Dyspnoea Laryngea. Roaring. Hemiplegia Laryngea. 137 



plains moreover why roaring should not be heard in quiet breath- 

 ing and why it should increase in force and in pitch as the 

 respiration becomes more and more hurried. It further accounts 

 for the noise being heard only during the act of inspiration, the 

 outward rush of the air in the expiratory act being of itself suffi- 

 cient to carry this valvular cartilage out of the passage and secure 

 a free and unimpeded current. 



The paralysis and wasting of the left recurrent nerve and 

 muscles are in their turn due to very varied morbid states. 



It may commence in the larynx from distortion of its cartilages 

 and inflammation of the mucosa, in which case the wasting of the 

 nerve is probably a result of its prolonged inactivity. This mode 

 of origin is strongly insisted on by Percivall, and no doubt occa- 

 sionally arises. Under this explanation, however, it is difficult 

 satisfactorily to account for its almost invariable occurrence on the 

 left side. The mere fact that the horse is habitually approached 

 on this side and more commonly turned toward it is a most in- 

 sufficient reason. 



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. 



Let 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 beneath the jugular vein in com- 

 pany 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 its reaches 

 the larynx, to the muscles on the right side of which it is dis- 



