114 Roaring hi Horses. 



depressing and pushing inwards the vocal cord and aryte- 

 noid cartilage ; at the same time that this encroachment 

 narrows, to a serious extent, the " portal of the breath of 

 life," the upper division, or aditus, is correspondingly 

 diminished in size by the passing forward of this cartilage 

 into its cavity. So that after a certain amount of weaken- 

 ing of the dilator muscle, and resulting protrusion of cord 

 and cartilage into the breathing space, the condition be- 

 comes yet more aggravated by the pressure exercised on 

 them by the air which should pass into the lungs, but 

 which drives against their upper surface instead. The more 

 frequently a horse affected with Roaring undergoes severe 

 exertion, so the more rapidly will the morbid displacements 

 of these parts become serious and permanent, and especially 

 if the animal is young — for then their softness allows 

 them to yield all the more readily to the pressure of the air. 

 In this way can the striking distortion observed in the 

 larynges of horses which have been for some time bad 

 Roarers be accounted for ; the atrophy of the dilator muscle 

 would not alone explain it. 



It will now be understood why limiting the admission of 

 air at the nostrils during active exertion benefits, rather 

 than inconveniences, a horse which is a Roarer. 



When the vocal cord does not advance to the middle of 

 the larynx, but is only rather relaxed, it is thrown into 

 coarse vibrations by the forced inspiration, and the horse 

 then " roars " ; when still less advanced, and the cord is 

 more tense, he whistles—the constrictor muscles of the 

 same side probably acting, though feebly, in shortening the 

 cord. But when cord and cartilage pass to or beyond the 

 median line, then on slight exertion the inspiration is nois)', 

 and when the exertion is increased, the sound becomes 

 painfully loud and shrill ; at the same time, the right dilator 

 muscle has to act in a continuous and spasmodic manner 

 to allow as much air as possible to reach the luno-s, and 

 also to pass from them. 



