568 PHYSIOLOGY. 



Its action is, in contracting, to draw the arytenoid cartilages 

 together so that the respiratory part of the glottis is closed. If the 

 contraction be simultaneous with that of the lateral crico-arytenoid 

 muscles, respiration is entirely interrupted. 



THE EXTRINSIC MUSCLES are those of the anterior region of the 

 neck ; those in the suprahyoid as well as those in the subhyoid region. 

 By the action of these muscles the entire larynx is moved upward 

 and downward. 



THE CAVITY of the larynx is lined with a mucous membrane. 

 The mucous membrane is continuous with that of the trachea. It is 

 covered with the prismatic or ciliated epithelium in all places ex- 

 cept over the vocal cords and epiglottis. In these special areas it is 

 stratified. 



THE VOCAL CORDS comprise two sets, as was previously men- 

 tioned; the upper, false cords, composed of folds of mucous mem- 

 brane, take no part in voice production; the lower, true cords, are 

 composed of a mucous membrane with pavement epithelium, a lamina 

 of elastic fibers, and the thyro-arytenoid muscle. 



Opening the cavity of the pharynx and raising the epiglottis, the 

 whole extent of the glottis is seen; that is, the slit left by the two 

 superior cords. This has the shape of a much elongated triangle 

 apex in front, base at the back. The limited anterior part of the 

 triangle is called the vocal part of the glottis ; whereas the posterior 

 part is called the respiratory portion. It does not participate in 

 phonation, but only in the passage of air. 



NERVE-SUPPLY. The nerves which are distributed to the larynx 

 come from the pneumogastric. The superior laryngeal nerve supplies 

 the mucous membrane of the larynx and gives the external laryngeal 

 branch to the crico-thyroid muscle. The inferior, or recurrent, laryn- 

 geal nerve supplies all of the muscles except the crico-thyroid. The 

 ganglia which preside over the motor innervation of the larynx are 

 seated in the floor of the fourth ventricle. 



Laryngoscopy. The laryngoscope is an instrument used to 

 bring to view various parts of the pharynx, larynx, and trachea. It 

 comprises a small mirror fastened to a long handle. The angle that 

 the mirror makes with its handle is from 125 to 130 degrees. 



CONDITION OF THE VOCAL CORDS. By observations made with 

 the laryngoscope it has been determined that, while in respiration 

 the vocal cords are inclined from each other, and the glottis is wide 

 open, in speaking or vocalization the cords are seen to approximate 

 and vibrate. In ordinary quiet breathing there is a wide, triangular- 



