824 SPECIAL MUSCULAR MECHANISMS. 



side ; 3, the thyro-arytenoideus interims, 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 carti- 

 lage 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 upward and backward to the outer angle of the ary- 

 tenoid, by pulling this outer angle forward throws the processus vocalis 

 inward, and so also narrows the glottis. 



760. Widening of the glottis. The crico-arytenoideus posticus, passing 

 from the posterior surface of the cricoid cartilage to the outer angle of the 

 arytenoid cartilage behind the attachment of the lateral crico-arytenoideus, 

 pulls back this outer angle, and so causing the processus vocalis to move 

 outward, widens the glottis. The arytenoideus postieus, acting alone, has a 

 similar effect. 



761. Tightening of the vocal cords. The crico-thyroideus pulls the 

 thyroid downward and forward, and so increases the distance between that 

 cartilage and the arytenoids when the latter are fixed. Supposing, then, the 

 arytenoideus and crico-arytenoideus posticus to fix the arytenoids, the effect 

 of the contraction of the crico-thyroideus would be to tighten the vocal 

 cords. 



762. Slackening of the vocal cords. This is effected by the whole 

 sphincter group just mentioned, but more especially by the thyro-arytenoidei 

 externus and internus ; these, acting alone, supposing the arytenoid cartilages 

 to be fixed, would pull the thyroid cartilage upward and backward, and so 

 shorten the distance between the processus vocalis and that body. 



763. Thus almost every movement of the larynx is effected not by one 

 muscle only, but by several, or at least by more than one, acting in concert. 

 The movements which give rise to the voice are preeminently combined and 

 coordinate movements. When we remember how a very slight variation in 

 the tension of the vocal cords must give rise to a marked difference in the 

 pitch of the note uttered, and yet what a multitude of fine differences of 

 pitch are at the command of a singer of even moderate ability, it appears 

 exceedingly probable that the various muscular combinations required to 

 produce the possible variations in pitch are of such a kind that frequently a 

 part only, possibly a few fibres only, of a particular muscle may be thrown 

 into contraction, while all the rest of the muscle remains quiet. Taking 

 into view, moreover, the great range of pitch possessed by even common 

 voices, as compared with the possible variations of tension of which the 

 vocal cords in their natural length are capable, it has been suggested that 

 some of the fibres of the thyro-arytenoideus internus, which, passing either 

 from the thyroid or from the arytenoid, appear to end in the vocal cords 

 themselves, may, by fixing particular points of the cords, so to speak, "stop" 

 them ; and by thus artificially shortening the length actually thrown into 

 vibration, produce higher notes than the cords in their natural length are 

 capable of producing. It has also been suggested that the processus vocales 

 may overlap each other, and thereby shorten the length of cord available 

 for vibration. 



764. These various muscles are supplied by the vagus nerve, or rather 

 by spinal accessory fibres running in the vagus trunk. The superior laryn- 

 geal is the afferent nerve supplying the mucous membrane, but it also con- 

 tains the motor fibres distributed to the crico-thyroid muscle ; hence, when 

 this nerve is divided on one side the corresponding vocal cord is relaxed and 

 high notes become impossible. It is worthy of notice that this, the chief 



