; they extend in 

 amongst the viscera wherever there is 

 room. If these processes have any res- 

 piratory function, it is at most only a 

 very slight one. An indication of a 

 similar arrangement is seen in the lungs 

 of Testudo, in which a single thin- walled 

 process extends backwards to the pelvic 

 region. These processes seem to fore- 

 shadow a condition which reaches its 

 highest development in Birds. 



A uniform ground-plan is to be 

 observed in the arrangement of the 

 intra-pulmonary bronchial system 

 through the whole series of the 

 Amniota, from Crocodiles onwards. 

 A continuation of the bronchus, 

 which is almost straight, always 

 passes through the lung to its pos- 

 terior end. This may be called the 

 main bronchus; from it a series of 

 lateral bronchi arise. 



Birds. — The respiratory appar- yi,-. 236. 

 atus of Birds presents so many 

 -remarkable peculiarities, both as 

 regards the structure of the lungs 

 and in the presence of air-sacs, that it must be considered in some 

 detail. 



The comparatively small but highly vascular lungs (Figs. 237 

 and 238) are closely applied to the thoracic vertebrae and heads of 



u 2 



— Lungs of Chamceteo 

 monachus. 



T — trachea. 



