Moving on along the vocal tract
As we continue our journey we come across an organ that has a crucial role in the process of speaking: the larynx, or voice box. This is a cartilaginous pyramidal organ characterized by a remarkable structural complexity and situated at the top of the trachea. As all speech organs, it primarily performs a vital role, namely it acts as a valve that closes, thus blocking the entrance to the windpipe and preventing food or drink from entering the respiratory ducts while we are eating. They are instead directed down the pharynx and the esophagus.
Close up of the larynx
Source of the pictures: logopaediewiki.de
The larynx is the first speech organ proper along the tract that we are following, as it interferes with the outgoing stream of air and establishes some of the essential features of the sounds that we produce. However, it is not the larynx proper (that is the organ in its entirety) that performs this important role within the speech mechanism, but two muscular folds inside it, called the vocal cords. You can see them closing and opening in the back in the following video...
Curious enough to watch a larynx in action? (not for the squeamish)
As mentioned above, the larynx consists of a number of cartilaginous structures that interact in an ingenious way enabling the larynx to perform its important respiratory and articulatory functions. The thyroid cartilage is made of two (left and right) rectangular flat plates that form an angle anteriorly, resembling the covers of a book that is not entirely open. The aperture of the angle, oriented posteriorly, varies with the sex. It is a right angle in men (90°) while in women it is 120°. The angle is more visible, because more acute, in the former situation and the cartilage is popularly known as “Adam’s apple”.
Posteriorly, each of the plates has two horns (an inferior and a superior one) called cornua, through which the thyroid cartilage is connected with the cricoid cartilage. The joint that the two cartilages form, resembling a sort of hinges, allows the cricoid one to move anteriorly and posteriorly with respect to the thyroid one, thus controlling the degree of tension in the vocal cords. One of the main functions of the thyroid cartilage is to protect the larynx and particularly the vocal cords. The cricoid cartilage is made of a ring-shaped structure, situated anteriorly and of a blade situated posteriorly and represents the base of the larynx, controlling communication with the trachea. On top of its blade, on the left and right side respectively, another pair of cartilages are situated: the arytenoid muscles.
The last important cartilage in the process of phonation or speech production is the epiglottis which is a spoon-shaped cartilage (in the video visible in the front) also playing an important role in keeping the food away from the respiratory tract. It is between the arytenoid cartilages and the thyroid cartilage that the two vocal cords mentioned above stretch. The vocal cords are each made of a so-called vocal ligament and a vocal muscle. They are covered in mucous membranes or skin folds also known as the vocal folds. They connect the lower part of the thyroid cartilage to the anterior part of the arytenoid cartilages. The opening between the folds and the arytenoid cartilages represents the glottal aperture, more commonly called the glottis.
Source of the picture: wikipedia.org
The length of the vocal folds varies with the age and the sex. They become longer at the age of puberty and are longer in men (24-26 mm) than in women (17-20 mm). During breathing, the two folds part (remember the video), letting the air come into the larynx or go out. During phonation they come closer, having an important role in establishing some of the main characteristics of the sounds we articulate. By the complex action of adjacent anatomical structures (the cartilages described above and a number of laryngeal muscles) the two vocal cords can be brought together or parted. They thus interfere to various extents with the outgoing airstream. They can obstruct the passage completely, as in the case of the so-called glottal stop, or their participation in the uttering of a given sound can be minimal (as in the case of many hissing sounds).
The rapid and intermittent opening and closing of the vocal cords, which results in the vibration of the two organs, plays a key role in one of the most important phonetic processes, that of voicing. Thus, vowels and vowel-like sounds, as well as a number of consonants, are produced with the vibration of the cords and are consequently voiced. The absence of vibration in the vocal cords is characteristic for voiceless obstruents. (More details about the process will be given in the following chapters.) The amplitude of the vibration is also essential for the degree of loudness of the voice: thus the intensity of the sound that is uttered depends on the pressure of the air that is expelled. The rate at which the vocal cords vibrate has also important consequences as far as the pitch of the voice is concerned; this is closely linked to the pressure exerted on the vocal cords. When we produce acute (high-pitched or shrill) sounds the vocal cords come closer to each other, while during the articulation of grave sounds the vocal cords leave a greater space between them.
The next stop on our way along the vocal tract is the pharynx, an organ situated at a kind of crossroads along the above-mentioned tract. Move on to the following page to finish your journey along the vocal tract.