e of the
other organs, while it is positively detrimental as the seat of goitre.
This unsightly disease is due to its enlargement, either by a great
increase of its blood-vessels or a development of the capsules and
increase of their contained jelly. Dr. S. V. Clevenger considers these
organs to have had a branchial or respiratory origin, saying that there
are many reasons for believing them to be rudimentary gills. Owen says
that the thymus appears in vertebrates with the establishment of the
lung as the main or exclusive respiratory organ. It is wanting in all
fishes, also in the gill-bearing batrachians, siren and proteus. The
thyroid appears in fishes, and Gegenbaur believes that it may have been
a useful organ to the Tunicata in their former state of existence.

Dr. Clevenger, in the _American Naturalist_ for January, 1884, points
out another curious structure in man, whose significance does not seem
to have been previously observed. This is a strange and striking fact
relating to the formation of the veins. It is well known that these
organs possess valves, which permit the free upward flow of the blood
toward the heart, but resist its descent through the action of gravity,
in this way aiding its return from the extremities. The rule holds good
throughout the quadrupeds that the vertical veins possess valves, while
they are absent from the horizontal veins, in which they would be of no
utility. But the singular fact exists that in the human trunk the valves
occur in the horizontal and are absent from the vertical veins. In other
words, they exist where they are useless for their apparent purpose and
are absent where they would be useful.

The only conclusion that can reasonably be drawn from this strange fact
is that we are here dealing with a fossilized structure, a functionless
survival. It leads irresistibly to the inference that man has descended
from a quadruped ancestor, and that when his body took the upright
position the structure of the veins, not being seriously detrimental,
remained unchanged. Those which had been vertical became horizontal, and
retained their now useless valves; those which had been horizontal
became vertical, and remained destitute of valves. The veins of the arms
and legs, vertical in both forms, retained their valves.

Dr. Clevenger points out that the intercostal veins, which carry blood
almost horizontally backward to the azygos veins and which would run
vertically upward in qua