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. [Illustration: 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. ] [Illustration: PLATE IX. SIDE VIEW OF THE VOICEBOX, OR LARYNX, SHOWING THE INTERIOR OF THE LEFT HALF. 1, 2, 3. SHIELD-PYRAMID MUSCLE (THYRO-ARYTENOIDEUS). 4, 5. RING-PYRAMID MUSCLE (CRICO-ARYTENOIDEUS). 6. SHIELD. 7. LEFT UPPER HORN. 8. PYRAMID. 9. RING. 10. WINDPIPE. ] It may be observed here that it is impossible to imitate, in the dead subject, the contraction of the vocal muscles. All conclusions, therefore, drawn from experiments upon exsected larynges, with regard to tone-production in living man are necessarily quite untrustworthy, and cannot for one moment be admitted as evidence against observations made upon singers with the laryngoscope. These two pairs of muscles, then, namely the ring-shield muscles (pl. VIII, 1, 2) and the shield-pyramid muscles (pl. IX, 1, 2, 3) by stretching, slackening, and compressing the vocal ligaments, mainly govern the pitch of the tones produced by their vibrations. The ring-shield muscles receive some assistance in stretching the vocal ligaments from another quarter, of which we shall speak later on.