234 



ANATOMY AND PHYSIOLOGY 



structed of fibre-cartilages connected with each other by ligaments 

 and lined by mucous membrane. The largest fibro-cartilage is the 

 thyroid, which forms the prominence of the larynx known as 

 "Adam's apple." Below the thyroid is the cricoid cartilage, 

 shaped like a seal ring, and placed with the broad part at the 

 back, where two small pyramid-shaped 

 cartilages rest upon it; they are the aryte- 

 noids. These are all connected by gliding 

 joints. (Other cartilages, very minute, 

 are not mentioned.) 



The epiglottis is a leaf-shaped flexible 

 cartilage extending upward from the thy- 

 roid in front, and resting against the base 

 of the tongue. During swallowing this is 

 bent backward over the entrance of the 

 larynx by the action of small muscles, to 

 allow the food to pass over it into the 

 esophagus. (For the Larynx, the Organ 

 of the Voice, see page 344.) 



FIG. 157. INTERIOR OF 

 LARYNX (LEFT SIDE RE- 

 MOVED) . (Sappey) . 



2, Epiglottis; 5, so-called 

 "false vocal cord"; 9, vocal 

 band; 13, thyroid cartilage; 

 14, arytenoid cartilage. 

 The other figures refer to 

 parts not mentioned in the 

 text. 



THE TRACHEA 



The trachea is a flexible tube about 

 one inch in diameter and four and one- 

 half inches long, extending downward from the larynx to the 

 level of the fourth thoracic vertebra. It is fibrous and elastic, 

 and stiffened with rings of cartilage which are incomplete at the 

 back; unstriped muscle fibers take their place, constituting the 

 tracheal muscle. 



The tracheal muscle makes the tube soft where the esophagus lies next to 

 it, and by the action of its fibers varies the size of the trachea. 



The trachea divides into two branches called bronchi. The 

 right bronchus is one inch long; the left is two inches long (it 

 passes under the arch of the aorta). 



The bronchi divide into branches called bronchial tubes which 

 subdivide again and again until the smallest tubes, called bron- 

 chioles, are formed. These lead to the spaces called alveoli, and 

 the air cells clustered about them. 



