THE THYROID GLAND The thyroid body or gland is a deep red glandular mass consist ing of two lobes which lie one on each side of the upper part of the.trachea and lower part of the larynx; these are joined across the middle line by the isthmus which lies in front of the second and third rings of the trachea. The gland is relatively larger in women and children than in men. It is enclosed in a capsule of cervical fascia and is supplied by the superior and inferior thyroid arteries. Microscopically, the gland consists of closed tubular alveoli filled with colloid material, and lined by cuboidal epithelial cells, un supported by a basement membrane. The lymphatic vessels are large and numerous, and contain the same colloid as the alveoli. Accessory thyroids, close to the main gland, are often found.
The median part of the gland is developed from the thyro-glossal duct which grows down in the middle line from the junction of the buccal and pharyngeal parts of the tongue (q.v.), between the first and second branchial arches. The de velopment of the hyoid bone obliterates the middle part of the duct, leaving its upper part as the foramen caecum of the tongue, while its lower part bifurcates. The lateral parts of the gland are developed from the entoderm of the fourth visceral clefts, and, joining the median part, lose their pharyngeal connection.
The endostyle or hypobranchial groove of Tunicata and Acrania is regarded as the first appear ance of the median thyroid; this is a median entodermal groove in the floor of the pharynx, secreting a glairy fluid in which food particles become entangled and so pass into the intestine. In the larval lamprey the connection with the pharynx is present, but in the adult as in all adult vertebrates, this connection is lost. In Elasmobranchs, the single median thyroid lies close to the man dibular symphysis, but in Teleostei it is paired. In Dipnoi there is also an indication of a division into two lobes. In Amphibia the thyroid forms numerous vesicles close to the anterior end of the pericardium. In Reptilia it lies close to the trachea, and in Chelonia and Crocodilia is paired. In birds it is also paired and lies near the origin of the carotid arteries. In Mammalia the lateral lobes make their first appearance. In the lower orders of this class the isthmus is often absent.
