208 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



come tense, and the edges are observed to vibrate. (4) 

 If the vocal sound is high-pitched, the chink becomes 

 very narrow, the cords very tense, and the vibration 

 caused by air driven .through very rapid (Fig. 135, B). 

 (5) In the highest soprano head notes the cords are still 



Fig. 135.— Glottis as seen with the laryngoscope : A, in simple breathing ; 

 B and C, in singing ; D, in straining. 



tenser, more pressed together, so that the air is driven 

 through only a small opening in the middle, and the 

 vibration is, of course, still more rapid (Fig. 135, C). 

 (6) Finally, in violent straining or strong muscular 

 effort the glottis closes absolutely air-tight (Fig. 135, D). 

 We first fill the lungs, then close the glottis, so as to fix 



