CHAP, vii.] SPECIAL MUSCULAR MECHANISMS. 1077 



faces of both arytenoid cartilages ; the fibres starting from the 

 outer edge of one cartilage run transversely across to the outer 

 edge of the other cartilage, and the belly of the muscle occupies 

 the concave hind surfaces of the two cartilages together with the 

 intervening space. The effect of the contraction of this muscle 

 is to bring the two cartilages closer together and so to narrow 

 the glottis; indeed if in an animal it be divided, the glottis 

 remains widely open behind. It is an important closer of the 

 glottis, adductor of the vocal cords. When it is not contracting 

 the cartilages come apart through the elastic reaction of their 

 connections. 



Most important is a mass of muscular fibres, which starting 

 from the lower part of the reentering angle of the thyroid pass 

 horizontally but inclined somewhat upwards to the arytenoids at 

 about the level of the vocal cords. The whole mass is described 

 as forming two muscles. The outer or lateral part ending in the 

 outer edge of the arytenoid and upper part of its processus mus- 

 cularis is called the external thyro-arytenoid (M. thyro-aryte- 

 noideus externus) (Figs. 189, m.th.ar.e. 186 B.) The direction 

 of the muscle as a whole is horizontally backwards, though in- 

 clined outwards and upwards, but the constituent individual 

 bundles run in various ways and some even pass vertically 



mth.ar.i 



rn.th.ar.e~/- 

 m.th.ar.ep-f;'^ 



h _^ , 

 pm Oe mart 



FIG. 189. DIAGRAM TO ILLUSTRATE THE THTRO-ARYTENOID MUSCLES. 



The figure represents a transverse section of the Larynx through the bases of 

 the arytenoid cartilages. 



Ary. arytenoid cartilage, p.m. processus muscularis. p.v. processus vocalis. 

 Th. thyroid cartilage, c.v. vocal cords. Oe is placed in the oesophagus. 



m.th.ar.i. internal thyro-arytenoid muscle, m.th.ar.e. external thyro-aryte- 

 noid muscle, m.th.ar.ep. part of the thyro-ary-epiglottic muscle cut more or less 

 transversely. 



into the ventricular bands. To the inner or median side 

 of this external muscle, between it and the corresponding 

 vocal chord, lies the inner muscle which running from the 

 reentering angle of the thyroid to the processus vocalis 

 and outer surface of the arytenoid forms a wedge-shaped 

 mass, the thin edge of which is covered by the actual vocal 



