RESPIRATORY ORGANS. 243 



III. RESPIRATORY ORGANS. APPARATUS RESPIRA- 



TORIUS. 



The organs of respiration consist of the nasal cavity, the 

 nasopharynx, the pharynx (also a food-passage), the larynx 

 (also the organ of the voice), the trachea, the bronchi, and the 

 lungs. With them are usually described also the thyroid and 

 thymus glands. 



I. The Nasal Cavity. Cavum nasi. — The osseous frame- 

 work of the nasal cavity has already been described (page 59), 

 and in connection with this description the boundaries of the 

 cavity and its connections with other cavities have been given. 

 It consists essentially of a large cavity bounded by the facial 

 bones and divided by a longitudinal partition into two lateral 

 halves. The two cavities thus formed are nearly filled by (i) 

 the labyrinths of the ethmoid (ethmoturbinals), (2) the superior 

 nasal conchae or nasoturbinals, projecting into the dorsal part 

 hom the ventral surface of the nasal bones, and (3) the inferior 

 nasal concha:, or maxilloturbinals, projecting into the ventral 

 portion from the medial surfaces of the maxillaries. 



There remain to be considered, in addition to the bones, 

 the cartilaginous framework of certain parts of the nose, and 

 the mucous membrane. The lamina perpendicularis is con- 

 tinued by cartilage, especially craniad, in such a way as to 

 make a complete septum separating the two cavities. This 

 septum extends from the septum of the external nose caudad 

 to the lamina cribrosa, and from the internasal suture ventrad 

 to the vomer and the suture of the premaxillaries. All parts 

 of the nasal cavity are lined by mucous membrane. This is 

 continuous at the nares with the integument, while at the 

 choana; it passes into the mucosa of the pharynx. It covers 

 the conchae nasales and the labyrinths of the ethmoid, passing 

 into the cellules of the latter. Owing to the crowding together 

 of the conchae nasales and the labyrinths the nose is almost 

 completely filled, only three narrow passageways being distin- 

 guishable. The ventral one of these, known as the ventral or 

 inferior meatus of the nose, lies ventrad of the inferior nasal 

 concha, next to the nasal septum. It passes caudad beneath 



