OF THE LARYNX, AND ITS ACTIONS. 



931 



Fig. 235. 



by a moderate degree of tension, and be fixed in such a manner as to be advan- 

 tageously acted on by a current of air, it will give a distinct tone. It is ob- 

 served by Miiller, that membranous tongues made elastic by tension may have 

 either of three different forms : I. That of a band extended by a cord, and in- 

 cluded between two firm plates, so that there is a cleft for the passage of air on 

 each side of the tongue, n. The elastic membrane may be stretched over the 

 half or any portion of the end of a short tube, the other part being occupied by a 

 solid plate, between which and the elastic membrane a narrow fissure is left. in. 

 Two elastic membranes may be extended across the mouth of a short tube, each 

 covering a portion of the opening, and having a chink left open between them. 

 This last is evidently the form most allied to the Human 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 in- 

 strument, 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. 235, 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 dou- 

 bled round this point, and secured by 

 being bound round the pipe at D, with 

 strong thread, as in Fig. 235, 6, 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 direction 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 elas- 

 ticity 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 experimenters, 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 ligaments 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 towards 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, 930), completely pre- 

 vents any sonorous vibrations from being produced. 



933. The pitch of the note 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 considerably ; and 



Artificial Glottis. 



