172 THE BIOLOGY OF THE FROG CHAP. 



According to Baglioni the external aperture of the nares 

 does not remain closed during the last phases of the elevation 

 of the floor of the mouth ; nevertheless, air does not escape 

 from the buccal cavity, as may be shown by placing the nose 

 of the frog beneath water, when no bubbles arise from the 

 nostrils. The muscles which draw the hyoid apparatus and 

 tongue forward and upward cause the tip of the jaw to be 

 depressed when a certain position of these organs has been 

 reached and the nares open. Why, then, does not air pass 

 out of the nares as the floor of the mouth continues to be 

 raised? As Baglioni maintains, this is because the nasal 

 passages are closed from behind by means of the anterior 

 processes of the hyoid cartilage, which are so formed and 

 situated that they fit neatly into the posterior nares as the 

 hyoid apparatus is drawn upward and forward in the act 

 of inspiration. 



Changes in the Blood in Respiration. - The respiratory 

 movements that have been described are subsidiary to keep- 

 ing fresh air in close relation with the blood. On the one 

 hand we have the organs of respiration and the distribution 

 within them of the blood vessels, which are so arranged that 

 the blood is brought very close to the surface over a large 

 area. And on the other hand we have a complicated and 

 beautifully adaptive mechanism for keeping the large portion 

 of the respiratory surface included in the lungs in contact 

 with pure air. These devices facilitate the exchange of 

 gases which takes place between the air and the blood by 

 means of diffusion across the intervening membranes. The 

 blood receives oxygen from the air and gives off carbon 

 dioxide, so that the air which has been expired from the 

 lungs or buccal cavity always contains less of the former and 

 more of the latter gas. The process of respiration falls 

 into two phases : (i) external respiration, or the exchange 



