DISEASES OF THE RESPIRATORY ORGANS. 135 



elapse before it recurs after being ouce heard. As the 

 muscles continue to waste, there is a corresponding and 

 permanent loss of power, and what at first was intermittent 

 is now established. 



Most commonly, however, the noise is not intermittent. 

 It is usually slight at first, and then gradually increases 

 with the waste, and consequent inability of the muscles to 

 open the laryngeal orifice. 



Causes of Roaring. — Heredity plays a very prominent 

 part in the transmission of roaring. Professor Williams 

 mentions a breed of horses in which nearly all the animals 

 of both sexes are roarers. Horses and geldings are, how- 

 ever, more likely to become roarers than mares, which are 

 rarely afilicted unless the hereditary predisposition is very 

 marked. 



Small ponies are rarely if ever afi"ected. An animal 

 predisposed to roaring in most cases suff'ers from laryn- 

 geal alFections from very slight causes, and the infirmity 

 usually appears after several attacks of sore throat and 

 cold. 



There are other causes of roaring besides those resulting 

 from the condition of the laryngeal muscles above men- 

 tioned. 



These are not so frequently met with, but are of great 

 importance. 



The characteristic sound may be due to disease of the 

 nose : nasal polypi ; osseous tumours in the nostrils, or 

 hypertrophy of the turbinated bones; constriction of the 

 laryngeal orifice -, tumours on the posterior nares, falling 

 into the glottis and causing intermittent roaring ; disten- 

 sion of the guttural pouches; tumours in the thorax; 

 disease of the lymphatic glands of the pharynx ; injury or 

 disease of the trachea, or, finally, any distortion of the 

 larynx. 



