The parathyroid glands are four small glands in the neck that produce parathyroid hormone. They are usually located on the back of the thyroid, but can be located anywhere near it. Each gland is about 6mm long and 4mm wide. They share arteries and veins with each other and with the thyroid, and also share a lymphatic system. Although the glands were found in a rhinoceros in 1852, they were not described in humans until 1880. Their role in the endocrine system was developed from 1891 to 1908.