918 THE LARYNX. 



cords ; the upper parts of these muscles, being attached higher up on the arytenoid 

 cartilages, depress them. 



The lateral crico-arytenoid muscles, by pulling forwards the outer angles of the 

 arytenoid cartilages, approximate the vocal cords to the middle line. The posterior 

 crico-arytenoid muscles pull backwards the outer angles of the arytenoid cartilages, and 

 thus draw asunder the posterior extremities of the vocal cords, and dilate the glottis 

 to its greatest extent ; they are likewise the elevators of the arytenoid cartilages, 

 being inserted above the articulation. 



The arytenoid muscle draws the arytenoid cartilages together, and, from the struc- 

 ture of the crico-arytenoid joints, this approximation when complete is necessarily 

 accompanied with depression. The aryteno-epiglottidean muscles at once depress and 

 approximate the arytenoid cartilages, which they include in their embrace, and draw 

 down the epiglottis, so as to contract the whole superior aperture of the larynx. 



With the aid of the laryngoscope it may be seen that in ordinary breathing the rima 

 glottidis is widely opn, and that in vocalisation the vocal cords come closely together ; 

 which is effected principally, no doubt, by the action of the lateral crico-arytenoid 

 muscles, assisted by the arytenoid and perhaps by the thyro-arytenoid, and accom- 

 panied with a varying amount of contraction of the crico-thyroid muscles. The 

 regulation of the tension of the vocal cords and of the width of the aperture of the 

 glottis, in the production of high and low pitched notes, is probably accomplished by 

 the crico-thyroid and thyro-arytenoid muscles. The movement of the thyroid on the 

 cricoid cartilage, effected by these muscles during the passage of the voice from one 

 extreme of the scale to the other, may be detected by placing the tip of a finger over 

 the crico-thyroid ligament. The arytenoid and aryteno-epiglottidean muscles come 

 into action in spasmodic closure of the upper aperture of the larynx ; the complete 

 descent of the epiglottis, however, can only take place when the tongue is retracted 

 as in the act of swallowing. 



The manner in which the larynx is affected by the extrinsic muscles, in the acts of 

 deglutition and vocalisation, has been mentioned at pages 191 and 193. 



It is remarked by Henle that, with the exception of the crico-thyroid and posterior 

 crico-arytenoid, the muscles of the larynx, namely, those " which lie in the space 

 enclosed by the laminae of the thyroid cartilage, and above the cricoid, the fibres of 

 which are substantially horizontal, may be regarded in their totality as a kind of 

 sphincter. Such a sphincter is found in its simple form embracing the entrance of 

 the larynx in reptiles ; and the complication which it attains in the higher vertebrates 

 arises, like the complication of the muscles generally, from the fibres finding various 

 points of attachment in their course, by which means they are broken up and 

 divided." 



The mucous membrane and glands of the larynx. The laryngeal mucous 

 membrane is thin and of a pale pink colour. la some situations it adheres 

 intimately to the subjacent parts, especially on the epiglottis, and still more 

 in passing over the true vocal cords, on which it is extremely thin and most 

 closely adherent. About the upper part of the larynx, above the glottis, 

 it is extremely sensitive. In or near the aryteno-epiglottidean folds it 

 covers a quantity of loose areolar tissue, which is liable in disease to in- 

 filtration, constituting oedema of the glottis. Like the mucous membrane 

 in the rest of the air-passages, that of the larynx is covered in the greater 

 part of its extent with a columnar ciliated epithelium, by the vibratory 

 action of which the mucus is urged upwards. The cilia are found higher 

 up in front than on each side and behind, reaching in the former direction 

 as high as the widest portion of the epiglottis, and in the other directions to 

 a line or two above the border of the superior vocal cords : above these 

 points the epithelium loses its cilia, and gradually assumes a squamous 

 form, like that of the pharynx and mouth. Upon the vocal cords also the 

 epithelium is squamous, although both above and below them it is ciliated. 



Glands. The lining membrane of the larynx is provided with numerous 

 glands, which secrete an abundant mucus ; and the orifices of which may 



