The fibrous organs arranged as membranes are, 1st, the fibrous
membranes, properly so called; 2d, the fibrous capsules; 3d, the
tendinous sheaths; 4th, the aponeuroses.
1st. The fibrous membranes comprehend the periosteum, the dura-mater,
the sclerotica, the albuginea, the peculiar membranes of the kidney,
the spleen, &c. &c. They are in general destined to form the covering
of certain organs, into the structure of which they enter.
2d. The fibrous capsules, very distinct, as we shall see, from the
synovial surfaces, are a kind of cylindrical sacs, which are found
around certain articulations, especially those of the humerus and
the femur, whose connexions with the scapula and the ilium they
strengthen, by embracing with their two extremities, both surfaces of
the articulation.
3d. The fibrous sheaths are destined to confine the tendons in their
passage upon the bones, where they are reflected, and generally
wherever by muscular contraction they would be made to deviate and
consequently transmit with difficulty to the bones the motion they
receive from the muscles. They may be divided into two kinds; one
receives and transmits the tendons of many muscles united, as is seen
at the wrist, the instep, &c.; the other, like that of the fingers, is
destined for a single tendon, or two only.
4th. The aponeuroses are a kind of fibrous nets more or less broad,
entering always into the system of locomotion, and so arranged that
they sometimes form coverings for different parts, and sometimes
furnish the muscles with points of insertion. Hence the aponeuroses of
covering and the aponeuroses of insertion; each of them is divided into
species.
The aponeuroses of covering are placed sometimes around a muscle, to
which they serve as a general sheath, as we see on the thigh, the
fore-arm, &c.; sometimes upon certain muscles which they partially
retain in their respective places, as that which goes from the
posterior and superior serratus minor to the anterior and inferior, as
the abdominal aponeurosis, as that situated anteriorly to the solæus,
behind the deep muscles of the leg, &c.
The aponeuroses of insertion are sometimes with surfaces more or less
broad, as in the attachments of the triceps femoris, the rectus, the
biceps, &c.; sometimes with fibres separate from each other, and
giving attachment by each of these fibres to a fleshy fibre, as at
the superior insertion of the iliacus, of the anterior tibialis, &c.;
sometimes finally in the form of an arch, and then at the same time
that they give the muscles points of insertion, they allow vessels to
pass under them, as in the diaphragm, the solæus, &c.