494 THE HUMAN BODY 



vocal cord. Over most of the interior of the larynx its mucous 

 membrane is thick and covered by ciliated epithelium, and has 

 many mucous glands embedded in it. Over the vocal cords, 

 however, it is represented only by a thin layer of flat non- 

 ciliated cells, and contains no glands. In quiet breathing, and 

 after death, the free inner edges of the vocal cords are thick and 

 rounded, and seem very unsuitable for being readily set in vibra- 

 tion. They are also tolerably widely separated behind, the aryte- 

 noid cartilages, to which their posterior ends are attached, being 

 separated. Air under these conditions passes through without pro- 

 ducing voice. If they are watched with the laryngoscope during 

 phonation, it is seen that the cords approximate behind so as to 

 narrow the glottis; at the same time they become more tense, and 

 their inner edges project more sharply and form a better-defined 

 margin to the glottis, and their vibrations can be seen. These 

 changes are brought about by the delicately coordinated activity 

 of a number of small muscles, which move the cartilages to which 

 the cords are fixed. 



The Muscles of the Larynx. In describing the direction and 

 action of these it is convenient to use the words front or anterior 

 and back or posterior with reference to the larynx itself (that is, 

 as equivalent to ventral and dorsal) and not with reference to the 

 head, as usual. The base of each arytenoid cartilage is triangular 

 and fits on a surface of the cricoid, on which it can slip to and fro 

 to some extent, the ligaments of the joint being lax. One corner 

 of the triangular base is directed inwards and forwards (i. e., to- 

 wards the thyroid) and is called the vocal process (Pv, Fig. 145) , as 

 to it the vocal cords are fixed. The outer posterior angle (Pm, 

 Fig. 145) has several muscles inserted on it and is called the mus- 

 cular process. If it be pulled back and towards the middle line 

 the arytenoid cartilage will rotate on its vertical axis, and roll 

 its vocal processes forwards and outwards, and so widen the 

 glottis; the reverse will happen if the muscular process be drawn 

 forwards. The muscle producing the former movement is the 

 posterior crico-arytenoid (Cap, Fig. 147); it arises from the back 

 of the cricoid cartilage, and narrows to its insertion into the mus- 

 cular process of the arytenoid on the same side. The opponent 

 of this muscle is the lateral crico-arytenoid, which arises from the 

 side of the cricoid cartilage, on its inner surface, and passes up- 



