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. 11, 1895)
and of W. Edmunds (_Proc. 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.