1348 THE DUCTLESS GLANDS. 



In a majority of cases the isthmus covers the second, third, and fourth rings of the 

 trachea, but it may cover the cricoid cartilage, or the fourth, fifth, and sixth tracheal 

 rings. Anteriorly, the gland is clothed by the pretracheal fascia, which separates it from 

 the sterno-thyreoid, sterno-hyoid, and omo-hyoid muscles. Extensions of this fascial layer 

 form an indefinite capsule for the gland. Postero-medially, the thyreoid gland is moulded 

 by the sides of the trachea and lower part of the larynx, and, when large, comes into 

 contact, behind them, with the pharynx and oesophagus. Postero-laterally, it is in 

 relation to the common carotid arteries, and when large is in intimate relation to the 

 recurrent nerves. Further, it has important relations to the parathyreoid glands (see 

 Parathyreoid Glands, Relations). 



Blood and Lymph Vessels. The blood supply is effected through the superior thyreoid 

 arteries, branches of the external carotids, and Jhrough the inferior thyreoid arteries, branches 

 of the thyreo-cervical trunks. Occasionally a fifth artery is present, the thyreoidea ima, a 

 branch of the innominate. The pyramidal lobe, if well developed, receives a special branch 

 from one of the superior thyreoids, usually the left. These arteries are remarkable for their 

 large size and for the frequence and freedom of their anastomoses. An anastomosing trunk 

 courses up the posterior aspect of each lateral lobe, uniting the inferior and superior thyreoid 

 arteries. It is of interest in connexion with the recognition of the parathyreoid glands. 

 Typically, three pairs of veins drain the gland. The upper two pairs, the superior and middle 

 thyreoid veins, join the internal jugulars ; the lower pair, the inferior thyreoid veins, join the 

 left innominate. These vessels take origin from a venous plexus on the surface of the gland or, 

 in the case of the inferior, from a downward extension of the plexus in front of the trachea. 

 When the gland is very large, accessory veins are present, sometimes in considerable numbers. 

 Most of these pass to the internal jugulars. A free, transverse, venous anastomosis is effected 

 along the borders of the isthmus through the superior and inferior communicating veins. 



The lymph vessels anastomose freely in the substance and on the surface of the gland. Most 

 pass direct to the deep cervical lymph glands, a few descend in front of the trachea to the 

 pretracheal lymph glands. 



Nerves. The nerves are derived from the middle and inferior cervical ganglia of the 

 sympathetic. They accompany the blood-vessels. 



Structure. The gland is enclosed in a fibrous capsule (tunica propria) which sends 

 prolongations inward to form a framework for the gland tissue proper. This consists of 

 spheroidal vesicles, -04 mm. to 1 mm. in diameter, lined with cubical epithelium, and filled with 

 " colloid." The size, shape, and cellular characters of the vesicles vary with diet and environ- 

 ment. The vesicles are surrounded by networks of blood capillaries and of lymph vessels. 



Development. The thyreoid gland takes origin from a single median outgrowth from 

 the pharyngeal floor (entoderm). It is recognisable as a shallow bay in a 1'8-mm. embryo, 

 practically simultaneously with the demarkation of the foregut. As the bud grows its end 

 expands whilst its stalk narrows to form the thyreo-glossal duct. 



In a 4-mm. embryo an elevation is present round the pharyngeal opening of the duct. This 

 is the tuberculum impar. It migrates forwards, the duct backwards, so that in a 5-mm. embryo 

 the duct opens into the furrow immediately caudal to the tuberculum (see Tongue, Development). 

 At about this stage the duct begins to obliterate. This process proceeds slowly and is seldom 

 quite complete, a vestige of the duct, the foramen ccecum of the tongue, remaining in the adult, 



While these processes are proceeding growth and lateral expansion of the bud continue. It 

 becomes bilobed and has a divided lumen, and all the while it undergoes a continuous relative 

 displacement caudalwards. Soon its lumen disappears. In a 9-mm. embryo the developing 

 thyreoid gland is a transverse bar composed of transversely disposed cell columns. At about the 

 tenth week of development, 55-mm. embryo, the formation of vesicles commences but is not com- 

 pleted until after birth. The remaining changes are due to simple growth and the moulding 

 effects of the pressure of surrounding structures. 



The thyreoid gland does not arise in any of its parts from any of the pharyngeal pouches 

 (see Ultimo-branchial Bodies). 



The developmental history of the gland affords a ready explanation of its variations in the 

 adult. Thus the development of a pyramidal lobe and its variations, partial and complete 

 duplication, are due to the development of gland tissue from that part of the thyreo-glossal duct 

 which has a double lumen and the more or less complete fusion or separation of the masses thus 

 formed. Accessory thyreoid glands near the hyoid bone are the result of a similar process in 

 connexion with isolated remnants of the duct. 



The occurrence in the adult of a duct leading from the foramen caecum to, or towards, the 

 hyoid bone (lingual . duct) is due to a persistence of the upper part of the thyreo-glossal duct. 

 Similarly, thyreo-glossal cysts are due to the persistence of short intermediate lengths of the duct. 



' (ii.) GLANDULE PARATHYREOIDE^E. 



The parathyreoid glands (O.T. parathyroid bodies ; Synonyms : epithelial 

 bodies, parathymic glands, branchiogenie glands) are finely granular,' yellowish 

 brown, lenticular or spheroidal structures, from 2 to 20 rnm. in diameter and from 



