LECT. XVIII. VOCAL CORDS. 337 



cries whenever we pinch any part of its body. You per- 

 ceive that, at the same time, the crico-thyroid muscles con- 

 tract. By the contraction of these muscles the vocal cords 

 are rendered tense, and brought nearer together. After 

 having divided the nerves going to these muscles, their 

 contraction can no longer be effected ; and if we then pinch 

 the animal it no longer cries, and we can hear only some 

 very grave and hoarse sounds. We may, therefore, con- 

 clude that the glottis constitutes the organ of the voice ; 

 that the trachea may be compared to the bellows' pipe of 

 an organ ; and that the upper part of the larynx, and all 

 the parts situated above the epiglottis, including the cavities 

 of the mouth and nose, form the upper tube of this instru- 

 ment, which serves merely to modify the sound, as may be 

 easily shown by effecting some alteration in this cavity. 



Vocal Cords. The vocal cords are formed of that elas- 

 tic tissue, which is remarkable for its yellow colour, for the 

 arrangement of its fibres, and for forming the middle coat 

 of the arteries, and a great number of ligaments. It is 

 decidedly the most elastic of all the tissues of the human 

 body. The movements of the cartilaginous pieces of the 

 larynx vary the degree of tension of the vocal cords, and 

 the transverse and oblique diameter of the glottis. In 

 general, the latter becomes narrower when it emits sounds. 

 The vocal cords, either as cords or as stretched membranes, 

 attached at one side, evolve sounds when a column of air 

 causes them to vibrate; and these sounds necessarily vary 

 according to the tension and length of the cords, and the 

 force of the current of air. A current of air traversing with 

 a certain rapidity an orifice whose diameter is variable, may 

 also, independently of the elasticity of the lips of the orifice, 

 produce different sounds, as in the so-called wind musical 

 instruments. The tube which rises above the glottis, and 

 which consists of the ventricles, the pharynx, and the mouth, 

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