368 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the throat almost directly into two large sacs; only 

 each sac is sacculated on the interior, so as to increase 

 its surface (Fig. 249). The aeration of the blood, there- 

 fore, is still more imperfect than in reptiles. (2) Again 

 amphibians not only have no diaphragm, but they have 

 no ribs. The mechanics of their breathing is entirely 

 different from all previously men- 

 tioned. It is not diaphragmatic nor 

 yet costal. It is a throat respiration. 

 The submaxillary space, or space be- 

 tween the branches of the lower jaw, 

 is large. This space is pressed down 

 by the hyoid or tongue bone, and 

 thus draws air through the nostrils 

 and fills the throat. The nostrils are 

 then closed, the throat is brought up, 

 and the air is forced down into the 

 lung sacs. This, of course, swells 

 the walls of the abdomen, which, re- 

 acting by contraction, drive out the air again through 

 the nostrils. 



This is the lowest form of lung respiration and 

 grades into gill respiration. In fact, all amphibians in 

 their early larval life breathe by gills, and only after- 

 ward by lungs. This, therefore, leads us naturally to 

 gill respiration. 



Fig. 249.— One lung of 

 a frog : &r, bronchus. 



2. GILL RESPIRATION. 



The most perfect form of gills, or branchial, is found 

 in the ordinary typical fishes (teleosts). We take this as 

 a type. 



Take any typical fish, such as a perch or a salmon 

 (Fig. 250). We observe on each side of the head an 

 opening extending downward and forward to beneath 

 the chin. It is covered with a' movable flap, the oper- 



