THE LARYNX 909 



muscle; internally it is covered by mucous membrane. The upper portion is con- 

 tiguous to the thyro-arytenoid, with which it is occasionally blended. It is su}i})li('il 

 by the recurrent hirvn^cal nerve 



The thyro-arytenoid muscle, which is i)laced above the foregoing, arises 

 broadly from th(; lower two-thirds of the inner surface of the thyroid cartilage 

 close to its retiring angle, and sHghtly from the external surface of the crico-thy- 

 roid meml)rane. Narrowing as it passes outwards and backwards, it is inserted 

 into the anterior surface of the arytenoid cartilage, and also into its base close to 



Fig. 531. — Scheme Showing Action of Thyro-arytenoid drawing the Vocal Cords 

 AND Vocal Processes from II to I. (Modified froiu Stirling.) 



the attachment of the lateral crico- arytenoid muscle. Some few of its fibres pass 

 onwards and become continuous with the oblique fibres of the arytenoid muscle. 



The lower portion of the muscle lies parallel to and blends Avith the outer 

 surface of the vocal cord. The upper and thinner portion is placed immediately 

 beneath the mucous membrane, and overlies the ventricle and laiyngeal pouch. 



These muscles by rotating the arytenoid cartilages draAV the vocal processes 

 downwards and inwards, and thus approximate the vocal cords. By pulling 

 forward the cartilages, they relax the cord as a Avhole. According to some 

 authorities, the fil)res attached to the outer border of the vocal cord act upon it l)y 



Fig. 532.— Scheme showing Action of Arytenoidet's drawing Arytenoid Cartilage 

 FROM Neutral Position I to II. (Modified from Stirling.) 



modifying its elasticity, tightening a portion in front and relaxing the remainder, 

 in this somewhat resembling the stop-action of thi' finger on a violin string. 



The fibres lying near the true vocal cords, in the angle l)etween the midtlle and 

 lower sections of the laryngeal cavity, are frequently described as a separate portion 

 called the thyro-arytenoideus internus; they form a triangular prismatic bundle, 

 attached anteriorly to the thyroid cartilage and the anterior part of the true vocal 

 cord, and posteruirly to the arytenoid cartilage in the neighbourhood of the vocal 

 process. The remainder of the muscle, the thyro-arytenoideus externus, is a 



