EOAEING. 379 



NATURE AND CAUSES OF THE PARALYTIC FORM OF 

 ROARING. — This disease is due to paralysis of one or both of 

 the muscles which open the larynx, and are called the crico-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 

 limgs during inspiration, and consequently the noise is heard, and 

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

 opening for the admission of air into the larynx is reduced to 

 about two-fifths of its normal width in bad cases of roaring, in 

 which both sides are affected. As a rule, only the muscle of tlie 

 left side becomes paralysed. 



In all cases, this paralysis is caused by interference with the 

 nervous supply of the laryngeal muscles, which, failing to be 

 stimulated by their nerves, gradually wither away from disuse. 

 Division of the nerve in healthy horses immediately causes 

 paralysis of the laryngeal muscles of that side, and consequently 

 makes the animal a roarer. In roarers, the interference in question 

 is generally caused by pressure on the affected nerve by timiours, 

 swollen glands, etc., which aje usually due to diseases of the organs 

 of breathing. The greater liability of the left nerve (the left 

 recurrent 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 easily the 

 glands of the throat become swollen from cold, etc. The right 

 nerve does not go near the glands of its side of the neck. Many 

 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 recurrent 

 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 forward 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, there is 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 ; 



