ARTICLE SEVENTH.
OF THE FIBROUS SHEATHS.
The fibrous sheaths are, as we have said, partial or general.
I. Partial Fibrous Sheaths.
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.
II. General Fibrous Sheaths.
The general sheaths are seen especially at the wrist and the instep, where they have the name of annular ligaments. They are destined to confine many tendons together. As in these two places, all those of the hand and the foot pass in a very narrow space, it is necessary that they should be strongly supported. Besides, these sheaths serve also sometimes to change their direction, as we see in those that go to the thumb, whether to its palmar or its dorsal face, and which evidently make an angle at the place of their passage under the sheath. The tendons of the little finger have also an analogous arrangement.