CHAP. VIIL] ORGANS OF RESPIRATION AND SECRETION. 229 



The aryteno-epiglottic, which might he called." crico-epiglottic," is a 

 thick fold of membrane (Fig. 107, fa) which proceeds from the base 

 of the outside of each arytenoid and contiguous part of the cricoid, to 

 the side of the epiglottis. The thyro-hyoid connects the os hyoides 

 with the thyroid, while the crico-thyroid connects the latter with the 

 cricoid, filling up the interval between them and containing much 

 elastic tissue. 



The most important ligaments, however, are those called the 

 " vocal cords," of which there are two kinds on either side of the 

 cavity of the larynx. The superior or fake vocal cords called also 

 the superior thyro-arytenoid ligaments are folds of membrane which 

 pass (one on each side) from the anterior aspect of the arytenoid 

 cartilages downwards and forwards to the mucous membrane of the 

 middle of the posterior (dorsal) surface of the epiglottis and thyroid 

 (Fig. 107, ac). These are very prominent folds of membrane, and it 

 is by their vibrations that the sound of " purring " is said to be 

 produced. 



The inferior or true vocal cords called also the inferior thyro- 

 arytenoid ligaments have a similar but lower origin and insertion 

 to the false vocal cords (Fig. 107, v) ; they are less prominent and 

 sharply edged. It is these true vocal cords which by their vibra- 

 tions are said to produce the mewing and howling sounds. These 

 cords leave between them an aperture termed the rima glottidis, and 

 they tend to form a horizontal partition dividing the cavity of the 

 larynx into an upper (anterior) and a lower (posterior) portion. 

 The whole larynx opens into the pharynx by its superior aperture 

 the glottis which is bounded in front by the epiglottis, behind by the 

 arytenoid cartilages, and on each side by the aryteno-epiglottidean 

 folds, and by the false vocal cords. Between the false and true vocal 

 cords of each side is a small depression called a " ventricle" These 

 two depressions, however, are so slight as hardly to deserve notice. 



7. The VOICE is produced by the vibration of the edges of the 

 vocal cords (when stretched and approximated), which vibration is 

 effected through the passage outwards of a stream of air, according 

 to the laws which regulate the vibrations of strings and membranes 

 Accordingly, certain muscles act upon the arytenoid cartilages, and 

 by t their action put the membranes on the stretch, and so alter 

 the shape of the opening of the glottis by contracting it (Fig. 107, 

 A & B). The more the cords are stretched, and the narrower the 

 aperture, the shriller are the sounds emitted. 



Though the vocal cords are the main agents in the production of 

 sounds, the tone and qualities of these sounds are modified by shape 

 of the cavities of the larynx, pharynx, and even of the air- cavities of 

 the skull, and by the physical qualities of all these parts. To effect the 

 needful changes, appropriate motor agents are needed, and there 

 are no less than eight pairs of laryngeal muscles. The erieo-thyroid 

 muscles extend from the upper border of the cricoid to the outside 

 of the thyroid and its lower cornua. Their action is to rotate the 

 thyroid downwards and to stretch the vocal cords. Two pairs of 



