With each shortening of the vibrating parts of the cords the pitch is correspondingly raised. The portion of the vocal range thus produced is called the medium register. The highest note of the medium register is reached when the forward spurs of the arytenoid cartilages are in contact at their tips. Beyond this point there can be no further shortening of the vocal cords in this way. The remaining notes of the compass, known as the head register, are secured through a shortening of the effective length of the vocal cords at their forward ends, as well as by a further increase in their tension. Both these actions are accomplished by the contraction of the thyro-arytenoids. [Illustration: FIGURE V] Figure V shows a laryngoscopic view of the larynx, as it appears in quiet breathing. It will be observed that the vocal cords are widely separated, and the glottis is opened to its full extent. A similar view of the larynx during the production of tone is given in Figure VI. The vocal cords are closely approximated and the glottis is narrowed to a tiny slit-like opening. [Illustration: FIGURE VI] III What was said about the uncertainty of our present knowledge concerning the laryngeal action applies with almost equal force to the other operations of tone production. Beyond the basic facts that the tones of the voice are produced by the vibration of the vocal cords and are reinforced and modified by the influence of the resonance cavities, little can be stated with absolute certainty. There is, of course, no question as to the definiteness of our knowledge of the anatomical structure of the parts involved. But, with regard to the muscular operations of the resonance cavities and the application of acoustic and mechanical principles in these operations, the same uncertainty is encountered as in the laryngeal actions. We shall continue therefore to outline the most widely accepted theories, without entering into a discussion as to their soundness. Considered acoustically, the voice is a wind instrument of the reed class. It differs, however, from all other reed instruments in several particulars. It is capable of producing a wide range of pitches, covering more than three octaves in many cases, by the operation of a single pair of reeds. Further, it has command of an immense range of tone qualities, through the combined action of its reed mechanism and its resonating cavities.