out the size
of a pea, known as the pineal gland. It is so destitute of any evident
function that Descartes, in lack of any more probable explanation of its
presence, ascribed to it the noble duty of serving as the seat of the
soul. Late research has been more successful in tracking this organ to
its lair. It is larger in the embryo than in the adult man, still larger
in some lower vertebrates, and in certain lizards has been found to
exist as an eye, its parts plainly distinguishable under the microscope.
It is placed in the middle of the forehead, between the other eyes, and
was no doubt an active organ of vision in some ancient batrachians.

The pineal eye, as it is now named, once useful, long useless, has
persisted as a fossil structure through a far extended line of
development. No more convincing evidence that man gained his body
through descent from the lower animals could be asked for than the
survival in the human brain of this wonderfully significant remnant of
a formerly useful organ. Like various other vestiges of ancient organs,
it is not only useless but detrimental. It occasionally enlarges and
becomes the seat of large and complicated tumors, which may cause death
by their compression of the brain.

Two other structures common to most of the vertebrate animals exist in
man, though they render him little or no service. These are the thymus
and thyroid glands, apparently vestigial structures. The thymus gland
attains a considerable development in the embryo and shrinks away to the
merest vestige in the adult. It begins to form early in the embryo life
as an epithelial ingrowth from the throat, and extends from the neck
into the chest. It continues to grow after birth, but later begins to
shrink and nearly disappears in the adult.

The thyroid gland has a somewhat similar origin, it beginning as an
ingrowth from the lower section of the pharynx and extending down to the
lower part of the neck. It subsequently loses its connection with the
pharynx, and in adult life is a bilobed structure on either side of the
windpipe. Like the thymus it is a ductless gland, abundantly supplied
with blood-vessels, and possesses a vast number of small cavities, lined
with cells and containing an insoluble jelly. So far as appears, both
these glands are useless, or nearly so, to man; or if the thyroid
performs any useful service it is a minor and obscure one. Such
functions as it may have could probably be performed by som