5IO ARRANGEMENT OF THE LARYNX. 



the cords rapidly return to their former position, and are again pushed asunder, and caused to 

 vibrate. 



1. Thus, when a membrane vibrates, the air must be alternately condensed and rarefied. 

 The condensation and rarefaction are the chief cause of the tone or note (as in the siren), not so 

 much the membranes themselves (v. Hehnholtz). 



2. The,air-tube or "porte vente," conducting the air to the membranes in man is the lower 



tortion of the larynx, the trachea, and the whole bronchial system ; the bellows are represented 

 y the chest and lungs, which are forcibly diminished in size by the expiratory muscles. 



3. The cavities which lie above the membranes constitute "resonators," and consist of the 

 upper part of the larynx, pharynx, and also of the cavities of the nose and mouth, arranged, 

 as it were, in two stories, the one over the other, which can be closed alternately. 



The pitch of the tone produced by a membranous apparatus depends upon the following 

 factors : 



(a) On the length of the elastic membranes or plates. The pitch is inversely proportional to 

 the length of the elastic membrane, i.e., the shorter the membrane the higher the pitch, or the 

 greater the number of vibrations per second. Hence, the pitch of a child's vocal cords (shorter) 

 is higher than that of an adult. 



(6) The pitch of the tone is directly proportional to the square root of the amount of the 

 elasticity of the elastic membrane. In membranous reeds, and also with silk, it is directly pro- 

 portional to the square root of the extending weight, which in the case of the larynx is the 

 force of the muscles rendering the cords tense. 



(c) The tone of membranous reeds is not only strengthened by a more powerful blast, as the 

 amplitude of the vibrations is increased, but the pitch of the tone may also be raised at the same 

 time, because, owing to the great amplitude of the vibration, the inean tension of the elastic 

 membrane is increased. 



(d) The supra-laryngeal cavities, which act as resonators, are also inflated when the larynx 

 is in action, so that the tone produced by these cavities is added to and blended with the sound 

 of the elastic membranes, whereby certain partial tones of the latter are strengthened ( 415). 

 The characteristic timbre of the voice largely depends upon the form of the resonators. 



(c) When vocalising, the strongest resonance takes place in the air-tubes, as they contain 

 compressed air. It causes the vocal fremitus which is audible on placing the ear over the chest 

 (H7,6). 



(/) Narrowing or dilating the glottis has no effect on the pitch of the tone, only with a wide 

 glottis much more air must be driven through it, which, of course, greatly increases the work 

 of the thorax. 



313. ARRANGEMENT OF THE LARYNX. I. Cartilages and Ligaments. 



The fundamental part of the larynx consists of the cricoid cartilage, whose small 

 narrow portion is directed forwards and the broad plate backwards. The thyroid 

 cartilage articulates by its inferior cornu with the posterior lateral portion of the 

 cricoid. This permits of the thyroid cartilage rotating upon a horizontal axis 

 directed through both of the articular surfaces, so that the upper margin of the 

 thyroid passes forward and downward, while the joint is so constructed as to 

 permit also of a slight upward, downward, forward, and backward movement of 

 the thyroid upon the cricoid cartilage. The triangular arytenoid cartilages 

 articulate at some distance from the middle line, with oval, saddle-like, articular 

 surfaces placed upon the upper margin of the plate of the cricoid cartilage. The 

 articular surfaces permit two kinds of movements on the part of the arytenoid 

 cartilages ; first, rotation on their base around their vertical long axis, whereby 

 either the anterior angle or processus vocalis, which is directed forwards, is rotated 

 outwards ; while the processus muscularis, which is directed outwards and projects 

 over the margin of the cricoid cartilage, is rotated backwards and inwards, or 

 conversely. Further, the arytenoids may be slightly displaced upon their bases 

 either outwards or inwards. 



The true vocal cords, or thyroarytenoid ligaments, are in man about 15 milli- 

 metres, and in woman 11 millimetres in length, and consist of numerous elastic 

 fibres. They arise close to each other from near the middle of the inner angle of the 

 thyroid cartilage, and are inserted, each into the anterior angle or processus vocalis 

 of the arytenoid cartilages. The ventricles of Morgagni permit free vibration 

 of the true vocal cords, and separate them from the upper or false cords, which con- 

 sist of folds of mucous membrane. The false vocal cords are not concerned in 



