112 Roaring in Horses. 



healthy, but also because it is more energetically called into 

 action to compensate for the defectiveness of its fellow. 



In commencing Roaring, or in those cases in which it 

 remains at a less advanced stage, and in which the muscle 

 is only slightly affected, the sound is probably produced by 

 the vocal cord alone ; the thin border of this, projecting into 

 the cavity of the larynx, meets the inspired air and pro- 

 duces the characteristic sound. But in more advanced 

 cases, when the muscle is feebler, or altogether paralysed, 

 then, during exertion, there can be no doubt that the ary- 

 tsenoid cartilage shares with the vocal cord in obstructing 

 the admission of air, and renders the noise louder and of a 

 different tone. When the muscle is completely wasted, both 

 vocal cord and cartilage assume and maintain the " cada- 

 veric position" when the horse is at rest ; but during severe 

 exertion they even pass beyond this, and still further 

 constrict the air- passage, diminishing it by more than one- 

 half. 



This movement of these parts beyond the centre of the 

 cavity, is due to the pressure the air exerts upon them, and 

 also to the part played by the sac or ventricle on their 

 outer side. During inspiration, when the larynx is in a 

 healthy condition, this somewhat extensive sac is obli- 

 terated and its slit-Uke entrance shut ; but when the glottis 

 is closed, as during neighing, then it is inflated with 

 air, to allow the vocal cord more liberty of movement for 

 vibration, and to act, with the upper laryngeal cavity, as a 

 resonator. In Roaring, from its flaccid condition, its elas- 

 ticity, and its wide opening, it catches a portion of the air 

 which should pass into the trachea, and this inflates it to 

 a considerable degree, causing the vocal cord to be pushed 

 still further towards the opposite side (Fig. 7). At the 

 same time, this movement of the cord downwards and 

 inwards also compels the arytienoid cartilage to which it is 

 attached to be drawn in the same direction, this action 

 being supplemented by the air also pressing down the upper 



