144 THE FROG chap. 



Sespiration. — By these alternate movements of inspira- 

 tion and expiration fresh air passes into the lungs at regular 

 intervals, while part of the air already contained in them is 

 expelled. Now we saw, when studying the blood (p. 79), 

 that dark purple blood drawn from a vein becomes bright 

 scarlet when exposed to air, and we subsequently learnt 

 (p. 107) that this change is due to the absorption of oxygen 

 by the red corpuscles. 



The blood brought to the lungs by the pulmonary artery 

 is, as we have seen (p. 94), non-aerated, being the impure 

 blood returned by the three caval veins to the right auricle. 

 When this blood is pumped into the capillaries of the lungs 

 it is separated from the air contained in those organs only 

 by the extremely thin walls of the capillaries themselves 

 and the equally delicate pavement epithelium lining the 

 lungs (Fig. 23, p. 142, Ep. Ing, Cp. Ing). Under these 

 circumstances an interchange of gases takes place between 

 the air and the blood : the hsemoglobih of the red 

 corpuscles absorbs oxygen, and the carbon dioxide in the 

 blood, derived from the waste of the tissues, is given off into 

 the cavities of the lungs. The blood in the pulmonary 

 capillaries thus become aerated and is returned as red blood 

 to the left auricle : at the same time it loses carbon dioxide, 

 together with a certain amount of water, and these waste 

 substances are expelled from the body with the expired air. 



Voice. — It was mentioned above (p. 141) that the glottis 

 and laryngo-tracheal chamber are supported by cartilages. 

 The largest of these are a pair of semilunar arytenoid 

 cartilages (Fig. 44, ar), which bound the glottis to right and 

 left. The mucous membrane on the inner or adjacent faces 

 of the arytenoids is raised into a pair — right and left — of 

 horizontal folds, the vocal cords (v.c£). By means of muscles 

 these folds can be stretched and relaxed, and can be brought 



