290 



COMPARATIVE ANATOMY 



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consist of two delicate elongated sacs of unequal length, and con- 

 stricted in the middle ; in Proteus they extend much further 

 backwards than in Necturus. A difference in length between 



the two lungs is seen also in other Am- 

 phibia, such as Amphiuma and Siren, in 

 which the two cylindrical lungs lie near 

 togethei', close to the aorta. Their in- 

 ternal surface is raised into a network, 

 corresponding with the distribution of 

 the blood-vessels, the meshes being 

 much finer in Amphiuma, and still more 

 so in Menopoma, than in Siren. 



In many Salamanders {e.g., Salaman- 

 drinae, Amblystomatinse, Desmognathinge, 

 Plethodontinse) the lungs undergo a more 

 or less complete degeneration, even though 

 all traces of the gills disappear. The fact 

 that the floor of the mouth is continually 

 raised and lowered as in other Amphibians 

 which possess lungs, and that in some 

 cases, at any rate, the animal dies if these 

 respiratory movements are prevented, in- 

 dicates that a bucco-pharyngeal respiration 

 takes place, and that cutaneous respiration 

 (which occurs in most Amphibians) alone 

 is insufficient. In other Salamanders the 

 lungs are as a rule equal in size, and have 

 tlie form of cylindrical tubes extending 

 backwards as far as the end of the stomlach; 

 their internal surface is more or less 

 smooth. The lungs of the Gymnophiona are similar to those of 

 Salamanders, but the right alone is fully developed, and this 

 shows in its interior a complicated trabecular network : the left 

 is only a few millimetres long. 



The sac-like lungs of Anura are quite symmetrical. Their 

 internal surface, which is lined partly by ciliated epithelium,, 

 is raised up into a rich respiratory network of trabeculse, and 

 numerous smooth muscular fibres are present in their walls. 



Reptiles. — In Reptiles, as in all other air-breathing Verte- 

 brates, the form of the lungs is to a great extent regulated by 

 that of the body. In the higher types, such as the Chelonia and 

 Crocodilia, their structure is much more complicated than in 

 Amphibia ; this complication finds expression in a very considerable 

 increase of the respiratory surface. With the exception of the 

 thin-walled lungs of Lizards, which retain a very primitive con- 

 dition, we no longer meet with a large central cavity, but the 

 organ becomes penetrated by a branched system of bronchi, which 



Pie. 235. — Lungs of Pro- 

 teus (A) AND Necturus 

 (B). The communication 

 with the vestibule is indi- 

 cated by a black spot 

 anteriorly. 



