566 



REPTILIA. 



vided into thorax and abdomen, as it is in Mammalia. From 

 the internal surface of the walls of each lung membranous septa 

 project inwards, so as partially to divide the interior of the organ 

 into numerous polygonal cells, which are themselves subdivided 

 into smaller compartments in a similar manner. This structure is 

 well seen in the lung of the Tortoise (Jig. 255). 



The pulmonary Fig. 255. 



cells are most nume- 

 rous and complete 

 towards the anterior 

 extremity of the 

 lung, and it is here 

 that the pulmonary 

 vessels principally 

 ramify : towards the 

 hinder part of the 

 viscus the cells be- 

 come larger, and 

 the breathing sur- 

 face proportionately 

 less extensive, until 

 in some cases, as in 

 Serpents, the cells 

 being quite oblite- 

 rated, the lung ter- 

 minates posteriorly 

 in a simple mem- 

 branous bladder. 



The air is brought 

 into the lungs 

 through a long tra- 

 chea composed, as 

 in other Vertebrata, 



of a series of cartilaginous rings ; but there is this peculiarity in 

 the construction of the Reptile lung, the trachea never divides into 

 bronchial ramifications, but terminates abruptly by one or more 

 orifices, which open at once into the general pulmonary cavity. 



It must be evident, from the whole construction of a lung of 

 this description, that, owing to the comparatively limited surface 

 that it presents internally, it is far less adapted efficiently to expose 

 the circulating fluid to the influence of the atmosphere than the 



