VERTEBRATED ANIMALS. 



299 



the Kespiratory organs, in Fishes the respiratory surface is formed by 

 an outward extension into fringes of gills, each of which consists of an 

 arch with straight laminae hanging down from it; and every one of these 

 laminae (Fig. 484) 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 sur- 

 face, like those of Mollusks and of the larva of the Water-Newt; the ne- 

 cessity for such a mode of renewing the fluid in contact with them being 

 superseded by the muscular apparatus with which their gill-chamber is 

 furnished. But in Reptiles the respiratory surface is formed by the walls 

 of an internal cavity, that of the lungs: these organs, however, are con- 

 structed on a plan very different from that which they present in higher 



FIG. 484. 



FIG. 485. 



Two branchial processes of the Gill of 

 the Eel, showing the branchial lamellae : 

 A, portion of one of these processes 

 enlarged, showing the capillary network 

 of the lamellae. 



Interior of upper part of Lung of Frog. 



Vertebrata, the great extension of surface which is effected in the lat- 

 ter by the minute subdivision of the cavity not being here necessary. In 

 the Frog (for example) the cavity of each lung is undivided; its wall?, 

 which are thin and membranous at the lower part, there present a sim- 

 ple 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 crowded with blood- 

 vessels (Fig. 485). 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 which 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 



