2 8o A MANUAL OF PHYSIOLOGY 



The pitch of a note, while it depends chiefly, as has been said, 

 on the tension of the vocal cords, rises and falls somewhat with 

 the strength of the expiratory blast ; the highest notes are only 

 reached with a strong expiratory effort. The intensity of all 

 vocal sounds is determined by the strength of the blast, for the 

 amplitude of vibration of the cords is proportional to this. 

 Besides pitch and intensity, the ear can still distinguish the 

 quality or timbre of sounds ; and the explanation is as follows : 

 Two simple tones of the same pitch and intensity, that is, the 

 sounds caused by two series of air-waves of the same period 

 and amplitude of the same frequency and height, to use less 

 technical terms would appear absolutely identical to the sense 

 of hearing ; just as the aerial disturbances on which they depend 

 would be absolutely alike to any physical test that could be 

 applied. But no musical instrument ever produces sound-waves of 

 one definite period, and one only ; and the same is true of the 

 voice. When a stretched string is displaced in any way from 

 its position of rest, it is set into vibration ; and not only does 

 the string vibrate as a whole, but portions of it vibrate inde- 

 pendently and give out separate tones. The tone corresponding 

 to the vibration period of the whole string is the lowest of all. 

 It is also the loudest, for it is more difficult to set up quick than 

 slow vibrations. The ear therefore picks it out from all the 

 rest ; and the pitch of the compound note is taken to be the pitch 

 of this, its fundamental tone. The others are called partial or 

 overtones, or harmonics of the fundamental tone, their vibration 

 frequency being twice, three times, four times, etc., that of the 

 latter. Now, the fundamental tone of a compound note or 

 clang produced by two musical instruments may be the same, 

 while the number, period, and intensity of the harmonics are 

 different ; and this difference the ear recognises as a difference 

 of timbre or quality. The timbre of the voice depends for the 

 most part on partial tones produced or intensified in the upper 

 resonance chambers. 



A great deal of our knowledge as to the mode and mechanism 

 of the production of voice has been acquired by means of the 

 laryngoscope (Fig. 121). This consists of a small plane mirror 

 mounted on a handle, which is held at the back of the mouth in 

 such a position that a beam of light, reflected from a larger 

 concave mirror fastened on the forehead of the observer, is 

 thrown into the larynx of the patient. The observer looks, 

 through a hole in the centre of the large mirror ; and an image 

 of the interior of the larynx is seen in the small mirror, in which 

 the parts that are anterior appear as posterior, the arytenoid 

 cartilages in front, the thyroid behind, and the vocal cords 

 stretching between. The small mirror is warmed to body- 



