580 



PHYSIOLOGY 



taken by any given muscle in the production of voice. The chief 

 muscles (Fig. 252) are as follows : 



(1) The crico-thyroid muscle is a short triangular muscle attached 

 below to the cricoid cartilage and above to the inferior border of the 

 thyroid cartilage ; the fibres pass from below upwards and back- 



FIG. 252. Muscles of the larynx. (SAPPBY.) 

 A, as shown in a view of the larynx from the right side. 



1, hyoid bone ; 2, 3, its cornua ; 4, right ala of thyroid cartilage ; 5, posterior 

 part of the same separated by oblique line from anterior part ; 6, 7, superior and 

 inferior tubercles at ends of oblique line ; 8, upper cornu of thyroid ; 9, thyro -hyoid 

 ligament ; 10, cartilage triticea ; 11, lower cornu of thyroid, articulating with the 

 cricoid ; 12, anterior part of cricoid ; 13, crico-thyroid membrane ; 14, crico-thyroid 

 muscle ; 15, posterior crico-arytenoid muscle, partly hidden by thyroid cartilage. 



B, as seen in a view of the larynx from behind. 



1, posterior crico-arytenoid ; 2, arytenoid muscle ; 3, 4, oblique fibres passing 

 around the edge of the arytenoid cartilage to join the thyro -arytenoid, and to form 

 the aryteno-epiglottic, 5. 



wards. When this muscle contracts, the cricoid cartilage is drawn 

 up under the anterior part of the thyroid cartilage so that its broad 

 expansion behind, with the arytenoid cartilages, is drawn downwards 

 and backwards, thus putting the vocal cords on the stretch. This 

 muscle is probably the most important in determining the tension of 

 the vocal cord. 



(2) The posterior crico-arytenoid muscle arises from a broad depres- 

 sion on the corresponding half of the posterior surface of the cricoid 

 cartilage. It passes upwards and outwards, its fibres converging, to 

 be inserted into the outer angle of the arytenoid cartilage. These 



