CHAP, vii.] SPECIAL MUSCULAR MECHANISMS. 1455 



(3) Quality. This, as we have seen ( 841), depends on the 

 number and character of the partial tones accompanying any 

 fundamental note sounded, and is determined by a variety of 

 circumstances, chief among which are, on the one hand the form, 

 thickness and other physical qualities of the cords, and on the 

 other hand, the disposition of the resonance chamber, or parts 

 of the respiratory passage other than the glottis itself. 



We may confine ourselves in the first instance to the conditions 

 which determine the mere utterance of the voice and to the 

 mechanisms which affect the tension of the vocal cords, and hence 

 the pitch of the voice. The problems therefore which we have 

 to attack 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, adduction or abduction of the edges of the glottis, and 

 tightening or relaxation of the vocal cords. We may first dwell 

 on the muscular aspects of the mechanisms by which these results 

 are brought about, taking the nervous factors into consideration 

 afterwards. The change of form of the glottis is best understood 

 when what we have already ( 901) said is borne in mind, namely, 

 that each arytenoid cartilage is, when seen in horizontal section 

 (Fig. 186), somewhat of the form of a triangle, with a 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 (Fig. 186 

 A'). When on the contrary the cartilages are wheeled round on 

 the pivots of their articulations, so that the processus vocales 

 diverge, and the internal surfaces of the cartilages form an 

 angle with each other, the glottis is widened (Fig. 186 B', C"). 

 Moreover the two cartilages may to a certain extent be bodily 

 drawn together, or dragged apart, the two hind angles, between 

 the median and posterior sides, being now close together, now 

 apart. 



906. The muscles of the larynx though small, are numerous 

 and complicated, and are so disposed in respect to their origins and 

 insertions and to the sweep of their fibres, that the effect of the 

 contraction of one muscle will depend upon whether or no and how 

 far other muscles are thrown into contraction at the same time ; 

 moreover in the case of some of the muscles at least the effect 

 is different according as the whole muscle or a part only 

 contracts. 



The first muscle to which we may call attention is the trans- 

 verse arytenoid (M. arytenoideus posticus s. trans versus) (Fig. 187). 

 This is a relatively thick muscle covering the hind surfaces of both 

 arytenoid cartilages ; the fibres starting from the outer edge of one 



