In Figure III these muscles are plainly shown. This cut presents a view of the interior of the left half of the larynx; the right half of the thyroid cartilage has been removed, together with the muscles which lie to the right of the middle line of the larynx. We see the muscles of the left side of the interior of the larynx and also the right posterior crico-arytenoideus and the right crico-thyroid. [Illustration: FIGURE IV] The thyro-arytenoideus muscles are attached in front to the interior angle of the thyroid cartilage, and at the back to the arytenoid cartilages. To their outer edges the vocal cords are attached and the space between them is called the glottis. The crico-thyroids are attached to the outer surfaces of the cartilages. The lateral and posterior crico-arytenoids have very complex attachments, which can be seen in Figure III. The arytenoideus connects the rear surfaces of the two arytenoid cartilages. In Figure IV a drawing of the larynx is presented, looked at from above, the surrounding parts having been dissected away to allow a clear view of the muscles just considered. It is impossible to state with absolute certainty just what office the various intrinsic laryngeal muscles have in the production of tones of different pitches and qualities. Most of our information regarding this subject has been obtained by means of the laryngoscope. People interested in singing are now very generally acquainted with this little instrument, although it was invented by Manuel García so recently as 1855. While it is generally used only by physicians, anyone who wishes to can easily procure one and by its means observe the actions of his own vocal cords. The laryngoscope consists of a little mirror fitted to a handle at an angle of about 100 degrees. When used by a physician, it is held in the back of the subject’s throat, the tongue being pulled forward and to one side. A ray of strong light is reflected into it from another mirror, which the observer straps to his forehead. This ray of light is again reflected by the laryngeal mirror so as to illuminate the vocal cords. At the same time the observer sees in the laryngeal mirror the image of the vocal cords and so studies their movements.