3oS VOICE AND SPEECH 



crico-arytenoid muscles and the consequent forward movement of 

 the muscular processes, the vocal cords are brought close together, 

 or adducted, and the rima is narrowed. The transverse or posterior 

 arytenoid muscle, which connects the two arytenoid cartilages 

 behind, also helps, by its contraction, to narrow the glottis by shift- 

 ing the cartilages on their articular surfaces somewhat nearer the 

 middle line. Running in each vocal cord, and, in fact, incorporated 

 with its elastic tissue, is a muscle, the thyro-arytenoid, the external 

 portion of which may to some extent cause inward rotation of the 

 vocal processes and adduction of the cords ; but the main function, 

 at least of its inner part, is to alter the tension of the cords. The 

 diagrams in Figs. 141 and 142 illustrate the action of the abductors 

 and adductors of the vocal cords. 



The crico-thyroid muscle and the deflectors of the epiglottis are 

 supplied by the superior laryngeal branch of the vagus, which also 



Fig. 141. Diagrammatic Hori- 

 zontal Section of Larynx to 

 show the Direction of Pull of 

 the Posterior Crico-Arytenoid 

 Muscles, which abduct the 

 Vocal Cords. Dotted lines 

 show position in abduction. 



Fig. 142. Direction of Pull- of 

 the Lateral Crico-Arytenoids, 

 which adduct the Vocal 

 Cords. Dotted lines show 

 position in adduction. 



contains the sensory fibres for the mucous membrane of the larynx 

 above the vocal cords. In the dog 'and rabbit motor fibres also reach 

 the crico-thyroid by the so-called middle laryngeal nerve which 

 arises from the superior pharyngeal branch of the vagus. All the 

 other intrinsic muscles are supplied by the recurrent laryngeal 

 branch of the vagus. It receives these motor fibres from the spinal 

 accessory, and supplies sensory fibres to the mucous membrane of 

 the larynx below the vocal cords and to the trachea. 



The voice is produced, like the sounds of a reed instrument, by 

 the rhythmical interruption of an expiratory blast of air by the 

 vibrating vocal cords. When a bell is struck, vibrations are set up 

 in the metal, which are communicated to the air. It is not the same 

 with the vibrations of the vocal cords; if they were plucked or 

 struck, they would only produce a feeble note. The air in the 

 mouth, pharynx, larynx, trachea, and lungs is the real sounding 



