VOICE AND SPEECH 581 



muscles rotate the outer angle of the arytenoid cartilages backwards 

 and inwards. They thus cause a movement outwards of the anterior 

 angles, so that the glottis is widened. During every act of expiration 

 there is a widening of the glottis, which is probably effected "by con- 

 traction of these muscles. If they are paralysed, the vocal cords are 

 approximated and tend to come together during inspiration, so that 

 dyspnoea may be produced. 



(3) The lateral crico-arytenoid muscle arises from the upper border 

 of the cricoid cartilage and passes backwards to be inserted into the 

 muscular process of the arytenoid cartilage. These muscles when 

 they contract pull the muscular process of the arytenoid cartilage for- 

 wards and downwards, thus approximating the vocal cords at their 

 posterior ends and antagonising the action of the posterior crico- 

 arytenoid muscles. 



(4) The arytenoid muscles consist of transverse fibres, some of 

 which decussate, uniting the posterior surface of the two arytenoid 

 cartilages. When they contract they draw the arytenoid cartilages 

 together. 



(5) The thyro-arytenoid muscles consist of two portions. The 

 outer fibres rise in front from the thyroid cartilage and pass back- 

 wards to be inserted into the lateral border and the muscular process 

 of the arytenoid cartilage. Some of the fibres pass obliquely up- 

 wards towards the aryteno-epiglottidean folds. These are often 

 spoken of as a separate muscle, the thyro-epiglottidean. By their 

 action they tend to draw the arytenoid cartilages forwards and to 

 relax the vocal cords. The upper fibres may also assist in depressing 

 the epiglottis. The inner fibres are called the musculus vocalis. They 

 arise from the lower half of the angle of the thyroid cartilage, and 

 passing backwards in the vocal cords are attached to the vocal pro- 

 cesses and to the adjacent parts of the outer surfaces of the arytenoid 

 cartilages. Many fibres do not run the whole distance, but end in 

 an attachment to some part of the vocal cord. Although their action 

 must be to draw the arytenoid cartilages forwards, yet, since they 

 are contained in the vibrating portion of the vocal cords, they cannot 

 by their contraction relax these cords. It is probable that they 

 play a great part in determining the tension of the vocal cords after 

 these have been put on the stretch by the action of the crico- thyroid 

 muscles. They may possibly act as a sort of fine adjustment of the 

 tension, the coarse adjustment being represented by the crico- thyroids. 



THE PRODUCTION OF VOICE 



In order to study the changes in the larynx which are associated 

 with voice production we must make use of the laryngoscope. The 

 principle of this instrument is very simple. A large concave mirror 



