108 DISEASES OF THE HORSE. 
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 ex- 
cessively inflamed and gangrenous. 
THICK WIND AND ROARING. 
Horses that are affected with 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 havins Deka idle and put to sudden exertion is not due 
to any disease and is only temporary. Very often a nervous, ex- 
citable 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 tempor ary, intermittent, 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; only when the disease is far advanced is a sound produced 
when the air is expelled, and even then it is not nearly so loud as ° 
- during inspiration. . 
In a normal condition the muscles dilate the aperture of the larynx 
by moving the cartilage and vocal cord outward, 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 i 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 ide, The reason why the muscles on the 
left side are the ones usually paralyzed i 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, 
