4 i2 THE DUCTLESS GLANDS 



upwards above the hyoid bone into the substance of the 

 tongue. Such cysts lie superficial to the larynx and are easily 

 felt in the anterior median line of the neck. They may lie 

 superficial to the hyoid bone, but, as the thyreo-glossal duct 

 frequently passes through the hyoid bone, they may be con- 

 stricted at this point. 



The thyreoid gland receives two arteries on each side. The 

 superior thyreoid, which arises from the external carotid, is 

 chiefly distributed to the upper pole and the upper border of the 

 isthmus, while the inferior thyreoid, which arises from the first 

 part of the subclavian artery, supplies the lower two-thirds of 

 the lateral lobe and the inferior border of the isthmus. The 

 superior and middle thyreoid veins pass laterally to join the 

 internal jugular vein, but the inferior group descend in front 

 of the trachea to terminate in the left innominate vein. The 

 position of the latter group is of great importance in connexion 

 with the operation of low tracheotomy, on account of the 

 danger of the inspiration of blood into the terminal bronchi. 



The lymph vessels of the thyreoid gland, for the most part, 

 terminate in the lower anterior group of the deep cervical 

 lymph glands (p. 352), but some of them descend along the 

 trachea and join the paratracheal and the mediastinal lymph 

 glands. 



The Parathyreoid Glands are four in number. They lie 

 within the fascial sheath of the thyreoid gland in close relation 

 to the posterior aspects of the lateral lobes, and it is believed 

 that they are able to assume and carry on the functions of 

 the thyreoid gland after thyreoidectomy. It has also been 

 suggested that the parathyreoid glands are at fault in tetany 

 and in paralysis agitans. 



The Thymus Gland. The thymus gland lies in the lower 

 part of the neck and in the superior mediastinum. Its history 

 is complicated by the numerous variations which may occur 

 from what is believed to be the normal standard. At birth 

 the gland is relatively large and is responsible for the large 

 area of dulness which is found on percussion over the upper 



