by virtue of the perfect elasticity of muscle the whole thickness of the cord, including this vocal muscle, can be stretched and thrown into vibration (vide fig. 8).  In the production of chest notes the whole vocal cord is vibrating, the difference in the pitch depending upon the tension produced by the contraction of the tensor (ring-shield) muscle.  When, however, the change from the lower to the upper register occurs, as the photographs taken by Dr. French and reproduced in a lecture at the Royal Institution by Sir Felix Semon show, the vocal cords become shorter, thicker, and rounder; and this can be explained by supposing that the inner portion of the vocal muscle contracts at the break from the lower to the upper register (vide fig. 11); and that as a result only the free edges of the cords vibrate, causing a change in the quality of the tone.  As the scale is ascended the photographs show that the cords become longer and tenser, which we may presume is due to the continued action of the tensor muscle.  Another explanation is possible, viz. that in the lower register the two edges of the vocal cords are comparatively thick strings.  When the break occurs, owing to the contraction of the inner portion of the vocal muscle, we have a transformation into thin strings, at first short, but as the pitch of the note rises, the thin string formed by the edge of the vocal cord is stretched and made longer by the tensor.  It should be mentioned that Aikin and many other good authorities do not hold this view.

[Illustration:  FIG. 7 A-A’, Ring cartilage.  B, Shield cartilage. 1, Pyramid cartilage. 2, Vocal process, with 2’, its position after contraction of muscle. 3, Postero-external base of pyramid, giving attachment to abductor and adductor muscles at rest, with 3’, its new position after contraction of the muscles. 4, Centre of movement of the pyramid cartilage. 5, The vocal cords at rest. 5’, Their new position after contraction of the abductor and adductor muscles, respectively seen in I and II. 6, The interligamentous, with 7, the intercartilaginous chink of the glottis. 8, The arrow indicating respectively in I and II the action of the abductor and adductor in opening and closing the glottis.]

[Description:  FIG. 7.—­Diagram after Testut (modified), showing:  (i.) the action of the abductor muscle upon the pyramid cartilages in separating the vocal cords; (ii.) the action of the adductor muscles in approximating the vocal cords.]

[Illustration:  FIG. 8]

[Description:  FIG. 8.—­Diagram after Testut (modified) with hinder portion of larynx and windpipe cut away, showing the conical cavity of the sound-pipe below the vocal cords.  The ventricle above the vocal cords is seen with the surface sloping upwards towards the mid line.]