The parathyroid glands are small brownish-red bodies, situated as a rule between the posterior borders of the lateral lobes of the thyroid gland and its capsule.
They differ from it in structure, being composed of masses of cells arranged in a more or less columnar fashion with numerous intervening capillaries.
They measure on an average about 6 mm. in length, and from 3 to 4 mm. in breadth, and usually present the appearance of flattened oval disks.
They are divided, according to their situation, into superior and inferior.
In man, they number four as a rule; fewer than four were found in less than 1 per cent. of over a thousand persons, but more than four in over 33 per cent. of 122 bodies examined by Civalleri. In addition, numerous minute islands of parathyroid tissue may be found scattered in the connective tissue and fat of the neck around the parathyroid glands proper, and quite distinct from them (accessory parathyroid glands).
Development.—The parathyroid bodies are developed as outgrowths from the third and fourth branchial pouches.
A pair of diverticula arise from the fifth branchial pouch and form what are termed theultimo-branchial bodies: these fuse with the thyroid gland, but probably contribute no true thyroid tissue.
Structure.—Microscopically the parathyroids consist of intercommunicating columns of cells supported by connective tissue containing a rich supply of blood capillaries. Most of the cells are clear, but some, larger in size, contain oxyphil granules.