DISEASES OF THE THROAT. 117 



CROUP AND DIPHTHERIA. 



Neither of these diseases affects the horse. But these names are 

 sometimes wrongly applied to severe laryngitis or pharyngitis, or to 

 forage poisoning, in which the throat is paralyzed and becomes exces- 

 sively inflamed and gangrenous. 



THICK WIND AND ROARING. 



Horses that are affected with a chronic disease that causes a loud 

 unnatural noise in breathing are said to have thick wind," or to be 

 roarers. This class does not include those affected with severe sore 

 throat, as in these cases the breathing is noisy only during the attack 

 of the acute disease. 



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

 air in some part of the respiratory tract. Nasal polypi, thickening of 

 the membrane, pharyngeal polypi, 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 only temporary. Very often a nervous, excitable 

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

 caused by the cramped position in which the head and neck are forced 

 in order to hold him back. 



Many other causes may occasion temporary, intermitting, or per- 

 manent noisy respiration, but chronic roaring is caused by paralysis 

 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 normally controlled by the 

 affected muscles lean into the tube of the larynx, so that when the air 

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

 the air is expelled from the lungs 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 are 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 paralyzed is owing to the difference in 



