144 THE FROG CHAP. 



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 con- 

 tained in those organs only by the extremely thin walls 

 of the capillaries themselves and the equally delicate 

 pavement epithelium lining the lungs (p. 142, and Fig. 23, 

 Cp. Ing, Ep. Ing). Under these circumstances an inter- 

 change of gases takes place between the air in the lungs 

 and the gases dissolved in the blood : the haemoglobin 

 of the red corpuscles absorbs oxygen, and the carbon 

 dioxide in the blood, derived from the waste of the 

 tissue, is given off into the cavities of the lungs. The 

 blood in the pulmonary capillaries thus becomes 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. cd). By means of muscles these folds can be 

 stretched and relaxed, and can be brought into either a 



