130 FHYSIOLOGY. 



these requires a separate contraction of muscular fibres, followed by 

 a relaxation of equal length. Each contraction must therefore have 

 occurred in one-tenth of a second. 



The production of voice, or vocal sounds, depends upon the appli- 

 cation of muscular power to the vocal instrument, and is therefore 

 properly considered under this head. 



OF THE VOICE AND SPEECH. 



The voice is produced when the air is expelled from the lungs ; 

 every one knows it is impossible to articulate sounds with the mouth 

 and nose closed. It must be remembered that vocal sounds, and 

 speech are two very different things ; the former may be produced in 

 great perfection, where there is no capability for the latter. The 

 voice is formed in the larynx ; the modifications of it by which 

 speech is formed, are effected in the cavity of the mouth. The na- 

 tural voice, or cry, exists in nearly all animals. Man alone, it is 

 believed, has the power of producing articulate sounds or language. 



The larynx consists of four cartilages. The cricoid, the thyroid, 

 and two arytenoid. The cricoid is the lowest of these ; it surmounts 

 the trachea, and is shaped like a seal-ring, having its deepest portion 

 behind. The thyroid is situated above the cricoid, with which it is 

 articulated by its lower cornua, in such a manner that its lower front 

 margin may be made to approach or recede from the upper margin 

 of the cricoid. The higher the tone, the more nearly do they ap- 

 proximate. The arytenoid cartilages are situated upon the posterior 

 and superior portion of the cricoid, with which they are articulated 

 also in a movable manner. From the tips of the arytenoid carti- 

 lages, to the inner front of the thyroid, are stretched the two pairs of 

 vocal ligaments, composed of yellow elastic tissue, and named the 

 superior and inferior vocal ligaments. The whole interior surface 

 of the larynx is lined by mucous membrane, which is reflected over 

 the vocal cords, dipping down into the spaces between them to form 

 the ventricles of the larynx, or, as they are sometimes called, ventri- 

 cles of Morgagni, or of Galen (Fig. 39). Upon the varying degree 

 of tension of the vocal ligaments depends the variety of tones of 

 which the voice is susceptible. For the production of vocal tones the 

 ligaments of the opposite sides are also required to be brought into 

 approaching parallelism with each other, by the approximation of the 

 points of the arytenoid cartilages, whilst in the intervals they are 

 separated from each other, and the rima glottidis, or fissure between 

 them, assumes the form of the letter V (Fig. 40). 



The muscles that are concerned in governing the pitch of the notes, 

 by regulating the tension of the cords, and those that govern the 

 aperture of the glottis are the following. (The table is derived from 

 Carpenter's Human Physiology.) 



