THE VOICE AND SPEECH. 53 



the top of the spinal column, but must accompany it in all its movements 

 as a whole. 



Ligaments. The thyroid cartilage is also connected with the cricoid, 

 not only by ligaments, but by two joints with synovial membrane (f, 

 [Pigs. 294 and 295); the lower cornua of the thyroid clasping, or nipping, 

 as it were, the cricoid between them, but not so tightly but that the thy- 

 roid can revolve, within a certain range, around an axis passing trans- 

 versely through the two joints at which the cricoid is clasped. The vocal 

 cords are attached (behind) to the front portion of the base of the arytenoid 

 cartilages, and (in front) to the re-entering angle at the back part of the 

 thyroid; it is evident, therefore, that all movements of either of these 

 cartilages must produce an effect on them of some kind or other. Inas- 

 much, too, as the arytenoid cartilages rest on the top of the back portion 

 of the cricoid cartilage (#, Fig. 298), and are connected with it by cap- 

 sular and other ligaments, all movements of the cricoid cartilage must 

 move the arytenoid cartilages, and also produce an effect on the vocal cords. 



Intrinsic Muscles. The so-called intrinsic muscles of the larynx, or 

 those which, in their action, have a direct action on the vocal cords, are 

 nine in number four pairs, and a single muscle; namely, two crico- 

 thyroid muscles, two thyro-arytenoid, two posterior crico-arytenoid, two 

 lateral crico-arytenoid, and one arytenoid muscle. Their actions are as 

 follows: When the crico-thyroid muscles (10, Fig. 297) contract, they 

 rotate the cricoid on the thyroid cartilage in such a manner that the 

 upper and back part of the former, and of necessity the arytenoid 

 cartilages on the top of it, are tipped backward, while the thyroid is in- 

 clined forward: and thus, of course, the vocal cords being attached in 

 front to one, and behind to the other, are "put on the stretch." 



The thyro-arytenoid muscles (7, Fig. 300) on the other hand, have an 

 opposite action, pulling the thyroid backward, and the arytenoid and 

 upper and back part of the cricoid cartilages forward, and thus relaxing 

 the vocal cords. 



The crico-arytenoidei posticus muscles (Fig. 299, V) dilate the glottis, 

 and separate the vocal cords, the one from the other, by an action on the 

 arytenoid cartilage which will be plain on reference to B' and 0', (Fig. 

 298). By their contraction they tend to pull together the outer angles 

 of the arytenoid cartilages in such a fashion as to rotate the latter at their 

 joint with the cricoid, and of course to throw asunder their anterior 

 angles to which the vocal cords are attached. 



These posterior crico-arytenoid muscles are opposed by the crico-anjte- 

 noidei laterales, which, pulling in the opposite direction from the other 

 side of the axis of rotation, have of course exactly the opposite effect, and 

 close the glottis (Fig. 300, 4 and 5). 



The aperture of the glottis can be also contracted by the an/fenoid 

 muscle (s, Fig. 299, and 6, Fig. 300), which, in its contraction, pulls 

 together the upper parts of the arytenoid cartilages between which it 

 extends. 



Nerve supply. In the performance of the functions of the larynx the 

 sensory filaments of the pneumogastric supply that acute sensibility by 

 which the glottis is guarded against the ingress of foreign bodies, or of 

 irrespirable gases. The contact of these stimulates the filaments of the 

 superior laryngeal branch of the pneumogastric; and the impression con- 

 veyed to the medulla oblongata, whether it produce sensation or not, is 

 reflected to the filaments of the recurrent or inferior laryngeal branch, 



