RESPIRATORY ORGANS 



1063 



is formed by an outward extension into fringes of gills, each of which 

 consists of an arch with straight lamime hanging down from it, and 

 every one of these lamina? (fig. 799) is furnished with a double row 

 of leaflets, which is most minutely supplied with blood-vessels, 

 their network (as seen at A.) 

 being so close that its meshes 

 (indicated by the dots in the 

 figure) cover less space than the 

 vessels themselves. The gills of 

 fish are not ciliated on their 

 surface, like those of molluscs 

 and of the larva of the water- 

 newt, the necessity for such a 

 mode of renewing the fluid in 

 contact with them being super- 

 seded by the muscular apparatus 

 with which their gill- chamber is 

 furnished. But in batrachians 

 and reptiles the respiratory sur- 

 face is formed by the walls of 

 an internal cavity, that of the 

 lungs : these organs, however, 

 are constructed on a plan very 

 different from that which they 

 present in higher Vertebrata, 

 the great extension of surface 

 which is effected in the latter 

 by the minute subdivision of 

 the cavity not being here neces- 

 sary. In the frog (for example) network of the lamellae, 

 the cavity of each lung is un- 

 divided ; its walls, which are 

 thin and membranous at the 

 lower part, there present a 

 simple smooth expanse ; and it 

 is only at the upper part, where 

 the extensions of the tracheal 

 cartilage form a network over 

 the interior, that its surface is 

 depressed into sacculi whose 

 lining is crow r ded with blood- 

 vessels (fig. 800). In this 

 manner a set of air-cells is 

 formed in the thickness of the 

 upper wall of the lung, which 

 communicate with the general 

 cavity, and very much increase 



the surface over whicli the blood comes into relation with the air ; 

 but each air-cell has a capillary network of its own, which lies 

 on one side against its wall, so as only to be exposed to the air 

 on its free surface. In the elongated lung of the snake the same 



r. 799. Two branchial processes of the 

 gill of the eel, showing the branchial 

 lamellae : A, portion of one of these pro- 

 cesses enlarged, showing the capillary 



FIG. 800. Interior of upper part of 

 lung of frcg. 



