ADRENAL  GLANDS may be either median or paired, and, as they are placed between the kidneys, are often spoken of as interrenals . In the

Amphibia The adrenal glands or suprarenal capsules are two conical the glands are sunk into the surface of the kidney . In reptiles and bodies, flattened from before backward, resting on the upper birds they are long lobulated bodies lying close to the testis or ovary poles of the kidneys close to the sides of the vertebral column; and receiving an adrenal portal vein . In the lower mammals they are not as closely connected with the kidneys as they are in man, each has an anterior and posterior surface and a concave base and their shape is usually oval or spherical which is in contact with the kidney . When viewed from in front the right gland is triangular and the left crescentic . On THE THYROID GLAND the anterior surface there is a transverse sulcus or hilum from which a large vein emerges . The arteries are less constant in The thyroid body or gland is a deep red glandular mass con-their points of entry, and are derived from three sources, the sisting of two lobes which lie one on each side of the upper part phrenic, the abdominal aorta and the renal arteries . The glands of the trachea and lower part of the larynx; these are joined are entirely retro-peritoneal, though the right one, even on its across the middle line by the isthmus which lies in front of the anterior surface,, is very little covered by peritoneum . In a second and third rings of the trachea . Occasionally, from the vertical transverse section each gland is seen to consist of two top of the isthmus, a nearly but not quite median pyramidal lobe parts, cortical and medullary . The cortical substance is corn- runs up toward the hyoid bone, while in other cases the isthmus posed of bundles of cells, separated by a stroma, which have a I may be absent . The gland is relatively larger in women and A B children than in the adult male .

It is enclosed in a

capsule of cervical fascia and is supplied by the superior and inferior thyroid arteries on each side, though occasionally a median thyroidea ima artery is present . On microscopical examination the gland shows a large number of closed tubular alveoli, lined by columnar epithelial cells, unsupported by a basement membrane, and filled with colloid or jelly-like material . These are supported by fibrous septa growing in from the true capsule, which is distinct from the capsule of cervical fascia . The lymphatic vessels are large and numerous, and have been shown by E . C . Bober (Phil . Trans., 1881) to contain the same colloid material as the alveoli . Accessory thyroids, close to the main gland, are often found . Embryology.—The median part of the gland is developed from a tube 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 . This tube is called the thyro-glossal duct and is entodermal in origin . The development 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, and so the asymmetrical arrangement of the pyramidal lobe is accounted for . A .

Kanthack (J . Anat. and Phys. vol.

xxv., 1891) has denied the existence of this duct, but on slender grounds . The lateral parts of the gland are developed from the entoderm of the fourth visceral clefts, and, joining the median part, lose their pharyngeal connexion . Nearly, but not quite, the whole of the lateral lobes probably belong to this part . (For literature and further details see Quain's Anatomy, London, 1892, and J . P . McMurrich's Development of the Human Body, London, 1906.) Comparative Anatomy.—The endostyle or hypobranchial groove of Tunicata (sea squirts) and Acrania (Amphioxus) is regarded as the first appearance 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 (Ammocoetes) among the Cyclostomata the connexion with the pharynx is present, but in the adult lamprey (Petromyzon), as in all adult vertebrates, this connexion is lost . In the Elasmobranchs the single median thyroid lies close to the mandibular symphysis, but in the bony fish (Teleostei) it is paired . In the mud fish (Dipnoi) there is also an indication of a division into two lobes . In the Amphibia the thyroid forms numerous vesicles close to the anterior end of the pericardium . In Reptilia it lies close to the trachea, and in the 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 . (For further details and literature see R .