(_a_) The _thyro-arytenoid[=e]us_ (pair), which by tilting the arytenoid cartilages forward relaxes the tension of the vocal bands. When they act with the adductors--_e.g._, the arytenoid[=e]us proprius--the result must be relaxation and approximation behind, which implies a greater or less degree of shortening, as usually happens when a certain point in an ascending scale is reached in persons whose methods of voice-production have not been in some way modified, and a new register begins, which in most female voices is marked by a more or less distinct and abrupt alteration of the quality of the tone. The crico-thyroids are the antagonists of the above-named muscles, and they may act either very much alone or, to some extent, in cooeperation with the above, to regulate or steady their action; for in movements so complicated as those required for voice-production it is highly probable that we are inclined to reduce our explanations of muscular action to a simplicity that is excessive, and to appreciate but inadequately the delicacy and complexity of the mechanism and the processes involved. It is quite certain that in the production of the highest tones of a tenor or soprano several muscles cooeperate, and one, especially, seems to be of great importance in the formation of such tones, most of all, perhaps, in high sopranos. The muscle referred to is the thyro-arytenoid already described. It is not only attached to the two cartilages indicated by its name, but also along the whole of the external or outer surface of the vocal band. It will be remembered that practically all the muscles are arranged in pairs, one on each side of the middle line. The muscle now under consideration, more, perhaps, than any other, is complex in its action. Apparently a very few of its fibres may act more or less independently of all the others at a particular moment and with a specific and very delicate result, a very slight change in pitch. Exactly how this is attained no one has as yet adequately explained; but it is doubtful whether any singer who does not possess a perfect control over this muscle can produce the highest tones of the soprano with ease and effectiveness. It is especially the muscle of the human birds of the higher flights. (_b_) To these thyro-arytenoids, which for most singers and all speakers are probably chiefly relaxing in action, must be added as aiding in this function another pair, the _lateral crico-arytenoids_. They are situated between the cricoid and arytenoid cartilages, and the direction of action is obliquely from below and forward, upward, and backward, so that the arytenoids are brought forward and also approximated more or less, which involves relaxed tension, at least, possibly also shortening of the vocal bands.