The broad muscles occupy in general the parietes of the cavities of the
animal economy, those of the thorax and abdomen especially. They form
in part, these parietes, defend the internal organs, and at the same
time by their motions assist their functions.
Their thickness is not great; most of them appear like muscular
membranes, sometimes arranged in layers, as in the abdomen, sometimes
covering the long muscles, as on the back; they are in the first case,
much more extensive superficially than deeper seated.
Whenever abroad muscle arises and terminates on one of the great
cavities, it preserves everywhere nearly its breadth, because it has a
large surface for its insertion. But if from a cavity it goes to a long
bone, or to a small apophysis, then its fibres gradually approximate;
it loses its breadth, increases in thickness, and terminates in an
angle which is succeeded by a tendon, which concentrates into a
very small space fibres widely scattered on the side of the cavity.
The great dorsal and pectoral muscles present us an example of this
arrangement, which is met with also in the iliacus, in the glutæi
medius and minimus. The broad muscles of the pectoral cavity have a
peculiar arrangement which the ribs require; their origin takes place
by serrated points fixed to these bones, and separated by spaces
between them.
The broad muscles are most often simple; many rarely unite to form
compound ones. Different cellular layers separate them, like the long
muscles; but they are hardly ever like them covered by aponeuroses; the
greatest number is merely subjacent to the integuments; the reason is
that their form naturally protects them from these displacements, of
which we have spoken in the article on aponeuroses, and which, without
these membranes, would be so frequent in the long muscles. I do not
know that the cramp has ever been observed in those of which we are
treating. When the abdominal muscles are laid bare by incisions made
through the integuments of a living animal, I have observed, that in
contracting, the bulk of each preserves the same place.