358 RESPIRATION. 



walls sufficiently rigid to resist the external pressure of the 

 air. 



Commencing with the larynx, it is seen that the cartilages 

 of which it is composed are sufficiently rigid and unyield- 

 ing to resist the pressure produced by any inspiratory 

 effort. Across its superior opening are the vocal cords, 

 which are four in number, and have a direction from before 

 backwards. The two superior are called the false vocal 

 cords, because they are not concerned in the production of 

 the voice. The two inferior are the true vocal cords. They are 

 ligamentous bands covered by folds of mucous membrane, 

 which is quite thick on the superior cords, and very thin and 

 delicate on the inferior. Anteriorly, they are attached to a 

 fixed point between the thyroid cartilages, and posteriorly, 

 to the movable arytenoid cartilages. Air is admitted to the 

 trachea through an opening between the cords, which is 

 called the rima glottidis. Little muscles, arising from the 

 thyroid and cricoid, and attached to the arytenoid cartilages, 

 are capable of separating and approximating the points 

 to which the vocal cords are attached posteriorly, so as to 

 open and close the rima glottidis. 



If the glottis be exposed in a living animal, certain regu- 

 lar movements are presented which are synchronous with the 

 acts of respiration. The larynx is widely opened at each in- 

 spiration by the action of the muscles referred to above, so 

 that the air has a free entrance to the trachea. At the ter- 

 mination of the inspiratory act, these muscles are relaxed, 

 the vocal cords fall together by their own elasticity, and in 

 expiration, the chink of the glottis returns to the condition 

 of a narrow slit. These respiratory movements of the glottis 

 are constant, and essential to the introduction of air in 

 proper quantity to the lungs. The expulsion of air from the 

 lungs is rather a passive process, and tends in itself to sepa- 

 rate the vocal cords; but inspiration, which is active and 

 more violent, were it not for the movements of the glottis, 

 would have a tendency to draw the vocal cords together. 



