in 1891 gley was the first to discover the importance of the parathyroid glands, which are four (or more) small endocrine glands lying close or embedded in the posterior surface of the thyroid gland. these glands had been recently discovered as an anatomical entity in 1880, however their importance was not understood at the time. gley realized that the cause of tetany after thyroid operations was the inadvertent destruction of the parathyroid glands. he demonstrated this by removing the parathyroid glands from laboratory animals and witnessing their deaths from tetany. because of his discovery, parathyroid glands have sometimes been referred to as "gley's glands".