TRACHEA AND (ESOPHAGUS 321 



and fourth rings of the trachea, and, therefore, nearer the 

 lower than the upper ends of the two lateral lobes. 



An -"additional lobe, the pyramidal or middle lobe, is 

 frequently present. It is an elongated slender process which 

 springs from the isthmus, on one or other side of the median 

 plane (more usually on the left side), and extends upwards 

 towards the hyoid bone. To this it may be connected by 

 fibrous tissue, or by a narrow slip of muscular fibres called 

 the levator glandulce thyreoidece. In some cases this little 

 muscle has an attachment to the thyreoid gland independ- 

 ently of the pyramidal process. The thyreoid gland is firmly 

 connected to the parts upon which it lies, and therefore 

 follows the larynx in all its movements. 



The dissector should note the great vascularity of the thyreoid gland. 

 Four large arteries, and occasionally a fifth smaller vessel, convey blood to 

 its substance. The two superior thyreoid branches of the external carotid 

 arteries divide at the apex of each lateral lobe into three branches for its 

 supply ; the two inferior thyreoid branches, from the thyreo-cervical trunks 

 of the subclavian arteries, distribute their terminal branches to the basal 

 portion and deep surface of each lateral lobe. The occasional artery is 

 the thyreoidea itna, a branch of the innominate or, more rarely, of the 

 common carotid or the aortic arch. It ascends upon the anterior aspect 

 of the trachea to reach the isthmus of the thyreoid gland. These thyreoid 

 arteries anastomose with each other. 



The veins which drain the blood away from the thyreoid gland are still 

 more numerous. They arise in part by tributaries which spring from a 

 venous network on the anterior face of the structure, but chiefly by branches 

 which emerge from its substance. They are three in number on each side 

 viz. the superior thyreoid, the middle thyreoid, and the inferior thyreoid. 

 The superior and middle thyreoid 'veins cross the common carotid artery and 

 join the internal jugular ; the inferior thyreoid vein descends on the trachea. 

 At the root of the neck it usually joins its fellow of the opposite side to 

 form a common stem which opens into the left innominate. 



Trachea and (Esophagus. The windpipe and the gullet 

 in the cervical portion of their course may now be studied. 

 Both begin at the level of the cricoid cartilage, anterior to 

 the sixth cervical vertebra. From this point they extend 

 downwards, anterior to the vertebral column, to the thoracic 

 cavity. 



The trachea, or windpipe, is a wide tube which is kept 

 constantly patent by the cartilaginous rings embedded in its 

 walls. These rings do not form complete circles ; posteriorly 

 they are deficient, and, in consequence, the posterior surface 

 of the trachea is flattened. Above, it is continuous with the 

 larynx, and, throughout its course in the neck, it is placed in 

 the median plane of the body. The anterior relations of the 



VOL. II 21 



