The vocal ligaments in the adult male are, in a state of rest, about three-quarters of an inch long, and in the female about half an inch. I pointed out before that the vocal ligaments are attached in front to the shield ([pl. VII], 6) and behind to the pyramids (pl. VII, 3, 3). Let it now be borne in mind—1st, That the pyramids, in their turn, are fastened to the upper border of the ring cartilage; and 2nd, That by drawing the shield downwards and forwards upon the ring, the distance between the upper border of the ring ([pl. VII], 5) and the front of the shield ([pl. VII], 6) is increased, and it will be easily seen that this movement must of necessity have the effect of stretching the vocal ligaments.
This drawing of the shield downwards and forwards upon the ring is brought about by a pair of muscles ascending on either side, in the shape of a fan, from the ring to the shield cartilage ([pl. VIII], 1, 2). These muscles we name the "Ring-Shield Muscles." In opposition to them there is another pair inside the shield, running parallel with the vocal ligaments (pl. IX, 1, 2, 3). They are attached (like the vocal ligaments) in front to the shield cartilage and behind to the pyramids. These muscles we will call the "Shield-Pyramid Muscles." They counteract the ring-shield muscles, and having overcome their resistance, pull the shield cartilage up again, thereby, of course, relaxing the vocal ligaments. The ring-shield muscles, therefore, stretch the vocal ligaments and the shield-pyramid muscles relax them. The shield-pyramid muscles have an additional function—that of pressing together the vocal ligaments, under certain circumstances, thereby narrowing the opening between them. They have therefore been, in these later days, called the Sphincter[G] muscle of the glottis. They have also been called the Vocal Muscles, since they play so important a part in the formation of all vocal tone that a paralysis of them causes total loss of voice.
Plate VIII.
SIDE VIEW OF THE VOICEBOX, OR LARYNX.
| 1, 2. | Ring-Shield Muscle (Crico-Thyroideus). |
| 3. | Lid. |
| 4. | Shield. |
| 5, 6. | Upper Horns of the Shield. |
| 7. | Ring. |
| 8. | Windpipe. |
Plate IX.