SPECIAL PHYSIOLOGY OF THE MUSCLES 211 



forward and a little downward and hence increases the ten- 

 sion of the vocal cords attached to the thyroid. 



2. The thyro-arytenoid muscle ', imbedded in the vocal 

 cords, function as the antagonist of the last-named muscle. 

 If it contracts simultaneously with the crico-thyroid muscle, 

 both it and the vocal cords which are formed by it are 

 rendered tense. Its contraction also aids in the 'closing of 

 the glottis, for the slightly outward-bent borders of the 

 vocal cords are stretched by its contraction. 



Innervation: The crico-thyroid is innervated by the 

 superior laryngeal nerve; all the others by the inferior 

 laryngeal. 



I. 



FIG. 16. REPRESENTATION OF THE POSITION OF THE VOCAL CORDS 



(DIAGRAMMATIC). 



a, anterior end of the vocal cord; b and t, base of the arytenoid cartilages; 

 I, position of rest; II, the arytenoid have approached each other and the vocal 

 cords are in position for voice formation; III, form of the glottis during con- 

 traction of the transverse and oblique arytenoid muscles and the posterior 

 crico-arytenoid; IV, form of the glottis during contraction of the lateral crico- 

 arytenoid muscles. 



(b) The pitch, range, and quality (timbre) of the human 

 voice. The pitch of a reed pipe depends on the number of 

 vibrations of the reed. In a membranous reed, it depends 

 upon the length, thickness, and tension of the reed, hence 

 the pitch of the human voice is the higher the less the 

 length and thickness and the greater the tension of the 

 vibrating parts of the vocal cords. 



Individual variations in the pitch of the voice are deter- 

 mined by the length and thickness of the vocal cords. The 

 vocal cords of a man, for example, are thicker and longer 

 (18 mm long) than those of a woman (12 mm long), hence 

 the man has a deeper voice than the woman. 



