» The Endocrine Glands
The paired thyroid glands (Fig. 37.37/5) of the chicken are reddish brown, oval, and about 10 mm long and 5 mm wide. In the budgerigar, in which thyroid disease is a major problem in iodine- deficient areas, they are paler and only 2 to 3 mm long and 1 to 2 mm wide.
The thyroid glands are located in the thoracic inlet, caudal to the crop and closely related to the common carotid artery, trachea, jugular vein, and vagus nerve (which accompanies the vein)-indeed, they lie just cranial to where these vessels are joined by the subclavian vessels (Fig. 37.37/16). Their color distinguishes them from the neighboring rather similar but pale thymic lobes.The parathyroid glands (Fig. 37.37/7), two or three on each side, are minute (1- to 3-mm) yellowish brown structures immediately caudal to the thyroid gland, to which one may be attached. They become enlarged (increased parathyroid hormone production) when the diet is deficient in calcium, which leads to decalcification of the bones. In African grey parrots there is a specific problem in which calcium fails to be mobilized from the skeleton despite a dietary deficiency. In this situation the bird will die of hypocalcemia, and much enlarged parathyroid glands will be found at necropsy.
The even more minute pink ultimobranchial glands (Fig. 37.37/8) lie next to the parathyroids.
The adrenal glands (Fig. 37.31/3') are yellowish brown, oval or triangular, and about 13 mm long and 8 mm wide. Each lies at the cranial pole of the corresponding kidney, related ventrally to the ovary (or epididymis). There is no distinct separation of cortex and medulla.
The hypophysis (or pituitary gland) (Fig. 37.38/7) is attached below the diencephalon and occupies the hypophyseal fossa in the base of the skull. It resembles that of mammals in its division and formation.