ich in their connection and
totality constitute the skeleton of an animal (see Plate XXIII). They
are of various forms, three of which--the long, the flat, and the
small--are recognized in the extremities. These are more or less regular
in their form, but present upon their surfaces a variety of aspects,
exhibiting in turn, according to the requirement of each case, a
roughened or smooth surface, variously marked with grooves, crests,
eminences, and depressions, for the necessary muscular attachments, and,
as before mentioned, are connected by articulations and joints, of which
some are immovable and others movable.

The substance of the bone is composed of a mass of combined earthy and
animal matter surrounded by a fine, fibrous enveloping membrane (the
periosteum) which is intimately adherent to the external surface of the
bone, and is, in fact, the secreting membrane of the bony structure. The
bony tissue proper is of two consistencies, the external portion being
hard and "compact," and called by the latter term, while the internal,
known as the "spongy" or "areolar tissue," corresponds to the
descriptive terms. Those of the bones that possess this latter
consistency contain also, in their spongy portion, the medullary
substance known as marrow, which is deposited in large quantities in the
interior of the long bones, and especially where a central cavity
exists, called, for that reason, the medullary cavity. The nourishment
of the bones is effected by means of what is known as the nutrient
foramen, an opening established for the passage of the blood vessels
which convey the nourishment necessary to the interior of the organ.
Concerning the nourishment of the skeleton, there are other minutiae,
such as the venous arrangement and the classification of their arterial
vessels into several orders, which, though of interest as an abstract
study, are not of sufficient practical value to refer to here.

The active organs of locomotion, the muscles (see Plate XXIII), speaking
generally, form the fleshy covering of the external part of the skeleton
and surround the bones of the extremities. They vary greatly in shape
and size, being flat, triangular, long, short, or broad, and are
variously and capriciously named, some from their shape, some from their
situation, others from their use; and thus we have abductors and
adductors--the pyramidal, orbicular, the digastricus, the vastus, and so
on. Those which are under the control