The partial sheaths destined to a single tendon are of two sorts; one
runs a long course; such are those of the flexors of the foot and the
hand, which correspond to the whole concave surface of the phalanges;
the others form only a kind of rings, in which a tendon is reflected,
an example of which is seen in the great oblique muscle of the eye.
All in general form a semi-circle and make half of a canal which the
bone completes; so that the tendon slides in a canal half osseous
and half fibrous. This canal is lined by a synovial membrane, the
attachment of which to the fibrous sheath is equal to that of the
articular synovial membrane to its capsule. By their external surface,
the fibrous sheaths correspond with the neighbouring organs, to which
they are united by a loose cellular texture.
All these sheaths are of a very dense and compact texture; they are
stronger in proportion to the effort which the tendons can exert upon
them, than the fibrous capsules are in relation to the different
impulses which the bones can communicate to them and which tend to
rupture these capsules. They are confounded with the periosteum at
their two edges. Those of the flexors unite also by their extremity
with the expansion of the tendons; hence the very considerable fibrous
interlacing that is observed at the extremity of the last phalanges.
In the limbs, the flexors only have these sheaths; the extensors
are destitute of them. This arises first from this, that there are
two tendons of the first kind to each finger, whilst there is only
one of the second, and that consequently more force is necessary to
retain them in the first direction. In the second place, each extensor
tendon receives on its sides the insertion of the small tendons of the
interosseous muscles and the lumbricales, which by drawing it in an
opposite direction in the great motions, retain it in its place, and
thus compensate for the fibrous sheaths that are wanting. Finally the
efforts of the extensors are much less than those of the flexors, of
which they are as it were but a kind of moderators.