57 6 TEXT-BOOK OF PHYSIOLOGY. 



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 becomes thinner, and is 

 finally attached anteriorly to the thyroid near the median line, and 

 posteriorly to the vocal process of the arytenoid, thus constituting 

 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 thin, free, reduplicated edge of the mucous membrane con- 

 stitutes the true vocal band. The surface of the mucous membrane 

 is covered by ciliated epithelium except in the immediate neighbor- 

 hood 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 cartilages. 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. 263 and 264). 



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, from which it takes its origin. The fibers of the 

 muscle pass upward and outward and in their course converge to 

 be inserted into the external angle of the arytenoid cartilage. The 

 superior and more horizontally directed fibers rotate the arytenoid 



