730 VOICE AND SPEECH. [CH. LIV. 



parallel to these are called the false rocal cords ; between the true and 

 false vocal cord on each side is a recess called the ventricle. 



Muscles. The muscles of the larynx are divided into intrinsic and 

 extrinsic. The intrinsic are named from their attachments to the various 

 cartilages ; the extrinsic are those which connect the larynx to other parts 

 like the hyoid bone. 



The intrinsic muscles of the larynx are as follows : 



1. Crico-thyroid. 



2. Posterior crico-arytenoid. 



3. Lateral crico-arytenoid. 



4. Thyro-arytenoid. 



5. Arytenoid. 



All these muscles except the arytenoid are in pairs. 

 Their attachments and actions are as follows : 



1. Crico-thyroid. This is a short, thick triangular muscle, attached below 

 to the cricoid cartilage ; this attachment extends from the middle line 

 backwards. The fibres pass upwards and outwards, diverging slightly to be 

 attached above to the inferior border of the thyroid cartilage, and to the 

 anterior border of its lower cornu. In the latter portion of the muscle, the 

 fibres are nearly horizontal. Some of the superficial fibres are continuous 

 with those of the inferior constrictor of the pharynx. 



The thyroid cartilage being fixed by extrinsic muscles, the contraction 

 of this muscle draws upwards the anterior part of the cricoid cartilage, and 

 depresses the posterior part, and with it the arytenoid cartilages, so that the 

 vocal cords are stretched. Paralysis of these muscles therefore causes an 

 inability to produce high-pitched tones. 



2. Posterior crico-arytenoid. This arises from the broad depression on 

 the corresponding half of the posterior surface of the cricoid cartilage : 

 its fibres converge upwards and outwards and are inserted into the outer 

 angle of the base of the arytenoid cartilage behind the attachment of the 

 lateral crico-arytenoid muscle. Near their insertion the upper fibres are 

 blended with the lower fibres of the arytenoid muscle. 



These muscles draw the outer angles of the arytenoid cartilages backwards 

 and inwards, and thus rotate the anterior or vocal processes outwards, and 

 widen the rima glottidis. They come into action during deep inspiration. 

 If they are paralysed, the lips of the glottis approach the middle line and 

 come in contact during each inspiration, so that dyspnoea is produced. 



3. Lateral crico-arytenoid. This arises from the sloping upper border of 

 the cricoid cartilage, and is inserted into the muscular process of the aryte- 

 noid cartilage and the adjacent part of its anterior surface. Its upper part 

 is more or less blended with the thyro-arytenoid, and a few of its fibres are 

 continuous round the outer side of the arytenoid cartilage with the arytenoid 

 muscle. 



These muscles draw the muscular processes of the arytenoid cartilages 

 forwards and downwards, and thus approximate the vocal cords. They are 

 antagonistic to the posterior crico-arytenoids. 



4. Thyro-arytenoid. This consists of two portions, inner and outer. The 

 inner portion arises in the lower half of the angle formed by the alae of the 

 thyroid cartilage, and passing backwards are attached behind to the vocal 

 process and to the adjacent parts of the outer surface of the arytenoid cartilage. 

 They are joined internally by short fibres which are attached in front to the 

 vocal cord, and behind to the vocal process. Some oblique fibres pass from 

 the sloping portion of the crico-thyroid membrane below the vocal cord, 

 upwards, outwards, and somewhat backwards, to end in the tissue of the 

 false vocal cord. The fibres of the outer portion arise in front from the 

 thyroid cartilage close to the origin of the inner portion and from the crico- 

 thyroid membrane. They pass backwards to be inserted in part into the 

 lateral border and muscular process of the arytenoid cartilage, and in part 

 they pass obliquely upwards towards the aryteno-epiglottidean fold, ending 



