400 



*"" L?:CTURE XXXIV. 



The human voice depends principally on the vibrations of the membranes 

 of the glottis, excited by a current of air, which they alternately intercept 

 and suffer to pass; the sounds being also modified in their subsequent 

 progress through the mouth. Perhaps the interception of the air by these 

 membranes is only partial ; or it may be more or less completely intercepted in 

 sounds of different kinds: the operation of the organs concerned is not 

 indeed perfectly understood, but from a knowledge of their structure, we 

 may judge in some measure of the manner in which they are employed. 



The trachea, or windpipe, conveys the air from the chest, which serves for 

 bellows: hence, it enters the larynx, which is principally composed of five 

 elastic cartilages. The lowest of these is the cricoid cartilage, a strong ring, 

 which forms the basis of the rest: to this are fixed, before, the thyreoid car- 

 tilage, and behind, the two arytaenoid cartilages, composing together the 

 cavity of the glottis, over which the epiglottis inclines backwards, as it 

 ascends from its origin at the upper part of the thyreoid cartilage. Within 

 the glottis arc extended its ligaments, contiguous to each other before, 

 where they are inserted into the thyreoid cartilage, but capable of diverging 

 considerably behind whenever the arytaenoid cartilages separate. These 

 ligaments, as they vary their tension, in consequence of the motions of the 

 arytaenoid cartilages, are susceptible of vibrations of various frequency, and 

 as they vibrate, produce a continuous sound. Properly speaking, there are 

 two ligaments on each side; but it is not fully understood how they operate; 

 probably one pair only performs the vibrations, and the other assists, by 

 means of the little cavity interposed, in enabling the air to act readily on 

 them, and in communicating the vibrations again to the air. (Plate XXVI. 

 Fig. 357, 358.) 



The vowels and semivowels are continuous sounds, chiefly formed by this 

 apparatus in the glottis, and modified either in their origin or in their pro- 

 cress by the various arrangements of the different parts of the mouth. Of 

 simple vowels sixteen or eighteen may be enumerated in different languages: 

 in the French nasal vowels the sound is in part transmitted through the 

 nostrils, by means of the depression of the soft palate: the perfect semivowels 

 differ from" the vowels only in the greater resistance which the air undergoes 

 in its passage through the mouth; there are also nasal and semiuasal semi- 



