103 



The author considers the thyroid gland as serving the purpose 

 of interrupting the vibrations of the cartilages of the trachea, to 

 which it is closely attached, especially when pressed against it by 

 the actions of the flat muscles which extend over it. If sound 

 were given out by the vibrations of the trachea, it would interfere 

 with the distinctness of the voice. The thyroid gland is, therefore, 

 placed low in the larynx, and on the upper part of the trachea, for 

 the purpose of checking the vibrations of that tube, and suppress- 

 ing the sounds that would thence be produced. In birds, where 

 the voice originates in the inferior larynx, and is propagated along 

 the trachea, the structure of that tube is adapted to vibration, the 

 circles of cartilage being complete, and there is no thyroid gland. 



In man, the primary source of the voice is the vibration of the 

 chordae vocales, or ligaments of the glottis, acted upon by the im- 

 pulsion of the air passing by them. The proper degree of tension 

 is given by these ligaments by the action of the muscles, which 

 adjust their position with the greatest accuracy. The movements 

 of the chest, which regulate the expulsion of the air, are at the 

 same time adapted with great nicety to the production of the re- 

 quired effect. The sacculi laryngis also contribute to give greater 

 extent and freedom to the vibrations of the glottis. It is by the 

 concurrent action of these organs that the breath, which under 

 ordinary circumstances of respiration is inaudible, becomes vocal- 

 ized, or thrown into sonorous undulations. • 



The author then gives an anatomical description of the pharynx 

 and mouth, which together constitute an irregular cavity, extend- 

 ing from the glottis to the lips and nostrils, and of which the va- 

 rious changes of dimension and of form effect corresponding 

 changes in the undulations into which the air is originally thrown 

 by the vibrations of the glottis, and produce the different modifi- 

 cations of the voice. The most important are those which con- 

 stitute articulate sounds. The simple vocal tones, or vowels, are 

 greatly modified by the mere elongation or shortening of this cavity. 

 But even in the formation of these, the contraction and appulse of 

 the pharynx acts an important part ; and in the articulation of the 

 consonants it is a principal agent; its smaller cavity being, upon a 

 w^ell-known hydrostatic principle, substituted with great economy 

 of power for the more capacious cavity of the chest. In pronoun- 

 cing the explosive consonants, such as B, D, and G, the velum 

 pendulum is raised, and, acting as a valve, closes the posterior nares ; 

 and the mouth being also closed, the vocalized breath, which con- 

 tinues to ascend through the glottis, suffers condensation, and gives 

 rise to the faint sound which precedes the explosion consequent 

 upon the opening of the closed cavity. This opening takes place, 

 either by the separation of the lips, or by the removal of the tongue 

 from the teeth, or palate, to which it had been applied. These pre- 

 vious actions of the pharynx and glottis are the circumstances which 

 distinguish the sound of these letters from their corresponding mute 

 consonants, P, T, and K. Thus the consonants, classed according 

 to their formation in the mouth, either by the closed lips ; the meet- 



