CHAPTER VII. 



ON SOME SPECIAL MUSCULAR MECHANISMS. 



SEC. 1. THE VOICE. 



899. IN the trachea the respiratory passage is of nearly 

 uniform bore ; in the larynx it is less regular, and at one part is 

 narrowed into a slit of variable width, stretching from front to 

 back, forming as it were a throat (glottis) to the passage below. 

 The mucous membrane lining the larynx is so modified at the 

 edges of this slit as to form two more or less parallel elastic 

 membranes capable of being thrown into vibrations when an 

 adequate blast of air is driven through the slit. These vibrations 

 communicated to the air give rise to the sound which we call the 

 voice, and the two membranous edges, which are thus the essential 

 means of producing the voice, are called the vocal cords, chordae 

 vocales. The blast of air is supplied by the respiratory mechan- 

 ism, the expiratory current being almost exclusively used because 

 it is more manageable, and more favourable for the conduction of 

 the sound outwards than is the inspiratory current. 



When a sound is produced, as is the voice, by the vibrations 

 of the membranous edges of a slit, placed in the course of a 

 tubular passage, the characters of the sound are determined in 

 part by the nature, arrangement and behaviour of the slit and its 

 membranous edges, but they are also determined by the length 

 and shape of the tubular passage, especially of that part through 

 which the sound is conveyed after its formation at the slit, and 

 which forms what is often called a "resonance tube" or "reson- 

 ance chamber." Hence in studying the voice we have to consider 

 on the one hand the essential parts of the apparatus, namely, 



