DISEASES OF THE HOESE. 117 



Thick wind is caused by an obstruction to the free jDassage of the 

 air in some part of the respiratory tract. Nasal polj^pi, thickening of 

 the membrane, pharyngeal pol3^pi, deformed bones, paralysis of the 

 wing of the nostril, etc., are occasional causes. The noisy breathing 

 of horses after having been idle and put to sudden exertion is not due 

 to any disease and is onh^ temporary. Ver}^ often a nervous, excitable 

 horse will make a noise for a short time when started off, generally 

 caused b}^ the cramped position in which the head and lieck are forced 

 in order to hold him back. 



Many other causes may occasion temporary", intermitting, or perma- 

 nent noisy respiration, but chronic roaring is caused by paralj'^sis of 

 the muscles of the larynx; and almost invariably it is the muscles of 

 the left side of the larynx that are affected. 



In chronic roaring the noise is made when the air is drawn into the 

 lungs; and only when the disease is far advanced is a sound produced 

 when the air is expelled, and even then it is not near so loud as during 

 inspiration. 



In a normal condition the muscles dilate the aperture of the larynx 

 by moving outward the cartilage and vocal cord, allowing a sufficient 

 volume of air to rush through. But when the muscles are paralyzed, 

 the cartilage and vocal cord that are normal 1}^ controlled by the affected 

 muscles lean into the tube of the larnyx, so that when the air rushes 

 in it meets this obstruction and the noise is produced. When the air 

 is expelled from the Itings its very force pushes the cartilage and vocal 

 cords out, and consequently noise is not produced in the expiratory act. 



The paralysis of the muscles is due to derangement of the nerve 

 that supplies them with energy. The muscles of both sides a.re not 

 supplied by the same nerve; there is a right and a left nerve, each 

 supplying its respective side. The reason why the muscles on the 

 left side are the ones usually parah^zed is owing to the difference in 

 the anatomical arrangement of the nerves. The left nerve is much 

 longer and more exposed to interference than the right nerve. 



In chronic roaring there is no evidence of anj^ disease of the larynx 

 other than the wasted condition of the muscles in question. The 

 disease of the nerve is generally located far from the larynx. Disease 

 of parts contiguous to the nerve along any part of its course may 

 interfere with its proper function. Enlargement of lymphatic glands 

 within the chest through which the nerve passes on its way back to 

 the larynx is the most frequent interruption of nervous supply, and 

 cojisequently roaring. When roaring becomes confirmed medical 

 treatment is entirely useless, as it is impossible to restore the wasted 

 muscle and at the same time remove the cause of the interruption of 

 the nervous supply. Before roaring becomes permanent the condi- 

 tion may be benefited by a course of iodide of potassium, if caused by 

 disease of the lymphatic glands. Electricit}^ has been used with indif- 



