582 PHYSIOLOGY 



with a central aperture is fixed before one eye of the observer, sitting 

 in front of the patient or person to be observed. The latter is directed 

 to throw his head slightly backwards and to open his mouth. In 

 order to keep the tongue out of the way the patient is made to hold 

 the end of it by means of a towel. The mirror is then so arranged 

 as to reflect light from a lamp into the cavity of the mouth. A small 

 mirror fixed in a handle is then warmed, so as to prevent the con- 

 densation of the patient's breath, and passed to the back of the mouth 

 until it rests upon and slightly raises the base of the uvula. By this 

 mirror the light reflected into the mouth from the large mirror is again 

 reflected down on to the larynx, and a reflection of the larynx and 

 trachea is seen in the mirror. By laryngoscopic examination we can 

 see the base of the tongue, behind which is the outline of the epi- 

 glottis. Behind this again in the middle line are seen the two vocal 

 cords, white and shining (Fig. 253). The cords appear to approximate 

 posteriorly ; between them is a narrow chink, the diameter of which 

 varies with each respiration, being wider during inspiration. On each 

 side of the true vocal cords are seen the pink false vocal cords. In some 

 cases the rings of the trachea, and even the bifurcation of the trachea 

 itself (Fig. 253, c), may be seen in the interval between the vocal cords. 



In order that the vocal cords may be set into vibration, they 

 must be put into a state of tension and the aperture of the glottis 

 narrowed, so as to afford resistance to the current of air. In the dead 

 larynx it is possible to produce sounds by forcing air from bellows 

 through the trachea, after the vocal cords have been put on the 

 stretch by pulling the arytenoid cartilages backwards. By experi- 

 menting on patients on whom tracheotomy has been performed, it 

 has been found that the pressure of air in the trachea, necessary to 

 cause production of voice, is, for a tone of ordinary loudness and 

 pitch, between 140 and 240 mm. of water, and with loud shouting the 

 pressure rises to as much as 945 mm. of water. This pressure is 

 furnished by the contraction of the expiratory muscles, i.e. of the 

 abdomen and of the thorax. Since the pitch of the note produced 

 rises with increasing force of the blast, while the tension of the cords 

 remains constant, it is evident that, in the act of ' swelling ' on a 

 note, the increased pressure necessary for the crescendo must be 

 associated with diminishing tension of the cords. It is the failure to 

 secure this muscular relaxation that so often causes a singer to sing 

 sharp when swelling on any given note. 



The voice, like the sound produced on any musical instrument, 

 may vary either in pitch, loiidness, or in quality or timbre. The 

 range of any individual voice is generally about two octaves. The 

 pitch of the voice usually employed is determined chiefly by the length 

 of the vocal cords. Thus in children the voice is high-pitched. Before 



