552 



VETERINARY PHYSIOLOGY 



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 aneurismal swellings. 

 Paralysis of this nerve 

 causes the vocal cord on 

 that side to assume the 

 cadaveric position, midway 

 between adduction and ab- 

 duction, 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 - thyreoid 

 muscle. Paralysis prevents 

 the stretching of the vocal 

 cords, makes the voice 

 hoarse, and renders it im- 

 possible 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 convolution. 

 Stimulation of this causes adduction of the cords as in 

 phonation, while destruction leads to no marked change. 



Fig. 224. — Cross Section of the Larynx, 

 to show the cricoid, Cr. ; thyreoid, 

 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 of the Voice. 



When a blast of air is forced between the vocal cords when 

 they are approximated by the lateral crico-arytenoids, they are 

 set in vibration both wholly and in segments like other vibrating 



