While laryngoscopic observation has thrown much valuable light on the operations of the laryngeal muscles, it has not by any means cleared up all the mysteries pertaining to this peculiarly intricate subject. All the muscles concerned are very small. Each one contains a vast and intricate number of tiny fibres, extending in widely varying directions. As each one of these sets of fibres can be contracted with a greater or less degree of strength than those most intimately connected with it, the possibility of variety in the combined actions of the laryngeal musculature is seen to be almost unlimited. Many conflicting theories have been offered to explain the details of the laryngeal action, but a comprehensive review of the subject is not called for here. Our purpose will be served by stating the most generally accepted theory, without committing ourselves as to its accuracy or sufficiency. This theory is as follows: In quiet breathing all the laryngeal muscles are in a state of relaxation, with the possible exception of the posterior crico-arytenoids. These muscles draw the arytenoid cartilages apart, and so open the glottis. For the production of a tone the glottis is closed by the contraction of the arytenoideus, which pulls the arytenoid cartilages together. The tension of the vocal cords is regulated by two sets of muscles: first, the thyro-arytenoids, to which the cords are directly attached; second, the lateral crico-arytenoids, which rotate the arytenoid cartilages, bringing their forward spurs together. The pitch of the tone is determined in two ways: first, by the tension of the vocal cords; second, by their effective length. In different parts of the vocal range the manner of adjustment for pitch varies. For the lowest notes of the voice, the chest register, the cords vibrate in their full length. As the pitch rises in singing an ascending scale passage in this part of the voice, the tension of the vocal cords is gradually increased by a corresponding increase in the strength of the contraction of the thyro-arytenoids. When the limit of the chest register has been reached, a further ascent in pitch is brought about by the gradual shortening of the vocal cords. This is effected by the contraction of the lateral crico-arytenoids, which rotate the arytenoid cartilages inward, as has just been described. The forward spurs of these cartilages are slightly curved in shape, so that, as they continue to rotate, their point of contact is gradually brought further and further forward. As the vocal cords are held tightly together behind this point, the portion of the cords left free to vibrate is shortened with each material increase in the tension of the lateral crico-arytenoids.