ISO ESSENTIALS OF PHYSIOLOGY. 



of air must have a certain pressure, and this is produced by contraction 

 of the muscles of the thorax and abdomen which are concerned in 

 expiration. 



The note produced may vary in loudness, pitch, and quality or 

 timbre. The degree of loudness varies with the force of the expiratory 

 current. The pitch is determined partly by the length and partly by 

 the tension of the vocal cords. In children the pitch is relatively high, 

 because the cords are short. At the time of puberty the larynx in- 

 creases considerably in size, more so in the male than in the female, and 

 as a result a boy's voice "breaks," that is, becomes much lower in pitch. 

 The possible variation in pitch in any individual is on an average about 

 two octaves. This is mainly due to variations in the tension of the 

 vocal cords brought about by the reciprocal action of the crico-thyroid 

 and thyro-arytenoid muscles. The pitch is also affected by the length 

 of the vocal cords free to vibrate, this being determined by the move- 

 ments of the arytenoid cartilages. Further, the force of the expiratory 

 current influences the pitch, the stronger the blast of air the higher 

 being the note produced. 



The quality of the note is due to the resonance produced in the 

 various resonating chambers, the air in the chest vibrating with the 

 lower notes and that in the mouth and pharynx and in the accessory 

 sinuses of the nose with the higher pitched notes. Hence the terms 

 chest notes and head notes used in connection with singing. 



The sounds which constitute speech are due to modifications of the 

 laryngeal simple note, and are brought about by alterations in the shape 

 of the mouth and in the adjustment of the lips and teeth. The vowel 

 sounds are continuous vibrations, whereas the formation of consonants 

 depends on the interruption of vibrations. 



For* the production of the broad "a " vowel sound, the mouth cavity 

 is widely open; for the "i" (ee) sound, the space between tongue and 

 palate is much reduced ; for "u " (oo), the posterior part of the tongue 

 is raised against the palate. 



Consonants are classified as dental, guttural, or labial, according to 

 the position at which the interruption of the laryngeal note takes place. 

 Thus "t" and "d " are dentals, "p" and "b" are labials, and "g" and 

 " k " are gutturals. 



In whispering there is no phonation ; the glottis is open, and the 

 words produced are the result of the modification of the air current by 

 the speech mechanism. 



