ORGANS OF RESPIRATION 377 



the lungs is forced between them, and produce a sound which is 

 the croaking of the frog. From the hinder part of the windpipe 

 an opening leads on each side to a short tube known as the 

 bronchus, which begins at once to expand into the lung. The latter 

 is a wide, thin-walled, elastic, highly vascular sac, whose internal 

 surface is increased by numerous folds. The lungs of the frog are 

 not enclosed, like those of man, in a ' chest ' shut off by a midriff, 

 but He free in the forepart of the common body cavity, and the 

 mode of breathing (air renewal) is correspondingly different 

 in the two cases. In man, as in the rabbit (p. 446), it consists in 

 an enlargement of the chest, which draws air into it, followed by 

 a collapse which drives it out. In the frog, air is pumped into the 

 lungs by the mouth. 



The buccal floor is lowered by muscles of the hyoid, and air 

 is drawn in through the open nostrils ; the external nares are 

 closed, and the raising of the buccal floor by the mylohyoid and 

 jaw muscles forces air into the lungs, which have emptied by 

 collapse of their elastic walls and pressure from abdominal 

 muscles. The air forced into the lungs must be a mixture of that 

 just expelled from them with fresh air from outside. When the frog 

 is at rest the lungs are only filled about once a minute, though 

 the beat of the buccal floor is more rapid. The jaws are closed 

 throughout. 



ARTERIAL AND VENOUS BLOOD 



In the lungs an exchange of oxygen for carbon dioxide takes 

 place through the thin walls between the air and the blood in 

 the lung capillaries. The same process goes on in the very vascular 

 mucous membrane of the mouth and in the skin. Indeed, when 

 the frog is at rest respiration is performed more through these 

 organs than through the lungs, and during hibernation the skin 

 alone is used. 



EXCRETORY ORGANS 



The respiratory organs are engaged, as we have seen, in ridding 

 the body of carbon dioxide, and some water is also lost in the 

 form of vapour through these organs. A further loss of water in 

 a hquid form and the excretion of solids dissolved in it takes 

 place through the kidneys (Figs. 294-297). These are a pair of 



M.Z. — 13 



