Dyspnoea Laryngea. Roaring. Hemiplegia Laryngea. 151 



Even if admitted it utterly fails to explain the immunity of the 

 muscles supplied by the superior laryngeal nerve. 



The fact that a horse has usually a hard and soft side of the 

 mouth and carries the head slightly to the latter is no better ex- 

 planation, as the tender side is not always the left. 



More commonly the disease arises at some other point near the 

 origin or in the course of the nerve, and the changes in the larynx 

 follow as the consequence of deficiency or entire absence of 

 motor innervation. Many cases can be cited, in which such an 

 origin was unquestionable, and on the hypothesis that this is the 

 true and constant history of the development of the malady, its 

 regular occurrence in the left side, and the absence of all signs of 

 wasting in the muscles supplied by the superior laryngeal nerve 

 are alike perfectly explainable. 



lyCt it be noted that the vagus nerves (right and left) of which 

 the recurrent laryngeal are branches originate from the base of 

 the brain, pass down the neck on the deep aspect of the jugular 

 vein in company with the carotid artery ; that on entering the 

 chest the right vagus nerve gives off its recurrent branch which 

 proceeds at once up the neck along the course of its parent trunk 

 till it reaches the larynx, to the muscles on the right side of which 

 it is distributed ; that the left vagus nerve on the other hand pro- 

 ceeds backward in company with the left innominate artery as far 

 as the base of the heart, where on about the level of the space be- 

 tween the sixth and seventh ribs it gives off the left recurrent 

 nerve ; that this left recurrent nerve closely applied at its point of 

 origin to the great parent arteries turns round the posterior aorta 

 enclosing it in a loop, and gaining the lower end of the windpipe 

 follows its course to the larynx. It will 'thus be understood how 

 many chest diseases may implicate the left recurrent nerve, and 

 from which the right, which extends no deeper than between the 

 two first ribs may be completely exempt. The frequent superven- 

 tion of roaring as a sequel of chest diseases receives in this an ample 

 explanation. Its connection with pleurisy becomes especially 

 probable, as the nerve lies in contact with the surface of the 

 pleura alike in its descending and ascending course within the 

 chest. 



Finally the loop encircling the posterior aorta exposes the nerve 

 to constant stretching and shocks from the heart's action during 



