•S74 A Manual of Veterinary Physiology. 



The cartilaginous box, the larynx, undergoes at its 

 glottal opening certain changes in shape during inspiration 

 and expiration. During ordinary inspiration it slightly 

 opens, and the vocal cords, which were towards the centre 

 of the tube, retire towards the side. During expiration the 

 glottal opening becomes smaller and the cords approxi- 

 mate. 



Movements of the Larynx. — These movements are carried 

 out by certain muscles in connection with the cartilages of 

 the larynx and vocal cords, and the muscles may be divided 

 into those which dilate the cavit}- and those which close it. 

 The sole dilator is the crico-arytenoideus posticus, a muscle 

 of considerable power, and strengthened by tendinous fibres 

 in its substance. Its function is to lift the arytenoid carti- 

 lage upwards and outwards, more especially during forced 

 respiration, when the glottis is dilated to its utmost. It is 

 the muscle of the larynx, and paralysis of it causes laryngeal 

 ' roaring,' owing to the immobile arytenoid cartilage being 

 drawn into the glottis, instead of being lifted up out of the 

 way of the incoming air. The paralysed vocal cord takes 

 no part in the production of the sound, which, it is to be 

 noticed, is almost invariably an inspiratory one. 



The larynx having been dilated, it is then closed partly 

 by its elastic recoil, but also through the medium of the 

 muscles antagonistic to the dilator, viz., the constrictors, or, 

 more correctly, the adductors, of which there are four on 

 either side, viz., the arytenoideus, thyro - arytenoideus 

 anticus and posticus, and crico-arytenoideus lateralis. By 

 the contraction of these muscles the glottis is closed, the 

 arytenoid cartilages depressed, their glottal surfaces closely 

 applied, and they also bring the vocal cords together, or 

 nearly so, forming a simple slit between them. 



The vocal cords, lying in close apposition to the thyro- 

 arytenoideus posticus muscle, are relaxed by a contraction 

 of this muscle, which approximates the arytenoid and 

 thyroid cartilages ; whilst the crico-thyroid places the cords 

 on the stretch, or renders them tense by the manner in 

 which it rotates the cricoid on the thyroid cartilage. The 



