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TEXT-BOOK OF PHYSIOLOGY. 



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

 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. 



FIG. 178. TRACHEA AND BRONCHIAL TUBES, i, 2, Larynx. 3, 3. trachea. 4. Bifurcation 

 ot trachea. 5. Right bronchus. 6. Left bronchus. 7. Bronchial division to upper lobe of right 

 lung. 8. Division to middle lobe. 9. Division to lower lobe. 10. Division to upper lobe of left 

 lung. ii. Division to lower lobe. 12, 12, 12, 12. Ultimate ramifications of bronchi. 13, 13, 13, 

 13. Lungs, represented in contour. 14, 14. Summit of lungs. 15, 15. Base of lungs. (Sappey.) 



The trachea is a tube, some 12 centimeters in length, from two to 

 two and a half centimeters in breadth, extending from the lower border 

 of the larynx to a point opposite the fifth dorsal vertebra. It consists of 

 an external fibrous and an internal mucous membrane, between which is 

 a series of superposed C-shaped arches or rings of elastic cartilage, some 

 1 8 or 20 in number. Between the fibrous and mucous coats posteriorly, 

 and occupying the space between and attached to the free ends of the car- 

 tilages, 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 cartil- 

 ages, either diminish or increase the caliber of the trachea. The surface of 



