A NOTE ON THE SEMIMEMBRANOSUS MUSCLE by A. J. E. Cave, M.D., D.Sc. and C. J. Porteous, M.B., B.S. Department of Anatomy, St. Bartholomew's Hospital Medical College CERTAIN CONSTANT FEATURES of the attachments of M.semimembranosus escape notice in the generality of formal descriptive accounts of this muscle. In British text-books the muscle origin is confined to the supero- lateral " facet" on the ischial tuberosity. Regarding the insertion, emphasis is upon the attachment to the " groove for semimembranosus " on the medial tibial condyle, concomitant attachments being treated as secondary expansions and an essential one being completely ignored. For this incompleteness of treatment Thane (1892) may be primarily responsible, since the authoritative 10th edition of Quain may have influenced all subsequent anatomical treatises. The French anatomists, notably Poirier and Charpy (1901), give the semimembranosus insertion at least a descriptive consideration more in accord with nature. The proximal and distal attachments of the semimembranosus were studied in twenty-one dissecting room subjects and the results of this exercise are presented briefly herein. They emphasize (1) the presence of a secondary but constant origin of the muscle; (2) the functional importance of the constant but commonly ignored vertical deep tendinous slip of insertion to a tuberculum quadratum tendinis of the tibia. Origin of the muscle On the tuber ischii the conventional areas of origin of (a) M.semimem- branosus and (b) the conjoint Mm.semitendinosus et biceps femoris are separated by an obtrusive linear elevation (Fig. Ia, x). This is generally regarded, by implication, as representing a demarcation between two contiguous areas of origin, but, in fact, results from the direct attachment of tendinous fibres of origin (Figs. lb, lc) including fibres of M.semi- membranosus. Attempted separation of the tendon of origin of M.semimembranosus from the conjoint tendon of origin of Mm.semitendinosus et biceps is resisted by three groups of connectant collagenous fibres. Of these the first, described by Weber and Collin (1904), is a sheet-like condensation, whose fibres arch transversely and superficially over the two tendons and gain attachment to the medial and lateral borders of the ischial tuberosity. The second is " un tissu dense dans lequel on trouve quelquefois une bourse sereuse" (Poirier and Charpy) found by Weber and Collin in the majority of their specimens. Its fibres arise from the deep aspect of either, or of both, of the components of the biceps-semitendinosus tendon and are inserted into the superficial aspect of the semimembranosus tendon. The third group comes into view after division of the first and second groups, and comprises fibres of origin of both the semimembranosus and the conjoint hamstring tendons. These fibres fan out beyond the con- 251 21 A. J. E. CAVE ventional tuberal areas of origin to become mutually interwoven in successive layers at and beyond their respective sides of the oblique tuberal line (Fig. lb). In this region, therefore, the semimembranosus tendon and the common hamstring tendon are inextricably conjoined and each has a strong attachment to the oblique line.

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Fig. 1. The ischial tuberosity and origin of M. semimembranosus. The primary origin of the M.semimembranosus is thus from the supero-lateral " facet " of the ischial tuberosity and from the tuberal oblique line. It is reinforced by a secondary origin in the form of a falciform tendinous expansion, some inch to inch-and-a-half long. This takes a linear origin from the roughened lateral border of the tuberosity and from the adjacent perineal border of the ischial ramus and is here situate behind M.quadratus femoris and in front of M.adductor magnus (see Fig. 2): the fibres of this falciform expansion (which resembles somewhat an open razor) spread down into the whole extent of the principal semimembranosus tendon, which becomes flattened below its origin. By virtue of this dual origin, M.semimembranosus has a firm hold upon the ischial tuberosity, which it grasps somewhat in the fashion of the fingers and thumb of a Pawlik's grip. From the medial margin of the principal semimembranosus tendon a fibrous expansion passes deep to M.adductor magnus to gain attachment to the vertical ridge (Fig. la, y) on the lower part of the posterior surface of the ischial tuberosity and to the contiguous deep edge of the perineal border of the ischial ramus (see Fig. 2).. Occasionally an additional fibrous expansion is present, of similar origin, but becoming continuous with M.adductor magnus. 252 A NOTE ON THE SEMIMEMBRANOSUS MUSCLE

Fig. 2. Origin of M. semimembranosus. Insertion of the muscle No descriptively adequate account of the semimembranosus insertion appears in British anatomical treatises and a substantial tendinous slip of functional importance is completely ignored. Perhaps the nearest approach to realism is that of Frazer (1920) from meticulous osteological observa- tion. The French anatomists' accounts of this insertion are superior in substance and emphasis, Poirier and Charpy (1901) recognizing the division of the main tendon of insertion into " tendons direct, recurrent et reflechi" and Testut (1904) noting " trois faisceaux divergents, descen- dant, recurrent et anterieur." As dissection reveals, the inferior tendon of M.semimembranosus sub- divides at about knee-joint level into four distinct and recognizable components. In order of importance these are: 253 21-2 A. J. E. CAVE 1. A short, stout, strong tendinous slip (Fig. 3a, x), descending in the line of the main insertion tendon and firmly attached to a prominent, squarish tubercle (tuberculum quadratum tendinis) on the postero-lateral

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Fig. 3a. Insertion of M. semimembranosus (diagrammatic). aspect of the medial tibial condyle (Fig. 3b, x). Poirier and Charpy, and likewise Testut, take notice of fibres attached here, derived from their " direct " or " descending " slip of insertion, but recognize neither a

I I Fig. 3b. Head of tibia, showing tuberculum quadratum tendinis. 254 A NOTE ON THE SEMIMEMBRANOSUS MUSCLE special tubercle (so evident on inspection) nor the functional importance of the " direct " slip. 2. An expanded, thin superficial tendon (Figs. 3a and 4), obscuring the deeply lying first component and proceeding infero-medially to an insertion upon the medial margin of the tibia immediately behind the medial ligament of the knee-joint, and commonly descending beyond the lower end of that ligament. This component takes the form of a sheaf of discrete, imbricated slips (Fig. 4b), which cross the medial inferior genicular vessels in their soft fatty bed and curve somewhat forwards to the tibial T

Fig. 4. Insertion of M. semimembranosus (semi-diagrammatic). 255 A. J. E. CAVE margin. Thereon they produce a distinct linear asperity, which in well- marked bones has the form of a discontinuous series of minute tubercles. The insertion of the imbricated slips, although contiguously deep to, is wholly independent of, the posterior free margin of the medial (internal lateral) ligament of the knee-joint. A substantial descending branch from the medial inferior genicular artery follows the posterior border of this sheaf-like component (Fig. 4). 3. A thin aponeurotic expansion covering M.popliteus and gaining an indirect attachment to the soleal (popliteal) line (Fig. 4, c). 4. A thick, funicular, recurrent tendon passing infero-anteriorly, deep to the medial ligament of the knee-joint, to gain attachment to the inferior lip and neighbourhood of the " groove for semimembranosus " on the medial tibial condyle (Fig. 4, a). This is the component commonly emphasized. 5. A variable number of irregularly disposed fasciculi, collectively constituting the oblique popliteal ligament (lig.posticum Winslowi) and arising from the deep aspect of the main tendon of insertion. This com- ponent requires no present detailed description. These several subdivisions of the main tendon of insertion of M.semi- membranosus are barely separate in the well-developed subject, being but parts of a single expanded tendon in the form of a flattened demi-cone, having deep and superficial parts. The fibres attached to the soleal line and to the tibial medial border are, in the early stages of tibial flexion, in the direct line of pull of M.semimembranosus: in the later stages they are doubled back upon the parent tendon. When upward traction is exerted upon the cadaveric muscle they are seen to be more tense early in flexion and to become much less so as flexion proceeds. The fibres of the recurrent slip to the horizontal groove are, on the contrary, in the direct line of muscle pull only when genuflexion has attained an amplitude of 90degs. At the beginning of flexion their direction is at a right angle to that of the parent tendon. When upward traction is exerted upon the muscle they are seen to be most tense in the later stages of flexion and least so in the earlier stages. The strong central slip to the tuberculum quadratum tendinis remains constantly in the direct line of muscle pull and is equally taut in all stages of genuflexion. Hence it must be recognized as being functionally the nain attachment of M.semimembranosus to the tibia.

REFERENCES FRAZER, J. E. (1920) Anatomy of the human skeleton. 2nd edit. London, Churchill. POIRIER, P., and CHARPY, A. (1901) Traite d'anatomie humaine 2 (1), 238-241. Paris, Masson. TESTUT, L. (1904) Traite d'anatomie humaine 1, 964-965. Paris, Doin. THANE, G. D. (1892) Quain's Elements of Anatomy 10th edit. 2 (2), 251. London, Longmans. WEBER, A., and COLLIN, R. (1904) Bibliog. anat. Paris 13 (3), 149-160. 256