CHAP. xv.J RESPIRATORY ORGANS IN ANIMAL KINGDOM. 237 



bathed the surface of the respiratory mucous membrane. The 

 amphibians have external branchiae under the form of foliations 

 or of ramified filaments, supported by the branchial arcs. 



The lung, in its most elementary form, is a 

 simple vascular sac. It appears first of all 

 among the amphibians, where it co-exists with 

 the branchiae, of which we have spoken above, 

 and which are sometimes temporary, some- 

 times permanent (perennibranchials). The 

 lungs of the proteus tribe are not more than 

 elongated sacs. Many reptiles have also 

 simple lungs. In the anourians the lung 

 is already subdivided into secondary cavities 

 or alveoli, which augment considerably the 

 respiratory surface. 



There has been a disposition to consider 

 as a rudimentary respiratory organ the 

 velatory bladder of fishes (Fig. 33). It is 

 certain that we find in this cavity carbonic 

 acid, azote, and hydrogen ; we also meet with 

 enormous proportions of oxygen. According 

 to Biot and Laroche, this proportion can rise 

 to 70 in 100 in the fishes caught at more than 

 50 metres deep, and only to from 26 to 29 

 in 100 in the others, though the deep layers 

 of water are not more oxygenised than the 

 layer on the surface. 



In reality, and making abstraction of the 

 general form of the apparatus, there is no 

 fundamental difference between the bran- 

 chiae and the lungs. The branchiae can even act in the open 

 air on the single condition of being kept in a state of sufficient 

 humidity, and on the other hand, the surface of the pulmonary 

 mucous membranes is constantly lubrified by a liquid secretion. 

 We can feed carps in the open air, in moss saturated with water ; 



FIG. 32. 



Vascular distribution in 

 the branchial laminar*: 

 o, section of osseous 

 branchial arc ; 6ft, the 

 branchial lamellae ; c, 

 branchial arteries ; c', 

 ramuscules of the 

 arteries in the lamel- 

 Ise ; d, branchial 

 veins ; d', d', venous 

 ramuscules in the la- 

 mellae. 



