ROARlNGf. 381 



iiiuscles which open the laiynx, and which are called the crica-aryte- 

 noidei postici. On account of this muscular inability, the larynx 

 cannot be opened to its full extent for the entrance of air into the 

 lungs during inspiration, 'and consequently the noise is heard, and 

 the distress in breathing is experienced, during that period. Dupuy 

 tells us that the opening for the admission of air into the larynx 

 is reduced to about two-fifths of its norm^al width in bad cases of 

 roaring, in which both sides are affected. As a rule, only the 

 muscle of the left side becomes paralysed. 



In all oases, this paralysis is caused by interference with the ner- 

 vous supply of the laryngeal muscles, which, failing to be stimu- 

 lated by their nerves, gradually wither away from disuse. Experi- 

 ments by Gallien, Legallois, Dapuy, and others show that division of 

 the nerve in healthy horses immediately causes pai-alysis of the laryn- 

 geal muscles of that side, and consequently makers the animal a 

 roarer. In roarers, the interference in question is generally caused 

 by pressui'e on the affected nei^e by tumours, swollen glands, etc., 

 which are usually due to diseases of the organs of breathing. The 

 greater liability of the left nerve (the left recm-rent branch of the 

 pneumogastric) to become affected than its fellow, seems to be due 

 to the fact that while passing, in its course, from the vicinity of 

 the heart to the larynx, it closely approaches the bronchial and 

 tracheal lymphatic glands, which, in the event of their becoming 

 swollen, would exert injurious pressure on it. We may judge from 

 our own experience, how e'asily the glands of the throat become 

 swollen from oold, etc. The right nerve does not go near the glands 

 of its side of the neck. Gunther, Franck, Martin, Sussdorf, and 

 other veterinary authorities maintain that this atrophy is caused 

 by the pressure of the aorta (the large artery -of general circulation 

 which issues from the left ventricle of the heart) on the left recur- 

 rent nerve during great exertion, and consequent violent action of 

 the heart, on account of its proximity to- the aorta, round the arch 

 of which it winds : the right nerve is not placed in this manner. 

 In support of this theory, the fact is put fonvard that a '' roaring " 

 respiration is a frequent symptom, in men, of aneurism of the arch 

 of the aorta, round which the left recurrent laryngeal nerve winds, 

 as in the horse. Consequently, when the artery becomes distended 

 by the aneurism, it presses on the nerve. Against this argument, 

 we have the unanswerable fact that roaring is dependent on climate 

 and not on the action of the heart or aortic pressure. For instance, 

 horses bred in India, South Africa, and other warm climates, even 

 from "noisy" parents, are worked, trained, and raced as hard as 

 anywhere else, and yet roaring is practically unknown among them. 

 Size cannot account for this immunity; for many of them are big 

 horses. From this we may also infer that aneurism of the aorta 



