The long muscles occupy in general the limbs, to the conformation
of which theirs is accommodated. Separated from the skin by the
aponeuroses, from the bone by the periosteum, they are situated in a
sort of fibrous gutter, which retains them powerfully, and in which
they are arranged in layers more or less numerous, the deep ones are
confined in their place by the superficial ones, which, in their turn,
have the aponeuroses to support them. These last are very long; they
commonly belong to the motions of three or four bones and even more,
examples of which we have in the sartorius, the semi-tendinosus and
membranosus, the biceps, the flexors and the extensors. As they become
deeper, they are also shorter and generally destined to the motions of
two bones only, as the adductors, the pectineus, &c. are a proof.
Cellular layers separate them; they are loose where great motions take
place, more compact where these motions are less, very thick where
vessels and nerves go between the muscular fasciculi. Often spaces more
or less considerable, filled with cellular texture, separate these
fasciculi from each other. We divide the long muscles into simple and
compound. They are simple when a single fasciculus forms them, compound
when they arise from the union of many. These fasciculi are found then
in two different manners; sometimes in fact its division is at the top
of the muscle, as we see in the brachial and femoral biceps; sometimes
it is at the inferior part, at the most moveable side that this
division is met with, as in the flexor and extensor muscles of the leg
and the fore-arm.
The long muscles often separate from each other, are sometimes held
together by means of aponeuroses, which confound a more or less
considerable portion of two, three and even four of these neighbouring
organs. The origin of the muscles of the internal and external
tuberosities of the humerus exhibits this arrangement, whence results
an essential advantage in the general motions of the limb. Then in fact
the contraction of each muscle serves, both to cause a motion below in
the moveable part to which it is attached, and to strengthen above the
fixed point of the neighbouring muscles which contract at the same time
with it.
Every long muscle is in general thicker in its middle than at its
extremities, a form which arises from the manner of the insertion of
the fleshy fibres, which arising above and terminating below, some
successively below others, are so much the less numerous as we approach
nearer each extremity, whilst in the middle they are all found in
juxta-position. The anterior rectus, the long supinator, the external
radial muscles, &c. have manifestly this arrangement.
There is a particular kind of long muscles, which has no analogy but
in external appearance with that of the muscles of the limbs. They
are those that are embedded in front and especially behind the spine.
Though they appear simple at first view, these muscles have as many
distinct fasciculi as there are vertebræ. The transversalis colli, the
sacro-lumbalis, the longus colli, &c. represent very well an elongated
fasciculus like the sartorius, the anterior rectus of the thigh, &c.;
but the structure of this fasciculus has nothing in common with that of
these muscles; it is a series of small fasciculi, which have each their
distinct origin and termination, and which appear to be confounded into
one muscle only because they are in juxta-position.