PARATHYROID GLANDS These little oval bodies are two on each side. The upper pair is embedded in the thyroid at the level of the lower border of the cricoid cartilage, while the lower pair may be embedded in the lateral lobes of the thyroid or lie below in relation to the inferior thyroid veins. They are often very difficult to find in the adult. Microscopically they consist of solid masses of epithelioid cells with numerous blood-vessels, while, embedded in their periphery, are often masses of thymic tissue including the concentric cor puscles of Hassall. They are developed from the entoderm of the third and fourth branchial grooves.
Parathyroids have been found in the orders of Primates, Cheiroptera, Carnivora, Ungulata and Rodentia among the Mam malia, and also in birds. In the other classes of vertebrates little is known of them.