of the will, known as the
voluntary muscles, appear in the form of fleshy structures, red in
color, and with fibers of various degrees of fineness, and are composed
of fasciculi, or bundles of fibers, united by connective or cellular
tissue, each fasciculus being composed of smaller ones but united in a
similar manner to compose the larger formations, each of which is
enveloped by a structure of similar nature known as the sarcolemma. Many
of the muscles are united to the bones by the direct contact of their
fleshy fibers, but in other instances the body of the muscle is more or
less gradually transformed into a cordy or membranous structure known as
the tendon or sinew, and the attachment is made by the very short
fibrous threads through the medium of a long tendinous band, which,
passing from a single one to several others of the bones, effects its
object at a point far distant from its original attachment. In thus
carrying its action from one bone to another, or from one region of a
limb to another, these tendons must necessarily have smooth surfaces
over which to glide, either upon the bones themselves or formed at their
articulations, and this need is supplied by the secretion of the
synovial fluid, a yellowish, unctuous substance, furnished by a peculiar
tendinous synovial sac designed for the purpose.

Illustrations in point of the agency of the synovial fluid in assisting
the sliding movements of the tendons may be found under their various
forms at the shoulder joint, at the upper part of the bone of the arm,
at the posterior part of the knee joint, and also at the fetlocks, on
their posterior part.

As the tendons, whether singly or in company with others, pass over
these natural pulleys they are retained in place by strong, fibrous
bands or sheaths, which are by no means exempt from danger of injury, as
will be readily inferred from a consideration of their important special
use as supports and reenforcements of the tendons themselves, with which
they must necessarily share the stress of whatever force or strain is
brought to bear upon both or either.

We have referred to that special formation of the external surface of a
bone by which it is adapted to form a joint or articulation, either
movable or fixed, and a concise examination of the formation and
structure of the movable articulations will here be in place. These are
formed generally by the extremities of the long bones, or may exist on
the surface