RESPIRATION AND EXCRETION 



345 



alveoli that the interchange of carbon dioxide and oxygen, 

 which results in freshening the blood, is made. 



Air, during respiration, enters through the nostrils into 

 the pharynx (figs. 156, 157). From the pharynx it passes 

 through the glottis into the larynx and trachea. The larynx 

 is the voice-box. The glottis is the opening of the voice-box, 

 closed at times by a flap called the epiglottis. 



The respiratory and the 

 digestive paths cross in the 

 pharynx. During respira- 

 tion, the epiglottis is open . 

 giving free passage for air 

 into the trachea. During 

 feeding it closes over the 

 glottis (fig. 157) so as to 

 admit the passage of food 

 into the oesophagus only. 



The heart lies in such a 

 position (fig. 175) as to be 

 able to send the blood 

 through the pulmonary ar- 

 teries directly into the lung 



tissues. The connective tis-^ ,„, . , ... ^ 



,.FiG- I'O- A bronchiole. B, entrance; 

 sue lymg between the alveoli a p, alveolar passage: Ac, air cells; I, 

 contains a mass of pulmo- infundibulum, or pouch into which 



nary capillaries, and each the air cells open. 



one of the infinite number of red blood corpuscles is exposed 



separately to the air in the air cells. By this means the 



haemoglobin of the blood corpuscles is kept saturated with 



oxygen. 



The mechanics of breathing movements. — ^Breathing 

 consists of inspiration, or taking the air through the air 

 passages into the air sacs of the lungs, and expiration, or 

 forcing air from the air sacs of the lungs out through the air 

 passages and nostrils. 



