480 MUSCLE-NERVE PHYSIOLOGY 



The Larynx. In nearly all air-breathing vertebrate animals there 

 are arrangements for the production of sound, or voice, in some parts of the 

 respiratory apparatus. In many animals, the sound admits of being variously 

 modified and altered during and after its production; and, in man, one such 

 modification occurring in obedience to dictates of the cerebrum, is speech. 



It has been proven by observations on living subjects, by means of the 

 laryngoscope, as well as by experiments on the larynx taken from the dead 

 body, that the sound of the human voice is the result of the vibration of the 

 inferior laryngeal ligaments, or the true vocal cords which bound the glottis, 

 caused by currents of expired air impelled over their edges. If a free opening 

 exists in the trachea, the sound of the voice ceases, but it returns if the opening 

 is closed. An opening into the air-passages above the glottis, on the con- 

 trary, does not prevent the voice being produced. By forcing a current of 

 air through the larynx in the dead subject, clear vocal sounds are elicited, 

 though the epiglottis, the upper ligaments of the larynx or false vocal cords, the 

 ventricles between the upper ligaments and the inferior ligaments, and the upper 

 part of the arytenoid cartilages, be all removed. But the true vocal cords must 

 remain entire with their points of attachment, and be kept tense and so 

 approximated that the fissure of the glottis may be narrow. 



The vocal ligaments or cords, therefore, are regarded as the proper organs 

 for the production of vocal sounds. The modifications of these sounds are 

 effected, as will be presently explained, by other parts, viz., by the tongue, 

 teeth, lips, etc. The structure of the vocal cords is adapted to enable them to 

 vibrate like tense membranes, for they are essentially composed of elastic tissue; 

 and they are so attached to the cartilaginous parts of the larynx that their 

 position and tension can be variously altered by the contraction of the muscles 

 which act on these parts. 



Thus it will be seen that the larynx is the organ of voice. It may be said 

 to consist essentially of the two vocal cords and the various cartilaginous, 

 muscular, and other apparatus by means of which not only can the aperture 

 of the larynx (rima glottidis) be closed against the entrance and exit of air 

 to or from the lungs, but also by means of which the cords themselves can be 

 stretched or relaxed, brought together and separated in accordance with the 

 conditions that may be necessary for the air in passing over them to set them 

 vibrating to produce the various sounds. Their action in respiration has been 

 already referred to. 



Anatomy of the Larynx. The principal parts entering into the formation of the larynx, 

 figures 342 and 343, are the thyroid cartilage; the cricoid cartilage; the two arytenoid 

 cartilages; and the two true vocal cords. The epiglottis, figure 343, has but little to do 

 with the voice, and is chiefly useful in protecting the upper part of the larynx from the 

 entrance of food and drink in deglutition. The false vocal cords and the ventricle of the 

 larynx, which is a space between the false and the true cord of either side, need be only 

 referred to. 



Cartilages, a, The thyroid cartilage, figure 342, i to 4, does not form a complete ring 



