510 MUSCLE-NERVE PHYSIOLOGY 



number of combined and harmoniously arranged muscular contractions 

 which are necessary for even the simplest acts of locomotion. 



II. THE PRODUCTION OF THE VOICE. 



Before commencing the consideration of the nervous system and the 

 special senses it will be convenient to consider first speech, the production of 

 the human voice, and the physiology of the larynx as a muscular apparatus. 



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 

 exist 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. 



