8o THE VITAL PROCESSES 



described in connection with the organs of digestion 

 (Chapter X). Air entering the pharynx, either by the 

 nostrils or by the mouth, passes through it into the larynx. 

 The larynx, being the special organ for the production of 

 the voice, is described later (Chapter XXI). The entrance 

 into the larynx is guarded by a movable lid of cartilage, 

 called the epiglottis, which prevents food particles and 

 liquids, on being swallowed, from passing into the lower air 

 tubes. The relations of the nostrils, mouth, pharynx, and 

 larynx are shown in Fig. 34. 



From the larynx the air enters the trachea, or windpipe. 

 This is a straight and nearly round tube, slightly less than 

 an inch in diameter and about four and one half inches in 

 length. Its walls contain from sixteen to twenty C-shaped, 

 cartilaginous rings, one above the other and encircling the 

 tube. These incomplete rings, with their openings directed 

 backward, are held in place by thin layers of connective 

 and muscular tissue. At the lower end the trachea divides 

 into two branches, called the bronchi, each of which closely 

 resembles it in structure. Each bronchus separates into a 

 number of smaller divisions, called the bronchial tubes, and 

 these in turn divide into still smaller branches, known as 

 the lesser bronchial tubes (Fig. 33). The lesser bronchial 

 tubes, and the branches into which they separate, are the 

 smallest of the air tubes. One of these joins, or expands 

 into, each of the minute lung sacs, orinfundibula. Mucous 

 membrane lines all of the air passages. 



General Condition of the Air Passages. One necessary 

 condition for the movement of the air into and from the 

 lungs is an unobstructed passageway. 1 The air passages 



1 In ordinary inspirations the force that causes the air to move through the 

 passages is scarcely an ounce to the square inch, while in forced inspirations it 

 does not exceed half a pound. On this account the closing of any of the air pas- 



