514 RELAXATION OF THE VOCAL CORDS. 



of the internal thyroarytenoid muscles (see 3) must be closed by the action of the 

 transverse and oblique arytenoid muscles. The vocal cords themselves must have 

 a concave margin, which is obtained through the action of the crico-thyroids and 

 posterior cricoarytenoids, so that the glottis vocalis presents the appearance of a 

 myrtle* leaf (J/enle), while the rima glottidis has the form of a linear slit (fig. 362). 

 The contraction of the internal thyroarytenoid converts the concave margin of the 

 vocal cords into a straight margin. This muscle adjusts the delicate variations of 

 tension of the vocal cords themselves, causing more especially such variations as are 

 necessary for the production of tones of slightly different pitch. As these muscles 

 come close to the margin of the cords, and are securely woven, as it were, amongst 

 the elastic fibres of which the cords consist, they are specially adapted for the above- 

 mentioned purpose. When the muscles contract, they give the necessary resistance 

 to the cords, thus favouring their vibration. As some of the muscular fibres end 

 in the elastic fibres of the cords, these fibres, when they contract, can . render 

 certain parts of the cords more tense than others, and thus favour the modifications 

 in the formation of the tones. The coarser variations in the tension of the vocal 

 cords are produced by the separation of the thyroid from the arytenoid cartilages, 

 while th& jiner variations of tension are produced by the thyro-arytenoid muscles. 

 The value of the elastic-tissue of the cords does not depend so much upon its 

 extensibility, as upon its property of shortening without forming folds and creases. 



Pathological. In paralysis of these muscles, the voice can only be produced by forcible 

 expiration, as much air escapes through the glottis ; the tones are at the same time deep and 

 impure. Paralysis of the muscle of one side causes napping of the vocal cord on that side 

 (Gerhardt). 



5. The relaxation of the vocal cords occurs spontaneously when the stretch- 

 ing forces cease to act ; the elasticity of the displaced thyroid and arytenoid carti- 

 lages comes into play, and restores them to their original position. The vocal 

 cords are also relaxed by the action of the thyro-arytenoid and lateral crico-arytenoid 

 muscles. 



It is evident, from the above statements, that tension of the vocal cords and 

 narrowing of the glottis are necessary for phonation. The tension is produced 

 by the crico-thyroids and posterior crico-arytenoids ; the narrowing of the glottis 

 respiratoria by the arytenoids, transverse and oblique, the glottis vocalis being 

 narrowed by the thyro-arytenoids and (1 lateral crico-arytenoids), the former muscles 

 causing the cords themselves to become tense. 



Nerves ( 352, 5). The crico-thyroid is supplied by the superior laryngeal 

 branch of the vagus, which at the same time is the sensory nerve of the mucous 

 membrane of the larynx. All the other intrinsic muscles of the larynx are supplied 

 by the inferior laryngeal. 



The mucous membrane of the larynx is richly supplied with elastic fibres, and so is the 

 sub-mucosa. The sub-mucosa is more lax near the entrance to the glottis and in the ventricles 

 of Morgagni, which explains the enormous swelling that sometimes occurs in these parts in 

 oedema glottidis. A thin clear limiting membrane lies under the epithelium. The epithelium 

 is stratified, cylindrical, and ciliated with intervening goblet cells. On the true vocal cords 

 and the anterior surface of the epiglottis, however, this is replaced by stratified squamous 

 epithelium, which covers the small papillae of the mucous membrane. Numerous branched 

 mucous glands occur over the cartilages of Wrisberg, the cushion of the epiglottis, and in the 

 ventricles of Morgagni ; in other situations, as on the posterior surface of the larynx, the glands 

 are more scattered. The blood-vessels form a dense capillary plexus under the membrana 

 propria of the mucous^ membrane ; under this, however, there are other two strata of blood- 

 vessels. The lymphatics form a superficial narrow mesh-work under the blood-capillaries, with 

 a deeper, coarser plexus. The medullated nerves have ganglia in their branches, but their 

 mode of termination is unknown. [W. Stirling has described a rich sub-epithelial plexus of 

 medullated nerve-fibres on the anterior surface of the epiglottis, while he finds that there are 

 ganglionic cells in the course of the superior laryngeal nerve.] 



Cartilages. The thyroid, cricoid, and nearly the whole of the arytenoid cartilages consist of 

 hyaline cartilage. The two former are prone to ossify. The apex and processus vocalis of the 



