328 HEAD AND NECK 



Distend the ventricle, and, if possible, the appendix, with cotton 

 wadding. This will greatly facilitate the subsequent dissection. 



The Lowest Subdivision of the Laryngeal Cavity (Fig. 1 18) 

 leads directly downwards into the trachea. Above, it is narrow 

 and compressed from side to side, but it gradually widens out 

 until in its lowest part it is circular. It is bounded by the 

 mucous membrane which covers the sloping medial surface 

 of the conus elasticus, and the inner aspect of the cricoid 

 cartilage. It is through the anterior wall of the lowest 

 compartment of the larynx that the opening is made in the 

 operation of laryngotomy. 



Mucous Membrane of the Larynx. The mucous 

 membrane of the larynx is continuous, above, with that lining 

 the pharynx, and below, with the mucous lining of the 

 trachea. Over the laryngeal or posterior surface of the 

 epiglottis it is closely adherent, but elsewhere, above the level 

 of the vocal folds, it is attached loosely by submucous 

 tissue to the adjacent structures. As it passes over the 

 vocal folds it is very thin and tightly bound down, and in 

 inflammatory conditions of the larynx, attended with oedema, 

 that attachment usually prevents the infiltration of the sub- 

 mucous tissue from extending downwards below the rima 

 glottidis. 



The mucous membrane of the larynx has a plentiful supply 

 of racemose glands which secrete mucus, but over the surface 

 of the vocal folds they are completely absent. 



Dissection. Place the larynx upon a block so that its anterior 

 surface looks upwards, and fix it in that position with pins. 

 The branches which the external laryngeal nerve gives to the 

 crico-thyreoid muscle should in the first place be followed out ; 

 and, carefully preserving the superior and inferior laryngeal 

 vessels and the internal and inferior laryngeal nerves, the 

 dissector should in the next place proceed to remove the thyreoid 

 gland, and the omo-hyoid, sterno-hyoid, sterno-thyreoid, and 

 thyreo-hyoid muscles. The fibres of origin of the inferior 

 constrictor muscle also should be taken away from the thyreoid 

 and cricoid cartilages. The thyreo-hyoid membrane, the crico- 

 thyreoid ligament, and the crico-thyreoid muscles are now 

 exposed, and their attachments may be defined. 



Membrana Hyo-thyreoidea. The thyreo-hyoid membrane 

 is a broad membranous sheet, which occupies the interval 

 between the hyoid bone and the thyreoid cartilage. It is not 

 equally strong throughout, but shows a central thick portion, 

 the median thyreo-hyoid ligament^ largely composed of elastic 



