KESPIRATION 325 



is raised on each side into a well-marked fold above each 

 true vocal cord the false vocal cord. Between this and the 

 true cord on each side is a cavity the ventricle of the larynx. 

 The other folds of mucous membrane, although of importance 

 in medicine, have no special physiological significance. 



The interior of the larynx may be examined during life by 

 the laryngoscope. (Practical Physiology.) 



5. Nerves. The muscles of the larynx are supplied chiefly 

 by the recurrent laryngeal branch of the vagus which comes 

 off in the thorax and arches upwards to the larynx. On the 

 left side, where it curves round the aorta, it is apt to be 

 pressed upon in aneurisrnal swellings. Paralysis of this nerve 

 causes the vocal cord on that side to assume the cadaveric 

 position, midway between adduction and abduction, and makes 

 the voice hoarse or abolishes it altogether. The superior 

 laryngeal is the great ingoing nerve, but it also supplies 

 motor fibres to the crico-thyroid muscle. Paralysis prevents 

 the stretching of the vocal cords, makes the voice hoarse, and 

 renders it impossible to produce a high note. 



Centre. These nerves are presided over by (a) a centre in 

 the medulla. When this is stimulated abduction of the vocal 

 cords is brought about. (&) This centre is controlled by a 

 cortical centre situated in the inferior frontal convolution 

 in man and apes. Stimulation of this causes adduction of 

 the cords as in phonation, while destruction leads to no 

 marked change. 



B. Physiology. When a blast of air is forced between 

 the vocal cords they are set in vibration both wholly and in 

 segments like other vibrating reeds, and sounds are thus 

 produced. These sounds may be varied in loudness, pitch, 

 and quality. 



The loudness, or amplitude of vibration, depends upon the 

 size of the larynx and upon the force of the blast of air 

 acting on the cords. 



The pitch, or number of vibrations per second, depends upon 

 the length and tension of the vocal cords. The tension of the 

 cords may be varied by the action of the crico-thyroid muscles. 



The power of varying the pitch of the voice differs greatly in 

 different animals. 



