CHAP. VIL] SPECIAL MUSCULAR MECHANISMS. 653 



is widened into a rhomboidal opening by the divergence of the 

 processus vocales (Fig. 88 C). When a note is about to be uttered, 

 the vocal cords are by the approximation of the processus vocales 

 brought into a position parallel to each other, and the whole rima 

 is narrowed (Fig. 88-4). By their parallelism and by the narrow- 

 ness of the interval between them the cords are rendered more 

 susceptible of being thrown into vibration by a moderate blast of 

 air. The problems we have to consider are, first, by what means 

 are the cords brought near to each other or drawn asunder as 

 occasion demands ; and secondly, by what means is the tension of 

 the cords made to vary. We may speak of these two actions as 

 narrowing or widening of the glottis, and tightening or relaxation 

 of the vocal cords. 



of the Glottis. The change of form of the glottis is 

 best understood when it is borne in mind that each arytenoid 

 cartilage is, when seen in horizontal section (Fig. 88), somewhat of 

 the form of a triangle, with an internal or median, an external, 

 and a posterior side, the processus vocalis being placed in the 

 anterior angle at the junction of the median and external sides. 

 When the cartilages are so placed that the processus vocales are 

 approximated to each other, and the internal surfaces of the 

 cartilages nearly parallel, the glottis is narrowed. When on the 

 contrary the cartilages are wheeled round on the pivots of their 

 articulations, so that the processus vocales diverge, and the inter- 

 nal surfaces of the cartilages form an angle with each other, the 

 glottis is widened. 



There are several muscles forming together a group, which has 

 been called by Henle the sphincter of the larynx. These are (1) 

 the thyro-ary-epiglotticus, proceeding from the inner surface of the 

 thyroid cartilage and from the arytenoid epiglottidean ligament, 

 and sweeping round the outer ridge of the arytenoid cartilage of 

 its own side to be inserted into the processus muscularis of the 

 arytenoid cartilage of the other side: (2) the thyro-arytenoideus 

 externus, passing from the reentrant angle of the thyroid cartilage 

 to be inserted into the outer edge of the arytenoid cartilage of the 

 same side : (3) the thyro-arytenoideus intemus, passing from the 

 angle of the thyroid cartilage to the processus vocalis and outer 

 side of the arytenoid cartilage : (4) the arytenoideus (posticus), 

 passing transversely from one arytenoid cartilage to another. All 

 these muscles, when they act together, grasp round the glottis and 

 tend to close it up : and each of them, acting alone, has, with the 

 exception of the last-named (arytenoideus), the same effect. In 

 addition to these, the crico-arytenoideus lateralis, which passes 

 from the lateral border of the cricoid cartilage upwards and back- 

 wards to the outer angle of the arytenoid,- by pulling this outer 

 angle forwards throws the processus vocalis inwards, and so also 

 narrows the glottis. 



