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Synovial Sheaths or Sacs

sheath, tendons, wrist, tendon, finger, knee-pan, hand, front and little

SYNOVIAL SHEATHS OR SACS Tendon will be readily under stood that the tendinous cords moving so fre quently on the surfaces of bone, and over bony prominences, would be liable to develop friction and heat to an extent that might be injurious, unless some means existed for rendering the motion as easy as possible. This is effected by means of sheaths which surround tendons. The sheaths form a double lining round the tendons, and the opposed surfaces are lined by synovial membrane, already noticed as one of the structures entering into the formation of joints. The membrane secretes the fluid synovia, which lubricates the sheath in which the ten don slides, and so facilitates the motion. Such synovial sheaths are specially well marked in the tendons of the hand and foot.

In the case of the palm of the hand there are no less than five of such tubular canals or sheaths. There is one for the tendon of the thumb, which begins high up on the wrist and through which the tendon runs, till the last joint of the thumb is reached, where the sheath stops, and the tendon passes on to its fixed point. There is also one for the little finger, starting even higher up on the forearm than that of the thumb, passing through the palm of the hand and running right along the little finger till it ends at the base Of the last joint. This sheath of the tendon of the little finger contains also the tendons of the index, middle, and ring fingers as far as the centre of the palm. Here these three fingers leave the common sheath, I and pass on. each to its own finger, uncovered by a sheath. But when each of these three tendons reaches the base of its own finger, it enters a sheath of its own, which extends over the front part of the first two bones of the finger, and ends at the base of the end bone of the finger. The tendons that run on the back of the hand are similarly clothed with synovial sheaths, but only for a short distance. On the back, the sheaths are situated over the wrist, extending only a little way above over the fore-aria, and below over the back of the hand. At this place all the tendons me bound down by a broad band of ligament, which crosses from one side of the wrist to the other. The tendons pass, side by side, underneath this band, and the sheaths form tunnels, as it were, under the band fur the passage of the tendons.

If these sheaths become inflamed, an increased quantity of the fluid—synovia—they secrete will be produced, and will distend the sheath, and will produce an appearance on the front or back of the wrist, of a narrow, elongated, swell ing, somewhat sausage-shaped. The wrist will be more or less deformed by these soft swellings under the skin. This is a frequent occurrence

in chronic rheumatism of the hands or feet. For it is the rheumatic poison that most fre quently produces such sheath inflammations, and that is the reason for the term chronic synovial rheumatism. But the inflammation may be confined to one sheath, perhaps due to an excessive amount of movement or strain on the particular tendon whose sheath it is. This is not an infrequent occurrence on the wrist of pianists or violinists, and one variety of it causes the round swelling, in shape like a boy's marble, on the back or front of the wrist (see Ganglion, p. 122).

There is a very similar arrangement of tendon sheaths on the back and sides of the foot, the sheaths being distributed over the back of the ankle, and on each side at the junction of foot and leg. In the sole of the foot the arrange ment resembles the palm of the hand.

Synovial Saes or sheaths lined with synovial membrane, little sacs simi larly lined, and containing fluid, exist in special places between two surfaces which move upon one another to any great extent. Such sacs are called synovial sacs, or synovial burs, or simply bursae. Bursa means a sac.

For instance, the movement of the knee-pan in front of the knee-joint has been already referred to. This motion, so constant in walk ing, would be sure to produce undue friction and heat, and consequent inflammation, were it not for the interposition of such a sac between the upper surface of the joint and the deep surface of the knee-pan.

Again, the knee-pan moves underneath the skin, and, to prevent friction between its upper surface and. the skin, another synovial sac or bursa is interposed. So that the knee-pan is between two burs, one superficial, just under the skin, the other deep beneath the bone. When these are inflamed, as they may be by injury, the large amount of increased secretion into the sacs distends the skin in every direction, and causes all the usual outlines of the joint to be lost. The knee-pan is thrust forward and seems to be floating on the surface of a bay of fluid, unless the bursa in front of the knee-pan is also involved, when the bone conies to lie between two sacs distended with fluid.

Similar, though smaller, sacs are found over the point of the elbow, over the knuckles, over the malleoli—the outer and inner projections of the ankle-joint,--and over various other prominent points, the great trochanter of the thigh bone, for example. They may also be present between two tendons or two muscles.

It is important to notice the bur.sfe, as they fulfil a very important duty, and are besides liable to disease, and specially to various forms of inflammation.