162 



Life and Health 



of its own wall and the pulmonary membrane. Hence a 

 gaseous interchange, the essential step in respiration, takes 

 place between the blood and the air, by which the latter 

 gains moisture and carbon dioxide, and loses its oxygen. 

 These changes in the lungs also restore to the dark blood 

 its bright red tint. 



The only condition absolutely necessary to respiration 

 is an organ having a delicate membrane, on one side of 

 which is a thin sheet of blood, while the 

 other side is in such contact with the air 

 that an exchange of gases can readily 

 take place. 



The demand for oxygen is, however, 

 so incessant, and the accumulation of 

 carbon dioxide is so rapid in every tis- 

 sue of the body, that an All-Wise Creator 

 has provided an extensive and compli- 

 cated accessory system of machinery to 

 effect this most remarkable and impor- 

 tant change in the blood. 



Let us now study the arrangement 

 and working of this machinery, or, in 

 other words, the organs of respiration. 



245. Passages from the Mouth. If we look into the 

 mouth of a friend, or into our own with the aid of a mirror, 

 we see at the back part an arch which is the boundary line 

 of the mouth proper. The funnel-shaped cavity beyond, 

 into which both the mouth and the posterior nasal pas- 

 sages open, is called the pharynx. In its lower part are 

 two openings : the trachea, or windpipe, in front, 'and the 

 cesophagus behind (Figs. 5 I and 45). 



246. The Larynx and the Epiglottis. The trachea, or wind- 

 pipe, is surmounted by a box-like structure of cartilage, about 



FIG. 83. 

 The Epiglottis. 



