ACTION OF THE LARYNGEAL MUSCLES. 



513 



placed the cross bundles (fig. 354) of the thyro-aryepiglotticus (or arytaenoidei 

 obliqui) ; they act like the foregoing. The action of these muscles is indicated in 

 fig. 357 ; the arrows point to the line of traction. 



Pathological. Paralysis of this muscle enfeebles the voice and makes it hoarse, as much air 

 escapes between the arytenoid cartilages during phonation. 



3. In order that the vocal cords be approximated to each other, which occurs 

 during phonation, the processus vocales of the arytenoid cartilages must be closely 

 apposed, whereby they must be rotated inwards and downwards. This result is 

 brought about by the processus musculares being moved in a forward and upward 

 direction by the thyro-arytenoid muscles. These muscles are applied to, and in 

 fact are imbedded in, the substance of the elastic vocal cords, and their fibres reach 

 to the external surface of the arytenoid cartilages. When they contract, they 

 rotate these cartilages so that the processus vocales must rotate inwards. The 

 glottis . vocalis is thereby narrowed to a mere slit (fig. 358), whilst the glottis 

 respiratoria remains as a broad triangular opening. The action of these muscles is 

 indicated in fig. 358. 



The lateral crico-arytenoid muscle is inserted into the anterior margin of the 

 articular surface of the arytenoid cartilage ; hence, it can only pull the cartilage 



Fig. 356. Fig. 357. Fig. 358. 



Fig. 356. Schematic horizontal section of the larynx. I, Position of the horizontally divided 

 arytenoid cartilages during respiration ; from their anterior processes run the converging 

 vocal cords. The arrows show the line of traction of the posterior crico-arytenoid muscles ; 

 II, II, the position of the arytenoid muscles as a result of this action. Fig. 357. 

 Schematic horizontal section of the larynx, to illustrate the action of the arytenoid muscle. 

 I, I, position of the arytenoid cartilages during quiet respiration. The arrows indicate the 

 direction of the contraction of the muscle ; II, II, the position of the arytenoid cartilages 

 after the arytenoideus contracts. Fig. 358. Scheme of the closure of the glottis by the 

 thyro-arytenoid muscles. II, II, position of the arytenoid cartilages during quiet respira- 

 tion. The arrows indicate the direction of the muscular traction. I, I, position of the 

 arytenoid cartilages after the muscles contract. 



forwards; but some have supposed that it can also rotate the arytenoid cartilage in 

 a manner similar to the thyro-arytenoid (?), with this difference, that the processus 

 vocales do not come so close to each other. 



Pathological. Paralysis of both thyro-arytenoid muscles causes loss of voice. 



4. The vocal cords are rendered tense by their points of attachment being 

 removed from each other by the action of muscles. The chief agents in this action 

 are the crico-thyroid muscles, which pull the thyroid cartilage forwards and down- 

 wards. At the same time, however, the posterior crico-arytenoids must pull the 

 arytenoid cartilages slightly backwards, and also keep them fixed. 



The genio-hyoid and thyro-hyoid, when they contract, pull the thyroid upwards and forwards 

 towards thechin, and also tend to increase the tension of the vocal cords (C. Mayer, Grutzner). 



Pathological. Paralysis of the crico-thyroid causes the voice to become harsh and deep, owing 

 to the vocal cords not being sufficiently tense. 



Position during Phonation. The tension of the vocal cords brought about in 

 this way is not of itself sufficient for phonation. The triangular aperture of the 

 glottis respiratoria between the arytenoid cartilages, produced by the unaided action 



2K 



