434 THE BODY AT WORK 



If at the same time the arytenoid muscle draws the cartilages 

 together, the glottis is reduced to a slit (Fig. 45, C). The 

 posterior crico-arytenoid muscles rotate the cartilages out- 

 wards. If the arytenoid muscle is at the same time relaxed, 

 the glottis gapes to its fullest extent (Fig. 45, A). The freer 

 the opening, the less is the resistance to the blast of air, 

 the gentler the vibrations of the cords, the lower the voice. 

 The closer the slit, the greater is the resistance which the air 

 in the windpipe has to overcome in passing through it, and 

 consequently the more ample the vibrations into which it 

 throws the vocal cords. 



The vocal cords are the tongues of a reed-pipe, which, com- 

 mencing in the chest at the point where the great bronchi join 

 to form the windpipe, comprises the larynx, and, above the 

 larynx, the complicated chambers of the throat, mouth, and 

 nasal cavities, including the spaces within the bones of the 

 head which open out of them. The pitch of the voice depends 

 upon (1) the length of the vocal cords, and (2) their tension. 

 The first factor is fixed for every individual. The voice is base, 

 baritone, tenor, in a man ; contralto, mezzo-soprano, soprano, 

 in a woman in proportion as the cords are long, of medium 

 length, or short. A man's vocal cords measure, on the average, 

 15 millimetres, a woman's 11 millimetres. When a boy is from 

 twelve to fifteen years of age his vocal cords double in length, 

 and the " breaking " of the voice occurs as he gives up trying 

 to get high notes out of his longer cords, and allows them to 

 produce manly tones of an octave lower. 



The lower posterior angles of the thyroid cartilages articulate 

 with the cricoid. If the four cartilages are freed from all soft 

 tissues without disturbing the thyro-cricoid, or crico-arytenoid 

 joints, and if, while the thyroid is held in one hand, a finger of 

 the other is placed on the front of the cricoid, it will be found 

 that as this is depressed the arytenoid cartilages which rest 

 upon its signet are tilted upwards and forwards within the 

 thyroid ; as it is raised, they are tilted away from it. In life 

 this movement is effected by a muscle the crico-thyroid 

 (Fig. 46) attached to the front of the cricoid cartilage and 

 to the under border of the lateral plate of the thyroid. This 

 is the muscle of supreme importance in the production of the 

 voice. The thyroid cartilage is slung in a fixed position by 



