THE NASAL CAVITY 293 



thereto, which, beginning with the nasal cavity, successively includes 

 the nasopharynx, larynx, trachea, arid bronchi of gradually diminishing 

 caliber, and which finally ends in the terminal bronchioles leading to the 

 air sacs and pulmonary alveoli. 



The arrangement of these several portions of the respiratory system 

 has been frequently compared to the structure of the tubulo-acinar 

 glands. From this point of view the larynx and trachea form the duct 

 stem of the gland, the bronchi form the branching interlobar and inter- 

 lobular ducts, and the terminal bronchioles (intercalary ducts) end in 

 the numerous acinar air saccules of the lung. 



Development. The original anlage of the respiratory system, begin- 

 ning with the larynx, is a short linear ventromedial evagination from 

 the cephalic end of the primitive esophagus. This tracheal groove be- 

 comes separated, distally, thus forming a tube, which grows backward, 

 meanwhile dividing distally into the bronchi which undergo further 

 division to form the successively finer branches of the pulmonary system. 

 The respiratory epithelium is thus of entodermal origin, and becomes 

 enveloped in connective tissue of mesodermal origin. 



THE NASAL CAVITY 



This cavity is bounded by a cartilaginous 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 respira- 

 tory portion, and (3) the olfactory portion. 



Its external and internal openings are the nares and clioance respect- 

 ively. Communicating with the nasal chambers are the sphenoidal, maxil- 

 lary, frontal, and palatal accessory sinuses, and the ethmoidal air cells. 

 Their lining membrane is continuous with, and histologically essentially 

 like, though thinner than, that of the respiratory portion of the nostril. 



THE VESTIBULE 



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 res- 

 piratory portion. The vestibule is lined by stratified squamous epithe- 

 lium, which offers a gradual transition from the moist respiratory epithe- 

 lium to the dense horny epidermis of the skin. Near its external ori- 



