852 



OF THE VOICE AND SPEECH. 



Artificial Glottis. 



but it may be made to approximate still more closely, by prolonging the 

 membranes in a direction parallel to that of the current of air, so that not 

 merely their edges, but their whole planes shall be thrown into vibration. 

 Upon this principle, a kind of artificial glottis has been constructed by Mr. 

 Willis; the conditions of action and the effects of which are so nearly allied 

 to that of the real instrument, that the similar character of the two can 

 scarcely be doubted. The following is his description of it. " Let a wooden 

 pipe be prepared of the form of Fig. 305, a, having a foot, c, like that of an 



organ-pipe, and an upper opening, 

 long and narrow, as at B, with a 

 point, A, rising at one end of it. 

 If a piece of leather, or, still better, 

 of sheet India-rubber, be doubled 

 round this point, and secured by 

 being bound round the pipe at D 

 with strong thread, as in Fig. 

 305, b, it will give us an artificial 

 glottis with its upper edges G H, 

 which may be made to vibrate or 

 not, at pleasure, by inclining the 

 planes of the edges. A couple of 

 pieces of cork, E F, may be glued 

 to the corners, to make them more 

 manageable. From this machine 

 various notes may be obtained, by 

 stretching the edges in the direc- 

 tion of their length G H; the notes 

 rising in pitch with the increased 

 tension, although the length of the vibrating edge is increased. It is true 

 that a scale of notes equal in extent to that of the human voice cannot be 

 obtained from edges of leather; but this scale is much greater in India- 

 rubber than in leather, and the elasticity of them both is so much inferior 

 to that of the vocal ligaments, that we may readily infer that the greater 

 scale of the latter is due to its greater elastic powers." By other experi- 

 menters the tissue forming the middle coat of the arteries has been used for 

 this purpose, in the moist state, with great success; with this, the tissue of 

 the vocal ligament is nearly identical. It is worthy of remark that, in all 

 such experiments, it is found that the two membranes may be thrown into 

 vibration, when inclined toivards each other in various degrees, or even when 

 they are in parallel planes, and their edges only approximate; but that the 

 least inclination from each other (which is the position the vocal ligaments 

 have during the ordinary state of the glottis, 693), completely prevents 

 any sonorous vibrations from being produced. 



698. The pitch of the notes produced by membranous tongues may be 

 affected in several ways. Thus, an increase in the strength of the blast, 

 which has little influence on metallic reeds, raises their pitch very consider- 

 ably; and in this manner the note of a membranous reed may be raised by 

 semitones, to as much as a fifth above the fundamental. The addition of a 

 pipe has nearly the same effect on their pitch, as on that of metallic reeds ; 

 but it cannot easily be determined with the same precision. Several different 

 notes may be produced with a pipe of the same length ; but there is a certain 

 length of the column of air, which is the one best adapted for each tone. It 

 has been recently ascertained, moreover, that the length of the pipe prefixed 

 to the reed has a considerable influence on its tone, rendering it deeper in 

 proportion as it is prolonged, down to nearly the octave of the fundamental 



