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Fig. 98.—Three diagrammatic longitudinal sections of the tendon at its entry into the sheath, illustrating the valve-like mechanism which permits the gliding of the tendon between the rigid fascia and the bone. I, The muscle at rest. II, The muscle in mid-contraction. III, The muscle fully contracted. Note that the sheath is divided into two portions by the downward projection of a tongue of tissue (the plica). As the muscle contracts, pulling the tendon upward, the deep pocket of the sheath becomes elongated until, in the phase of complete muscular contraction, this deep pocket, which was originally shallow, has become much longer than the superficial pocket.

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Fig. 99.—Diagrammatic longitudinal section illustrating the relation of the tendon to the surrounding connective tissue structures and to the sheath. The sheath is a sac containing fluid, interposed between tendon and fascia or bone, wherever the tendon changes its direction. It acts as a fluid buffer diminishing friction. To permit gliding of the tendon between the rigid fascia and the bone it is surrounded by a peculiarly elastic tissue known as the paratenon, which is well adapted to follow the movements of the tendon without rupture of its fibers. At the upper part of the sheath this tissue is prolonged downward as a tongue-like projection known as the plica. Within the sheath the paratenon serves to attach the tendon to the bone by a delicate membrane through which the blood vessels run, corresponding to the mesentery of the intestines. This is known as the mesotenon. The mesotenon expands on the deep surface of the tendon (see Fig. 100), forming a delicate

of its fibres. For the surgeon, it is of particular importance to conserve the gliding function of this tissue in the performance of tendon transplantations, since it is the means of preventing adhesion between the tendons and the unyielding fascia.

The tendon sheath is interposed between the tendon and the fascia or bone, wherever the tendon has to change its direction. When no change in direction is present, as in the case of the Achilles tendon, there is no sheath. Of course, I use the term sheath here in its strict technical sense, as a synovial-lined structure containing a synovial fluid. It corresponds therefore somewhat to a bursa, and in some ways to a joint. Its function is to act as a fluid buffer preventing friction of the tendon against ligament or bone (see Fig. 99).

To the surgeon who wishes to follow nature's method, it is clear that in tendon plastics, so important a structure as the sheath must not be overlooked, and that if possible the normal relation between it and the tendon should be maintained.

The Mesotenon.-When the sheath of a tendon is opened and the tendon lifted out, a delicate connective-tissue membrane is seen connecting the tendon with the floor of the sheath. This structure, known as the mesotenon, transmits bloodvessels to the tendon and corresponds roughly to the mesentery of the intestine (Fig. 100). That portion of the tendon into which it is inserted is termed the hilus. It is always on the surface of the tendon least exposed to friction. Here the connective tissue of the mesotenon expands on the surface of the tendon, forming the epitenon, and sends connectivetissue strands between the tendon bundles, thus forming the endotenon (Fig. 101).

The Blood-vessels of the Tendon.-The results of injection experiments have shown that the current conception of the tendon as practically bloodless is incorrect. Though much less vascular than muscle or the surrounding loose connective tissue, the tendon contains numerous vessels, except near its friction surface. Here practically no blood-vessels are visible. In general the vessels of the tendon are derived from three main sources: (1) from muscular branches; (2) from vessels

layer of connective tissue termed the epitenon. In this tissue the main blood vessels of the tendon have their longitudinal course. Paratenon, epitenon and mesotenon can be termed together the esotenon to differentiate them from the connective tissue within the tendon known as the endotenon. All the connective tissue structures associated with the tendon may be termed the peritenon.

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Fig. 100.- Microscopical cross-section of the tibialis anticus tendon drawn out of its sheath to show the relation of the mesotenon to the floor of the sheath and to the tendon.

running in the surrounding connective-tissue: paratenon, mesotenon, and the vincula; (3) from vessels of the bone and periosteum near the point of insertion of the tendon. These vessels travel in the hilus of the tendon, in the epitenon, and in the connective-tissue septa between the tendon bundles (the endotenon) and anastomose freely by transverse and oblique branches. Fig. 101, a transverse section through the flexor longus hallucis tendon, shows the longitudinal vessels cut transversely and some of the transverse and oblique

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