Causes of Chronic Roaring. Ill 



required amount of air. But in proportion as the need for 

 an increased supply of air becomes urgent, so the move- 

 ment of these parts is extended to afford more glottal space, 

 until, in forced and deep respiration, the glottis is dilated 

 to the utmost, by each aryta3noid cartilage being drawn by 

 its dilator muscle upwards and outwards, and the vocal 

 cord pulled close to the side of the thyroid cartilage; so that 

 the middle division formed by these in the organ has 

 almost entirely disappeared, and the lumen of the larynx 

 is nearly of the same dimensions throughout (Fig. 6). In- 

 spiration being accompUshed, the cartilages fall downwards 

 and inwards, chiefly by the dilator muscles ceasing their 

 contraction (though they are never entirely relaxed during 

 Hfe), and the natural elasticity of the cartilages and vocal 

 cords coming into play ; then they are in the passive posi- 

 tion assumed in tranquil respiration, in which the glottal 

 opening is wider than when they are in what is termed their 

 " cadaveric position." Expiration now taking place, the air 

 expelled from the lungs easily pushes cartilages and cords 

 aside, in its course through the larynx towards the nasal 

 passages. 



It will now be perceived what must happen when the 

 dilator muscles are weakened or paralysed, and the respira- 

 tion is increased. The vocal cords and aryta^noid cartilages 

 not being moved sufficiently out of the way of the inrushing 

 column of air, they form such an obstacle in the laryngeal 

 cavity as to allow only a portion to enter, and this is drawn 

 in with such force as to produce a noise, all the more shrill 

 the narrower the glottal space has become. It may be 

 observed that in man a similar condition is remarked ; 

 paralysis of the abductor muscles of the larynx is always 

 manifested by inspiratory stridor. 



When the dilator muscle of only one side is affected — the 

 left, for example — then the stridulous breathing is not so 

 marked or distressing on exertion ; as the opening of the 

 glottis is wider, not only because the other dilator muscle is 



