THE LUNGS OF KEPTILES. 



865 



Fig. 115. 



The lungs of the Frog, Fig. 115, 2, may be 

 described as subcellular ; the internal subdi- 

 vision of their surface into cells is more 

 simple than that of the Saurian reptiles, 

 though more complex than the alveolar 

 structure of the Ophidian lung. The cor- 

 responding bronchus opens at once into 

 this subcellular lung, the alveoli at the 

 upper part of which are supported by fine 

 cartilaginous trabeculse ; it has been aptly 

 compared to a single ultimate lobule of the 

 lung of the Bird ; but the layer of capillaries 

 is single, and is exposed to the air on one 

 side only. The respiration of the Amphibia 

 is comparatively imperfect, and their blood 

 is cold ; moreover, it is partly performed by 

 the surface of their moist skin, as has been 

 proved by their continuing to exhale car- 

 bonic acid after the removal of both lungs, 

 an operation which they for a time survive. 

 Even in temperate climates, the Amphibia 

 hibernate beneath the water ; and in that 

 state they respire solely by the skin ; but in 

 summer, simultaneously with a greater gen- 

 eral energy, their respiration becomes more 

 active, and then they breathe by the lungs 

 also. The necessity for respiration increases 

 in these and in other cold-blooded animals, 

 with the elevation of the temperature of the 

 surrounding medium, which excites them to 

 greater activity of body. In the Siren and 

 Arnphiuina, the interior of the lungs is only Fig. 115. i, Lungs of a serpent ; the right 



Slightly alveolar, presenting the last traces one only is fully developed; it is of such 

 Of a Structure like air-cells ; in the Proteus, length, that the upper and lower portions 



the lungs are smooth on their internal sur- 

 face like the air-sacs of the Fish ; the trachea 

 is membranous, and the glottis is a simple 

 cleft. 



In the early larval or tadpole state, all the 

 Amphibia breathe aquatically, and some 



species retain the means of so doing in the adult condition ; but all of them 

 have lungs, however simple. 



To Fishes the aquatic form of respiration is proper ; but, as the homologue of 

 the lungs of the other Vertebrata, the air-bladder of certain species requires men- 

 tion. The air-bladder of the Fish is a simple membranous sac or bag, placed be- 

 neath the spine, sometimes elongate, cylindrical, or fusiform, sometimes pyri- 

 form, sometimes provided with simple diverticula, more rarely with short 

 branched tubuli ; sometimes it is double, one part being longer than the other, 

 thus resembling the asymmetrical lung of the Serpents. The air-bladder always 

 contains some air. It is usually a closed sac ; but, in many cases, as in the pike, 

 carp, salmon, and herring, it communicates by means of a short open duct, 

 the ductus pneumaticus, with the oesophagus, near the stomach or higher up ; 

 in the Lepidosiren it communicates with the pharynx, and even opens into it 

 by a slit like a glottis ; it is also bifid and slightly alveolar, closely resembling, 

 therefore, a simple saccular lung ; it sometimes presents vascular projections 

 in its interior. The idea that the air-bladder is the homologue of the lungs is 

 supported by the fact, that in the embryos of the so-called pulmonated Yer- 

 tebrata, the primitive lungs originate as little buds developed from the sides 

 of the upper part of the oesophagus, which afterwards become hollowed out 

 and branched. There is no homology between the gills and lungs in the Ver- 

 tebrate series of animals ; they have analogous functions, but they are totally 

 distinct parts of the organism. 



55 



alone are represented ; the upper end is tra- 

 beculated and sacculated. 2, Lungs of the 

 frog, showing their simple cellular character. 

 3, Portion of the lung of a turtle, showing its 

 compound cellular structure. 



