THE ANTERIOR PART OF THE NECK 229 



of the omo-hyoid muscles. In this median intermuscular 

 interval the following structures will be found : (i) the median 

 part of the thyreo-hyoid membrane; (2) the anterior border 

 of the thyreoid cartilage with the projecting prominentia 

 laryngea (O.T. pomum adami) at its upper end ; (3) the 

 arch of the cricoid cartilage ; (4) the crico-thyreoid ligament 

 with the anastomosis between the crico-thyreoid arteries, and 

 the anterior ends of the crico-thyreoid muscles ; (5) the first 

 ring of the trachea with the anastomosis between the medial 

 terminal branches of the superior thyreoid arteries ; (6) the 

 isthmus of the thyreoid gland; (7) the inferior thyreoid veins, 

 and (8) the lower cervical rings of the trachea. Occasionally 

 the third or middle lobe of the thyreoid gland and the 

 levator glandulae thyreoidea,'or one or other of them, is found 

 extending upwards from the isthmus of the thyreoid gland. 

 When it is present the middle lobe either terminates above 

 in a pointed extremity or becomes continuous with a fibrous 

 cord, the remains of the thyreo -glossal duct, which disappears 

 posterior to the hyoid bone. The levator extends from the. 

 isthmus or from the third lobe, and is attached above to the 

 lower border of the hyoid bone. 



Dissection. To display these structures fully the fascia which covers and 

 binds together the infra-.hyoid muscles of opposite sides must be removed. 

 When this has been done the anterior part of the thyreoid cartilage will be 

 exposed, and, above it, in the thyreo-hyoid interval, lies some loose areolar 

 tissue. If an incision, directed upwards and posteriorly, is made into this 

 tissue, behind the lower border of the hyoid bone, the infra-hyoid bursa 

 will be opened, and when this and the areolar tissue are removed, the 

 thyreo-hyoid membrane will be exposed. Note that the membrane extends 

 from the upper border of the thyreoid cartilage posterior to the body of the 

 hyoid bone to its upper border. When the areolar tissue in the lower part 

 of the intermuscular interval is removed the inferior thyreoid veins will be 

 displayed ; they disappear, below, behind the sternum where they join the 

 innominate veins. Occasionally also a small unpaired artery, the thyreoidea 

 irna, will be found ascending, in the median plane, to the isthmus of the 

 thyreoid gland. Behind the inferior thyreoid veins lies another layer of 

 deep cervical fascia, the pretracheal layer. It covers the front of the 

 trachea, envelops the isthmus of the thyreoid gland, and is attached above 

 to the lower border of the thyreoid cartilage. The part which extends 

 from the isthmus of the thyreoid gland to the thyreoid cartilage acts as a 

 suspensory ligament of the isthmus. The dissectors will find that, so long 

 as the attachments of this part of the pretracheal fascia are not interfered 

 with, they cannot displace the isthmus of the thyreoid gland downwards. 

 On the other hand, if they cut through the attachment of the fascia to the 

 thyreoid cartilage, introduce the handle of the knife through the incision, 

 and press downwards, they will be able to displace the isthmus of the 

 thyreoid gland to a lower level and expose the upper rings of the trachea. 

 At the root of the neck the pretracheal fascia descends into the thorax, 

 n156 



