RESPIRATION IN REPTILES. 371 



surface and to swallow air ; and it would seem as if the interior of the 

 intestinal canal then served the purpose of a respiratory surface, the 

 air being expelled from the anus, deprived of a large part of its oxygen, 

 and highly charged with carbonic acid. 



667. In addition to their apparatus for aquatic respiration, many 

 Fishes are provided, in their air-bladder, with the rudiments of the air- 

 breathing apparatus of higher animals ; although it is only in certain 

 species, which approach Reptiles in their general organization, that 

 this really affords any aid in the aeration of the blood. The air- 

 bladder in its simplest condition is entirely closed ; and it is then 

 obviously incapable of taking any share in the respiratory function, 

 although it seems to be an organ of some importance to the animal, in 

 regulating its specific gravity, and altering its position in the water. 

 In other cases, it communicates with the intestinal tube by a short 

 wide canal, termed the ductus pneumaticus ; arid this may serve to 

 admit air, which is taken into the alimentary tube by the process of 

 swallowing just mentioned. In the Reptilian Fishes, just adverted to, 

 the air-bladder forms a double sac, which is evidently the representative 

 of the double lung of the air-breathing Vertebrata ; and it communi- 

 cates with the back of the mouth by a regular trachea or windpipe, 

 which has a muscular valve at its commencement, serving to open or 

 to close its orifice. Some of these fishes are able to live for a consider- 

 able time out of water, their respiration being maintained by these 

 rudimentary lungs ; and they can also make a hissing sound, by the 

 expulsion of the contents of the air-sacs through the narrow glottis, or 

 entrance to the trachea. 



668. The condition of the Respiratory apparatus, and the mode in 

 which the function is performed in the class of Reptiles, are peculiarly 

 interesting ; as it is in this class, that we first meet with the complete 

 adaptation of the Yertebrated structure to the aeration of the blood by 

 the direct influence of the atmosphere. Their general habits of life 

 require but a very feeble amount of aeration, especially at moderate 

 temperatures ; their muscular and nervous systems being usually exer- 

 cised in a very low degree ; their movements being sluggish, and their 

 perceptions obtuse. In fact, they may be considered, on the whole, 

 as the most vegetative of all Vertebrated animals. In accordance 

 with this character, the lungs are so constructed as not to expose any 

 very large amount of blood to the air at any one time ; and, as we have 

 already seen ( 563), only a portion of the stream of the circulation is- 

 diverted to the lungs ; the main current being sent to the system, with 

 only that amount of aeration, which it has derived from the admixture 

 of the portion of blood that has been aerated in the lungs, with the 

 venous current that has last been returned from the system. 



^69. The lungs of Reptiles are, for the most part, capacious sacs, 

 occupying a considerable part of the cavity of the trunk ; but they are 

 very slightly subdivided, so that the amount of surface they can expose 

 is really small. Where any subdivision exists, it is usually at the upper 

 extremity of the lung, near the point of entrance of the bronchial tube ; 

 and where there is no actual subdivision of the cavity, we usually find 

 that its surface is extended in this situation, by the formation of a 

 number of little depressions' or pouches in its walls, upon which the 



