RESPIRATION 



393 



bounded by the vocal bands, is also triangular in shape, though in this case 

 the base is directed backward, and the apex forward. (See chapter on Phona- 

 tion and Speech.) 



The introduction of the vocal bands narrows at this level the air-passage 

 and to some extent interferes with the free entrance of air. According to 

 the investigations of Semon, the area of the air-passage above and below 

 the phonatory apparatus is about 200 sq. mm.; while the area bounded 

 by the vocal apparatus is but 155 sq. mm. during quiet respiration. 



The Trachea. The trachea is a tube, some 12 centimeters in length, 

 from two to two and a half centimeters in width, and extends from the 

 lower border of the larynx to a point opposite the fifth thoracic vertebra. 

 It consists of an external fibrous and an internal mucous membrane, be- 

 tween which is a series of superposed C-shaped arches or rings of elastic 



FIG. 180. Diagram showing the larnyx, trachea, bronchi, and lungs; the ramification of the 

 bronchi, and their division into lobules. 



cartilage, some 18 or 20 in number. Between the fibrous and mucous coats 

 posteriorly, and occupying the space between and attached to the free ends 

 of the cartilages, there is a layer of transversely arranged non-striated 

 muscle-fibers, known as the tracheal muscle. The alternate contraction 

 and relaxation of this muscle would by varying the distance between the 

 ends of the cartilages, either diminish or increase the lumen of the trachea. 

 The surface of the mucous membrane is covered by a layer of stratified 

 columnar ciliated epithelium. In the submucous tissue there are a num- 

 ber of glands the ducts of which open on the free surface. 



Opposite the fifth thoracic vertebra the trachea divides into a right 

 and a left bronchus. Each bronchus again subdivides into two or three 

 branches, which penetrate the corresponding lung. 



The Lungs. The lungs, in the physiologic condition, occupy the greater 

 part of the cavity of the thorax. They are separated from each other by the 



