PARATHYROID  GLANDS These little

oval bodies, of considerable physiological importance, are two in number on each side . From their position they are spoken of as postero-superior and antero-inferior; the postero-superior are embedded in the thyroid at the level of the lower border of the cricoid cartilage, while the antero-inferior may be embedded in the lower edge of the lateral lobes of the thyroid or may be found a little distance below in relation to the inferior thyroid veins . They are often very difficult to find, but it is easiest to do so in a perfectly fresh, full-term foetus or young child . Microscopically they consist of solid masses of epithelioid cells with numerous blood-vessels between, while, embedded in their periphery, are often found masses of thymic tissue including the concentric corpuscles of Hassall . They have been regarded as undeveloped portions of thyroid tissue in an embryonic state, but the experiments of Gley (Comptes rendus de la Soc. de Biol . No . I1, 1895) and of W . Edmunds (Prot . Physiol . Soc.-Journ . Phys. vol. xviii., 1895) do not confirm this . 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 Mammalia, and also in Birds . In the other classes of vertebrates little is known of them . The fullest and most recent account of these bodies is that of D . A . Welsh in Journ . Anat. and Phys. vol . 32, 1898, pp . 292 and 380 .