62 MANUAL OF ELEMENTARY ZOOLOGY 



irregular ring, the cricoid cartilage. The lining of the 

 larynx is thrown into a pair of folds, the vocal chords. 

 Between these is a narrow slit, the rima glottidis, through 

 which the air must pass to and from the lungs. The 

 cartilages are supplied with muscles, by means of which 

 they can be moved, so as to tighten the vocal chords and 

 bring them close together. In this condition the chords 

 vibrate when air from 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 being thrown into numerous folds. 

 The lungs of the frog are not enclosed, like those of man, 

 in a " chest " shut off by a midriff, but lie free in the fore- 

 part of the common body cavity, and the mode of breathing 

 is correspondingly different in the two cases. In man, 

 breathing is brought about by an enlargement of the chest, 

 which draws air into it, followed by a collapse which drives 

 it out. In the frog, air is drawn into the mouth by a lower- 

 ing of the floor by the muscles of the hyoid, the nostrils 

 being open and the mouth and glottis closed. The floor of 

 the mouth is then raised by the mylohyoid and jaw muscles, 

 while the glottis is open and the mouth and nostrils closed. 

 The air is thus driven into the lungs, from which it is after- 

 wards expelled by the collapse of their elastic walls, aided 

 by contraction of the abdominal muscles. 



In the lungs an interchange of oxygen for carbon dioxide 



takes place through the thin walls between 

 venoiioBiooti. tne 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. 

 In the tissues of the body the blood undergoes a reverse 

 change, parting with its oxygen to the protoplasm and re- 

 ceiving from it carbon dioxide (see p. 60). The blood which 

 has thus become poor in oxygen and rich in carbon dioxide 

 returns to the heart through the veins. Such blood is 

 therefore known as venous blood. It is of a dark red colour. 

 On reaching the heart, this blood is directed, as we have 

 seen, principally to the lungs, skin, and mucous membrane 



