The functions of these muscles render any loss of their power as glottis-openers a matter of importance. It will be remembered that they are so situated that they not only rotate the arytenoids, turning the vocal processes away from each other, but they also serve to fix the cartilages, giving them a firm support as points of attachment for the vocal cords. The outer fibres tend also to draw the body of the arytenoids away from each other, as well as to fix them in a postero-lateral position. They are, more than any other of the muscles of the larynx, organs of respiration. They are also in constant action: with each inspiration they contract, and during expiration they {87} fall into rest. In this respect they resemble the other muscles of respiration and the central organ of the circulation. In some respects they also resemble the muscles of the heart in the degenerative changes to which they are subject. Their antagonists are the lateral crico-arytenoids. When both sets of muscles are paralyzed, the glottis is in what is known as the cadaveric condition; that is, the vocal cords are neither widely separated nor parallel to each other. There is an opening of a triangular shape as in the act of easy inspiration, not sufficiently approximated to admit of speech, but sufficiently open to admit of free inspiration. With this understanding of the physiology of the parts, we can readily appreciate the results of the loss of power of these muscles. As stated by Bosworth, the especial danger is in the integrity of the adductors, tending for the want of antagonism to keep the glottis closed. Of all the muscles of the larynx, these are therefore the most important so far as life is concerned. The disease is progressive (Lefferts, Semon, Bosworth). The first symptom which attracts attention is generally inspiratory dyspnoea while taking active exercise. The difficulty continues to increase till there is constant difficulty in the act of inspiration, usually with spasm. The dyspnoea is more marked during sleep than when awake. Death may occur at this period of the disease before the gravity of the trouble has been recognized. As a rule, tracheotomy will be required to prolong life, after which the dangers to the patient are passed.