222 OF EESPIRATIOX. 



tebral column, and, in general, communicating with the pha- 

 rynx, or stomach, by a membranous canal. Numerous blood- 

 vessels and nerves, derived from the eighth pair and the sym- 

 pathetic, are distributed on its walls ; this organ is most de- 

 veloped in those fishes which come frequently to the surface 

 of the water, and are remarkable for their vehement and 

 prolonged muscular movements, as the Lepidosteiis of the 

 American rivers. The air-sac in this fish is divided into two 

 chambers, the lining membrane presenting an arrangement 

 of cells like the lung of a reptile ; the duct from this air-sac, 

 surmounted by a rudimentary larynx, opens high up in the 

 throat, and, although a simple membranous tube, is the horno- 

 logue of the trachea of air-breathing vertebrata. In the Lepi- 

 dosiren the air-sac is a double organ, each division being divided 

 into several lobes ; it is situated behind the kidneys, against the 

 ribs, and is internally cellular, like the lung of a serpent ; an- 

 teriorly it opens by a tolerably long, narrow membranous 

 tube into the esophagus ; each division of the air-sac receives 

 a branch of the pulmonary artery, arising from the branchial 

 arteries. For these reasons ttye air-sac of fishes is regarded as 

 a rudimentary lung, performing an accessory part in the great 

 function of respiration. It is least developed, or even wanting, 

 in those species which live at the bottom, and burrow in sand 

 or mud, as the lampreys, rays, and Pleuronec ticks. Many 

 fishes respire by the intestinal canal, the air which they swallow 

 at the surface being employed for that purpose, as it escapes 

 from the intestine loaded with carbonic acid gas. The fact of 

 fishes swallowing air may be seen in the electric eel, and in 

 fishes kept in vases, the water of which has been deprived of 

 its air by their respiration. 



[ 384. The higher forms of reptiles, as serpents, lizards, 

 and turtles, breathe by lungs. In the amphibia, one group 

 comprising the frogs and salamanders, respire, during a term 

 of their embryonic development, by vascular tufted gills ; but 

 these organs are subsequently absorbed, as the lungs become 

 developed ; and, during adult life, they breathe air by lungs, 

 respiration being aided by the general surface of their smooth, 

 naked, tegumentary membrane. In another group the gills 

 are persistent through life, and co-exist with the lungs. Such 

 is the case in the Amphiuma, Menobranchus, Proteus, Siren, 



