CHAPTER XIV 

 THE RESPIRATORY SYSTEM 



THE respiratory system may be said to comprise a true respi- 

 ratory organ, the pulmonary alveoli, in which the interchange of 

 gases between the air and the blood occurs, and a system of duct- 

 like passages leading thereto, which, beginning with the nasal 

 cavity r successively includes the naso-pharynx, larynx, trachea, 

 and bronchi of gradually diminishing caliber, and which finally 

 ends in the terminal bronchioles leading to the pulmonary alveoli 

 or air saccules. 



The arrangement of these several portions of the respiratory 

 system has been frequently compared to the structure of the 

 tubulo-acinar secreting glands. From this point of view the 

 larynx and trachea form the duct stem of the gland, the bronchi 

 form the branching interlobular ducts, and the terminal bronchi- 

 oles end in the numerous acinar air saccules of the lung. 



THE NASAL CAVITY. This cavity is bounded by a cartila- 

 ginous and bony wall and is lined by a mucous membrane which, 

 according to the nature of its epithelium, may be divided into 

 three portions : (1) the vestibule, (2) the respiratory portion, and 

 (3) the olfactory portion. 



The VESTIBULE of the nose corresponds very closely to the 

 cartilaginous portion of the nasal wall. Its mucous membrane is 

 continuous anteriorly with the skin and posteriorly with the 

 mucous membrane of the respiratory portion. The vestibule is 

 lined by stratified squamous epithelium, which offers a gradual 

 transition from the moist respiratory epithelium to the dense 

 horny epidermis of the skin. Near its external orifice are numer- 

 ous coarse stiff hairs, vibrissae, connected with which are many 

 sebaceous glands. Some of these glands also open directly upon 

 the surface of the mucous membrane. 



The fibrous tunica propria of the vestibule is continuous with 

 the corium of the skin, and in it are embedded the deeper portions 

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