676 TEXT-BOOK OF PHYSIOLOGY 



with a synovial membrane and enclosed by a loose capsular ligament which 

 would permit of an extensive sliding of the arytenoid cartilage downward and 

 outward were it not prevented by the posterior crico-arytenoid ligament, 

 which is attached, on the one hand, to the cricoid, and, on the other, to the 

 inner angle of the arytenoid. The movements permitted at this joint are: 

 (i) Rotation of the arytenoid around a vertical axis which lies close to its 

 inner surface. (2) A sliding motion inward and forward with inward 

 rotation of the vocal process, or a sliding motion outward and backward 

 with outward rotation of the vocal process. In either case the process 

 describes an arc of a circle. (3) A sliding movement toward the median 

 line in consequence of which the inner surfaces of the arytenoids are 

 brought almost in contact. 



The crico-thyroid membrane is composed mainly of elastic tissue. It 

 may be divided into a mesial and two lateral portions. The mesial portion 

 is well developed, triangular in shape, and unites the contiguous borders of 

 the cricoid and thyroid cartilages. The lateral portion is attached below to 

 the superior border of the cricoid. From this attachment it passes upward 

 and inward under cover of the thyroid. As it ascends it elongates and be- 

 comes thinner, and is finally attached anteriorly to the thyroid near the 

 median line, and posteriorly to the vocal process of the arytenoid, thus con- 

 stituting the inferior thyro-arytenoid ligament. It is covered internally by 

 mucous membrane and externally by the internal thyro-arytenoid muscle. 

 The free edge of this ligament forms the basis of the true vocal band. A 

 superior thyro-arytenoid ligament forms the basis of the false vocal band. 



The thyro-hyoid membrane, composed of elastic tissue, unites the 

 superior border of the thyroid to the hyoid bone. 



The mucous membrane lining the larynx is thin and pale. As it passes 

 downward it is reflected over the superior thyro-arytenoid ligament, and 

 assists in the formation of the false vocal band; it then passes into and lines 

 the ventricle, after which it is reflected inward over the superior border of the 

 thyro-arytenoid muscle and ligament, and assists in the formation of the 

 true vocal band; it then returns upon itself and passes downward over the 

 lateral portion of the crico-thyroid membrane into the trachea. 



The Vocal Bands. The thin, free, reduplicated edge of the mucous 

 membrane constitutes the true vocal band. The surface of the mucous mem- 

 brane is covered by ciliated epithelium except in the immediate neighborhood 

 of the vocal bands. 



The vocal bands are attached anteriorly to the thyroid cartilage near the 

 receding angle and posteriorly to the vocal processes of the arytenoid cartil- 

 ages. They vary in length in the male from 20 to 25 mm. and in the female 

 from 15 to 20 mm. 



The Muscles of the Larynx. The muscles which have a direct action 

 on the cartilages of the larynx and determine the position of the vocal bands 

 both for respiratory and phonatory purposes, and which regulate their 

 tension as well, are nine in number and take their names from their points 

 of origin and insertion: viz., two posterior crico-arytenoids, two lateral crico- 

 arytenoids, two thyro-arytenoids, one arytenoid, and two crico-thyroids 

 (Figs. 273 and 274). 



The posterior crico-arytenoid muscle lies on the posterior surface of the 

 quadrate plate of the cricoid cartilage, on either side of the median line, 



