THE RESPIRATION 



309 



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 aneurismal swellings. Paralysis of this 

 nerve causes the vocal cord on that side to assume the 

 cadaveric position, midway between adduction and abduc- 

 tion, 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. 

 (6) This centre is controlled 

 by a cortical centre situated 

 in the inferior frontal con- 

 volution. Stimulation of 

 this causes adduction of the 

 cords as in phonation, while 

 destruction leads to no 



Cr. 



Ar. 



marked change. 



FIG. 142. Cross Section of the Larynx, 

 to show the Cricoid, Cr. ; Thyroid, 

 Th. ; Arytenoid Cartilages, Ar. The 

 continuous line shows the parts at 

 rest. The dotted line under the action 

 of the Lateral Crico-arytenoid Muscle, 

 and the dot-dash line under the action 

 of the Posterior Crico-arytenoid. 



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 greater 

 length of the vocal cords in the male, as compared with the 



