Anatomy and Physiology of the Larynx. 97 



Exner, experimenting on rabbits, concludes that, while the 

 external thyro-aryta^noid muscle is supplied chiefly, if not 

 exclusively, by the inferior or recurrent laryngeal nerve, 

 the internal thyro-aryta^noid derives a large share of its 

 innervation from the superior laryngeal nerve. He also 

 demonstrated the existence of a third or median laryngeal 

 nerve in the rabbit and dog, which is believed to be of great 

 motor importance. He and Mandelstamm, in rabbits, and 

 Weinzweig in man, have found that sometimes the re- 

 current nerve of one side sends fibres to that of the oppo- 

 site side. But we agree with Moeller, that further investi- 

 gations are needed to ascertain whether these variations 

 occur in the horse, and if they can be taken into account in 

 explaining the graduated differences in the respiratory 

 sounds which constitute Koaring. It appears certain, at 

 any rate, that the differences observed by Exner with 

 respect to the innervation of the dilator muscles in rabbits 

 — section of the recurrent nerves at one time leading to 

 degeneration of these muscles, at another time having no 

 such result — are not observed in horses ; for section of 

 these, nerves in them always brings about more or less pro- 

 nounced atrophy of the muscles, according to the duration 

 of their paralysis. 



The movements in the horse's larynx during respiration 

 can be readily noted when that organ is opened, as in the 

 operation for Koaring — the opening being made through 

 the middle crico-thyroid ligament, and also through the 

 cricoid cartilage and first tracheal ring if necessary. It will 

 then be seen that in gentle breathing the action of the 

 laryngeal muscles is slight, the aryttenoid cartilages and 

 vocal cords moving very little, the natural tension of the 

 dilator muscles ensuring sufficient width of the glottis. In 

 deep, forcible respiration, however, these muscles are most 

 active in effecting dilatation of the upper and middle 

 portions of the larynx, by their traction on the arytienoid 

 cartilages, as already described. As has been pointed out, 



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