nearer in the long line of descent to their remote animal-like progenitors.

In man, the os coccyx, together with certain other vertebrae hereafter to
be described, though functionless as a tail, plainly represent this part in
other vertebrate animals. At an early embryonic period it is free, and
projects beyond the lower extremities; as may be seen in the drawing (Fig.
1.) of a human embryo. Even after birth it has been known, in certain rare
and anomalous cases (52. Quatrefages has lately collected the evidence on
this subject. 'Revue des Cours Scientifiques,' 1867-1868, p. 625. In 1840
Fleischmann exhibited a human foetus bearing a free tail, which, as is not
always the case, included vertebral bodies; and this tail was critically
examined by the many anatomists present at the meeting of naturalists at
Erlangen (see Marshall in Niederlandischen Archiv fur Zoologie, December
1871).), to form a small external rudiment of a tail. The os coccyx is
short, usually including only four vertebrae, all anchylosed together: and
these are in a rudimentary condition, for they consist, with the exception
of the basal one, of the centrum alone. (53. Owen, 'On the Nature of
Limbs,' 1849, p. 114.) They are furnished with some small muscles; one of
which, as I am informed by Prof. Turner, has been expressly described by
Theile as a rudimentary repetition of the extensor of the tail, a muscle
which is so largely developed in many mammals.

The spinal cord in man extends only as far downwards as the last dorsal or
first lumbar vertebra; but a thread-like structure (the filum terminale)
runs down the axis of the sacral part of the spinal canal, and even along
the back of the coccygeal bones. The upper part of this filament, as Prof.
Turner informs me, is undoubtedly homologous with the spinal cord; but the
lower part apparently consists merely of the pia mater, or vascular
investing membrane. Even in this case the os coccyx may be said to possess
a vestige of so important a structure as the spinal cord, though no longer