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Anatomy of Tendons.—The easiest method of grasping the essentials of the anatomy of tendons is by study of a series of cross-sections taken through a tendon above the sheath, and at various points within the sheath. Fig. 93 represents a cross-section through the tendon of the extensor proprius hallucis, 1 inch above the intermalleolar line. It is observed
Tendon of extensor proprius hallucis Muscle fibre of extensor proprius hallucis
Anterior tibial artery
Fig. 93.— Microscopical cross-section through the calf of an infant eight months old just above the upper pole of the sheath of the extensor proprius hallucis. Note that between the fascia and the tendon there intervenes a layer of tissue known as the paratenon. Under higher magnification this tissue is seen to consist of fat cells and elastic fibers.
that the tendon is separated from the fascia by a band of tissue which under a high power is seen to be a loose fatty areolar structure containing a large number of elastic fibres. In the next section, taken 14 inch lower (see Fig. 94), a cleft is visible which corresponds to the upper pole of the tendon sheath. This cleft, however, does not come into immediate contact with the tendon, but is separated from it by tissue which exactly resembles that of the previous section lying between fascia and tendon. In the next section, again 14 inch distal (see Fig. 95), the cleft is divided into two portions by a transverse band; whereas in the next, Fig. 96, this band has disappeared and the cleft separates the fascia from the tendon. To render these microscopical sections clearer, and to correlate them, a series of corresponding cross-sections and a longitudinal section are given in Fig. 97. From these it is evident that the loose areolar tissue which separates the tendon from
Fig. 94.— Microscopical cross-section through the calf of an infant eight months old at the level of the upper pole of the sheath of the extensor proprius hallucis. Note that the cleft corresponding to the sheath is separated from the tendon by the same type of tissue, which in the preceding section, lay between tendon and fascia (the paratenon).
the fascia above the sheath, extends downward into the sheath as a tongue-like projection which divides the upper end of the sheath into two portions—a deep pocket between the tongue-like projection and the tendon, and a superficial pocket between it and the fascia. This projection is known as the plica, and the tissue above the sheath separating fascia from tendon is known as the paratenon.
The significance of paratenon and plica in the gliding mechanism of the tendon becomes evident as soon as the muscle contracts and the tendon glides. Then it is seen that
Friction surface of -
Deep pocket of the sheath
Fascial layer of
Fascial septum between
extensor proprius hallucis and tibial anticus
Muscle fibres of extensor proprius hallucis Fig. 95.—Microscopical cross-section through the calf of an infant eight months old 14 inch distal to the preceding section. The cleft corresponding to the sheath is divided into two portions by a transverse band of loose connective tissue similar in structure to the paratenon—the plica.
Fig. 96.- Microscopical cross-section through the calf of an infant eight months old, 22 inch distal to the preceding section. At this level the tendon sheath is directly interposed between the fascia and the tendon. The remnants of the paratenon are seen on each side as rudimentary structures.
the deep pocket of the sheath, namely that between plica and tendon, becomes elongated, whereas the superficial pocket becomes somewhat shallower (see Fig. 98). As the muscle contracts still further, these changes become more pronounced until when the muscle has reached the maximal point of contraction the deep pocket has become distinctly longer than the superficial. It is clear that a kind of invagination has occurred, and that the plica, acting as a kind of valve, has allowed the tendon to glide freely, and at the same time has maintained the sheath wall intact.
Tip of plica and deep pocket of
of plica to tendon sheath Fig. 97e.- Diagrammatic longitudinal section of the extensor proprius hallucis tendon to correlate the four preceding cross-sections. Note that the paratenon situated between fascia and tendon is prolonged downward as a tongue-like structure (the plica), which divides the upper pole of the sheath into two pockets: (1) a superficial, between fascia and plica, and (2) a deep, between plica and tendon.
That some such valve-mechanism must be present is necessitated by the anatomical relations of the fascia and the bones which form the immovable channel through which the tendon glides. To use a rough simile, the tendon corresponds
. to the piston of a cylinder represented by the rigid bone and the fascia attached to it. Just as it would be impossible for the piston to glide to and fro within the cylinder, were it rigidly attached at any point, so it would be impossible for the tendon to glide within its cylinder unless some mechanism were present which could, because of its elasticity, maintain the sheath as a closed cavity and yet be able to follow the excursion of the tendon.
The essential in this gliding mechanism is the paratenon. As before stated, this is a loose areolar tissue with an unusually large number of elastic fibres. Because of the elasticity thus imparted to it, it can be stretched 3 or 4 cm. without rupture
d c Fig. 97.—Diagrams representing the relations of the tendon to the paratenon and to the upper pole of the tendon sheath. a, Cross-section above the upper pole of the sheath corresponding to Fig. 93; note that the paratenon intervenes between the tendon and the fascia. b, Cross-section through the upper pole of the sheath corresponding to Fig. 94; note that the sheath is separated from the tendon by the paratenon. c, Cross-section 14 inch distal to the preceding, corresponding to Fig. 95; note that the sheath is divided into a superficial and a deep portion by a band of paratenon, termed the plica. d, Cross-section 12 inch distal to c, corresponding to Fig. 96; note that the sheath now intervenes between fascia and tendon.
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