330 HEAD AND NECK 



thyreoid lamina, and also into the anterior border of its 

 inferior cornu. As a general rule, it is divided into two parts. 

 The anterior or straight part is composed of those fibres which 

 are attached to the lamina of the thyreoid cartilage ; the 

 posterior or oblique part is formed of those fibres which are 

 inserted into the inferior cornu of the thyreoid cartilage. It 

 is closely associated with the origin of the inferior constrictor 

 muscle. The crico-thyreoid muscle is supplied by the ex- 

 ternal laryngeal branch oi the superior laryngeal nerve. The 

 crico-thyreoid muscles are the chief tensors of the vocal 

 ligaments. 



Conus Elasticus. Extending upwards, from the upper 

 border of the anterior and lateral parts of the cricoid cartilage 

 to the thyreoid and arytaenoid cartilages, is a strong elastic 

 membrane, the conus elasticus, which is separable into a 

 median and two lateral parts. The median part is the crico- 

 thyreoid ligament, which extends from the upper border of the 

 anterior part of the cricoid arch to the lower border of the 

 thyreoid cartilage. Each lateral part (O.T. lateral part of 

 crico-thyreoid membrane) runs upwards and medially and 

 terminates in a free, thickened border, called the ligamentum 

 vocale, which lies in the substance of the plica vocalis, and is 

 attached, posteriorly, to the vocal process of arytaenoid, and, 

 anteriorly, to the angle of union of the two laminae of the 

 thyreoid cartilage. The inner surface of the conus elasticus 

 is covered with the mucous membrane of the lowest section of 

 the cavity of the larynx, and the outer surfaces of the lateral 

 parts are in relation with the lateral crico-arytaenoid and the 

 thyreo-arytaenoideus muscles (Fig. 118). 



Dissection. The position of the larynx must now be reversed. 

 Fix it upon the block in such a manner that its posterior aspect 

 is directed upwards. The oesophagus should then be slit open 

 by a median incision through its posterior wall. Next, remove 

 the mucous membrane which covers the posterior aspect of the 

 cricoid and arytaenoid cartilages. Whilst doing that, bear in 

 mind that the inferior laryngeal artery and the inferior laryngeal 

 nerve pass upwards, between the thyreoid and cricoid cartilages, 

 and must be preserved. 



Upon the posterior aspect of the broad lamina of the cricoid 

 cartilage the dissector will now note the two posterior crico- 

 arytaenoid muscles, and the attachment of the tendinous band 

 through which the longitudinal fibres of the oesophagus are 

 fixed to the cricoid cartilage. The band takes origin from the 

 prominent median ridge on the posterior aspect of the cricoid 

 cartilage. ' On the posterior surface of the arytaenoid cartilages, 



