The _crico-thyroid_ also tends to open the glottis. Just as the diaphragm is the most important muscle of breathing, so is the crico-thyroid the most important in ordinary speaking and in singing in the lower register. It is a relatively large and strong muscle with an oblique direction in the main, though it is composed in reality of several sets of fibres some of which are much more oblique in direction than others (Fig. 28). As its name indicates, its points of attachment are to the thyroid and the cricoid cartilages, but the most fixed point (_origin_) is its point of attachment to the larger cartilage; hence its direction of pull is from the thyroid, with the result that the anterior part of the cricoid is drawn up, the posterior part down, and the arytenoid cartilage, resting on the upper part of the cricoid, backward, so that the vocal band is rendered longer and more tense (see especially Fig. 29). It is important to note that this is the muscle most used in singing the lower tones of the scale, and that its action must necessarily cease, to a great extent, when a certain point in the pitch is reached, as there is a limit to the degree of contraction of all muscles; and, besides, the crico-thyroid space is of very moderate size, and the cricoid cartilage can ascend only within the limits thus determined. It thus follows that Nature has provided in the change of mechanism for a new register, which is nothing else than a change of mechanism with a corresponding change of function. It will be at once apparent that the claim that registers are an invention of men, and without foundation in nature, is without support in anatomy and physiology. The crico-thyroid is probably, however, of much more importance to tragic actors and barytones than to tenors or sopranos. This, however, is no excuse for the neglect of its development by the latter class, as often happens, for without it the best tones of the lower register are impossible. On the other hand, the elocutionists who prescribe for students practices that involve the excessive use of this muscle, with a cramped position of the vocal organs, the larynx being greatly drawn down, with the view of producing disproportionately heavy lower tones, must take no comfort from the above anatomical and physiological facts. Art implies proportion, and it was one of the ambitions of all the best actors in the golden age of histrionic art to have an "even voice"--_i.e._, one equally good through the whole range required. The tragic actor, elocutionist, and public speaker, and the singer, whether soprano or bass, should neglect no muscle, though they may be justified in developing some in excess of others, but ever with a watchful eye on the weakest part. 2. The muscles which regulate the tension of the vocal bands are the following: