GENERAL ARRANGEMENT OF THE PHONATING ORGANS 547 



of the arytenoid cartilages as a whole, which brings their posterior 

 extremities closer together. 



The muscles involved in this process belong to the intrinsic group 

 of the laryngeal musculature, and present the following individual 

 actions: 



(a) The posterior crico-arytenoid muscle arises from the posterior surface of the 

 quadrate plate of the cricoid cartilage on either side of the median line and passes 

 obliquely upward and outward to be inserted upon the external angle of the 

 muscular process of the arytenoid cartilage (Fig. 271, 1). Its chief action is to 

 rotate the vocal process of the corresponding arytenoid upward and outward so 

 that the glottis is widened. This muscle, therefore, abducts the vocal cords. 



(b) The lateral crico-arytenoid muscle takes its origin upon the upper border of 

 the cricoid cartilage and, passing upward and backward, is inserted upon the 

 forepart of the muscular process of the arytenoid (Fig. 271, 2). Its contraction 

 gives rise to an inward and downward movement of the vocal process, insuring 

 thereby an adduction of the vocal cords chiefly at their posterior ends. 



\ 



FIQ. 271. DIAGRAM ILLUSTRATING THE ABDUCTION AND ADDUCTION OF THE VOCAL CORDS. 

 A, abduction; 1, point of insertion of the post, crico-arytenoid muscle; G, glottis; B, 

 adduction; 2, points of insertion of the lat. crico-arytenoid and thyro-arytenoid muscles; 

 3, point of insertion of the arytenoid muscles. The dot indicates the position of the 

 center of rotation of the arytenoid cartilages. 



(c) The thyro-arytenoid muscle extends between the inner surface of the thyroid 

 cartilage, post-external to the median line, and the anterior margin and external 

 angle of the arytenoid. Its inner fibers lie in close relation to the vocal cords and 

 are frequently designated as the musculus vocalis. When contracting, this muscle 

 rotates the corresponding arytenoid cartilage around its vertical axis, drawing the 

 vocal process forward and inward. It acts, therefore, as an aid to the lateral 

 crico-arytenoid muscle in causing the adduction of the vocal cords. 



(d) The arytenoid muscle extends from side to side, joining the two arytenoid 

 cartilages. It consists of two groups of fibers, one of which is directed horizontally 

 across the median line and the other obliquely (Fig. 271, 3). The ends of the 

 former are fastened to the outer margins of the arytenoids on each side, while the 

 latter unite the outer angle of one with the apex of the other. Obviously, 

 these fibers have to do with the approximation of the posterior ends of the ary- 

 tenoid cartilages, lessening the length of the rima glottidis. 



The Innervation of the Larynx. The nerve supply of the larynx 

 is derived from the systems of the right and left vagi nerves. The 



