372 OF RESPIRATION. 



blood-vessels are minutely distributed. The greatest amount of subdi- 

 vision is seen in the lungs of the Turtle tribe ; but even in these, the 

 partitions scarcely form a complete division at any part of the lungs ; 

 and the ultimate air-cells are of very large size (Fig. 105). The air- 

 sacs of Reptiles are not filled, like those of 

 Fig. 105. Mammalia, by an act of inspiration, but by a 



process of swallowing, which is comparatively 

 tedious ; and, from the small amount of aerating 

 surface, in proportion to the amount of air thus 

 received into the cavity, one inflation of the air- 

 sacs lasts for a considerable time. When the 

 replacement of oxygen by carbonic acid has 

 proceeded to an extent that renders the air no 

 longer fit to remain in the lungs, these cavities 

 are emptied by pressure exercised upon them 

 by the muscles of the trunk ; and the slow exit 

 of the air through the narrow glottis is accom- 

 panied by a prolonged hissing sound, which is 

 the only sort of voice that is possessed by the 

 greater part of the Reptile class. The lungs 

 are again filled by the swallowing-process ; and 

 all goes on as before. 



section of the Lung of the Turtle. 670. Now in the Frog tribe, which forms the 

 lowest order of Reptiles (and which is sometimes 



ranked as a distinct' class, under the title of Amphibia), the respiration 

 during the early or Tadpole state, is aquatic ; being carried on by 

 means of gills, and conducted exactly upon the plan of that of Fishes. 

 The lungs are not developed, until a period long subsequent to the ani- 

 mal's emersion from the egg ; and as soon as they are ready to come 

 into play, an alteration begins to take place in the circulating system, 

 by which the current of blood is diverted towards them, and away from 

 the gills ( 562). This change takes place to its full extent in the Frog, 

 Toad, Newt, and their allies ; which henceforth have a respiration and 

 a circulation exactly analogous to that of Reptiles in general ; but it is 

 checked in the Proteus, Siren, and other species, which form the peren- 

 nibranchiate group, so called from the persistent character of their 

 gills, which still remain in action, the lungs never being sufficiently de- 

 veloped to maintain the respiration by themselves. The curious influ- 

 ence which Light possesses on this metamorphosis, has been already re- 

 ferred to ( 95). 



671. This order BatracTiia is further distinguished from other Rep- 

 tiles, even when the metamorphosis is complete, by the softness and 

 nakedness of the skin, which is destitute of the scales and horny plates 

 that cover it in the Lizards, Serpents, and Tortoises. The skin of the 

 Frog tribe is a very important organ of respiration, being richly sup- 

 plied with blood-vessels, and exposing their contents to the influence of 

 the air, under circumstances nearly as favourable as those afforded by 

 the imperfectly-developed lungs of these animals. Thus a Frog, from 

 which the lungs have been removed, will live a considerable time at a 

 moderate temperature, if its skin be freely exposed to a moist air $ for, 

 in consequence of the peculiar mode in which the circulation is carried 



