THE ORGANS OF RESPIRATION. 



THE respiratory tract proper, that is excluding the nasal fossae (through 

 which the air passes when the mouth is closed) and the pharynx, includes 

 the organs concerned in effecting the interchange of the gases between the 

 blood and the inspired air and, in addition, the production of voice. It 

 comprises the larynx, the trachea and its subdivisions the bronchi, and the 

 lungs, together with the serous membranes, the pleurce, which surround 

 the lungs and line the spaces containing them. The respiratory tract is 

 developed as a ventral outgrowth from the primitive gut-tube and is lined, 

 therefore, by entodermic epithelium, all other parts of the organs being 

 derived from the mesoderm. 



THE LARYNX. 



The larynx consists of a fibro-cartilaginous framework lined with mucous 

 membrane and surrounded by muscles. By the action of the latter the 

 relative position of the cartilages is modified, thereby affecting the approxi- 

 mation and tension of two 

 folds of mucous membrane, 

 known as the vocal cords, 

 that cover the free edges of 

 fi bro-elastic membranes and 

 bound the cleft through 



. . , , . Thyro-thyroid 



Which the air passes tO and membrane 



Mass of fat 



Epiglottis 



Hyoid bone 



;'a? Hyoid cornu 



'* Thy ro-h void 



i-\ ligament 



VX Thyroid corni 



) i 



Cuneiform 

 cartilage 

 Corniculate 

 cartilage 



False vocal fold 

 Vocal fold 



Cricoid 

 cartilage 



from the windpipe. 



The cartilages of the 

 larynx include three single 

 ones: the cricoid, the thyroid , 



I., -/,, j "li_ Thyroid cartilage 



and the epiglottis ; and three 

 paired ones: the arytenoid, 

 the cornicular and the cunei- 

 form, the last being small and 

 sometimes wanting. Other 

 minute masses of cartilage 

 will be noted in connection 

 with the structures in which 

 they occur. In structure the 

 laryngeal cartilages corre- 

 spond to the hyaline variety, with the exception of the epiglottis, the tip and 

 vocal process of the arytenoid, the cuneiform and the corniculate, which 

 parts consist of elastic cartilage and seldom, if ever, exhibit replacement 

 by bone. Beginning about the twentieth year, more or less extensive ossi- 

 fication of the cartilages, especially the thryoid and cricoid, occurs as a 

 normal change. The arytenoid cartilages are less affected and then remain 

 unchanged in women much longer than in men. 



The mucous membrane lining the larynx is a prolongation of that 

 of the pharynx, and consists of the epithelium and tunica propria, with the 

 underlying submucous tissue by which the mucosa is attached to the sur- 

 rounding framework. The epithelium is, for the most part, stratified ciliated 



187 



Ventricle 



Cricoid cartilage 



Trachea 



FIG. 233. Median sagittal section of larynx ; right half, seen 

 from within. X K- 



