STRUCTURE OF AIR PASSAGES. 327 



connective tissue is the remarkably close-set meshwork of capilla- 

 ries, in which the blood is exposed to the air. The delicate wall 

 of the vessel and thin body of the epithelial lining cell are the 

 only structures interposed between the blood and the air. 



2. The air passages are kept permanently open during ordinary 

 breathing by the elasticity of more or less rigid tissues. The 

 trachea and bronchi have special cartilaginous springs for the 

 purpose. These are closely attached to the fibro-elastic tissues 

 which complete the general foundation of the walls of the tubes. 

 The air passages are throughout lined with ciliated cylindrical 



FIG. 146. 



Muscles of Larynx, viewed from above. Th. Thyroid cartilage. Or. Cricoid cartilage. 

 V. Edges of the vocal cords. Ary. ArytenoiJ cartilages. Th. A . Thyro-ary tenoid muscle. 

 C.a.l. Lateral crico-ary tenoid muscle. C.a.p. Posterior crico-arytenoid muscle. Ar.p. 

 Posterior arytenoid muscle. 



epithelium, which, at the entrance to the infundibula, loses its 

 cilia, and forms but a single layer of flattened cells. 



The air passages are supplied with muscle tissue of different 

 kinds. Besides the ordinary striated muscles that control the 

 opening of the anterior and posterior nares and pharynx, a special 

 set surrounds the upper part of the larynx, and is capable of 

 completely closing the glottis, and thus shutting off the lung 

 cavities and proper air passages from the outer air. ( V, Fig. 146.) 



In the trachea a special muscle exists which can narrow the 



