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(c) Those by which the sacrum articulates with the last lumbar vertebra (Lumbo-sacral joints);
(d) Those by which the hip bones are attached to the vertebral column (Sacro-iliac joints);
(e) That by which the hip bones are attached to each other (Symphysis pubis).
Lumbo-sacral Joints.-The articulation of the sacrum with the fifth lumbar vertebra is constructed precisely on the principle of the articulations between two typical vertebræ, and the usual ligaments associated with such joints are repeated. There is, however, an additional accessory ligament, termed the lateral lumbo-sacral ligament. This extends from the anterior aspect of the inferior border of the transverse process of the last lumbar vertebra, downwards and slightly laterally, to the front of the lateral aspect of the ala of the sacrum, close to the sacroiliac joint. Further, a variable membranous band extends between the lateral aspect of the inferior part of the body of the last lumbar vertebra and the front of the ala of the sacrum. This band lies in front of the anterior ramus of the fifth
i lumbar nerve.
Each hip bone articulates with the sacral section of the vertebral column on each side through the intervention of a diarthrosis, termed the sacro-iliac joint.
This joint is formed between the contiguous auricular surfaces of the sacrum and ilium. Each of these surfaces is more or less completely clothed with hyaline articular cartilage. The joint cavity, which is little more than a capillary interval, may be crossed by fibrous bands.
The cavum articulare (joint cavity) is surrounded by ligaments of varying thickness and strength, which constitute the fibrous stratum of its articular capsule. Thus, its anterior part is thin, and consists of short but strong fibres which pass between adjoining surfaces on the ala of the sacrum and the iliac fossa of the hip bone; they form the anterior sacro-iliac ligament. On the posterior aspect there are three ligaments. The interosseous sacro-iliac ligament (Fig. 313) consists of numerous strong fasciculi, which pass from the rough area on the medial aspect of the ilium, above and behind its auricular surface, downwards and medially to the tubercles of the transverse processes and the depressions behind the first and second segments of the sacrum. This ligament is of great strength, and with its
FIG. 314.-POSTERIOR VIEW OF THE PELVIC LIGAMENTS AND OF THE HIP-JOINT.
fellow it is responsible for suspending the sacrum and the weight of the superimposed trunk from the hip bones.
The long posterior sacro-iliac ligament (Fig. 314) is a superficial thickened portion of the interosseous ligament. It consists of a definite band of fibres passing from the posterior superior iliac spine to the tubercles of the transverse processes of the third and fourth segments of the sacrum.
The short posterior sacro-iliac ligament consists of superficial fibres of the interosseous ligament passing from the posterior superior iliac spine to the tubercles of the first and second transverse processes of the sacrum.
The articular cavity of this joint is very imperfect and rudimentary.
Several accessory ligaments are associated with the articulation of the hip bone to the sacral section of the vertebral column.
Lig. Iliolumbale. The ilio-lumbar ligament (Fig. 314), which is merely the thickened anterior lamina of the lumbo-dorsal fascia, extends from the tip of the transverse process of the last lumbar vertebra, almost horizontally laterally, to the inner lip of the iliac crest at a point a short distance behind its highest level. A proportion of these fibres is attached to the medial rough surface of the ilium between the iliac crest and the auricular impression. To these the name of the lig. iliolumbale inferius is applied.
Lig. Sacrotuberosum.-The sacro-tuberous ligament (O.T. great sacro-sciatic lig.) (Fig. 314) is somewhat triangular in outline. It occupies the interval between the sacrum and the hip bone, and is attached medially to the posterior inferior spine of the ilium; to the posterior aspects of the tubercles of the transverse processes and lateral margins of the third, fourth, and fifth segments of the sacrum, as well as to the side of the first segment of the coccyx. It passes downwards and laterally, becoming narrower as it approaches the ischium, near to which, however, it again expands, to be attached to the medial side of the tuber ischiadicum, immediately below the groove for the tendon of the obturator internus muscle, i.e. the lesser sciatic notch. A continuation of the medial border of the ligament-the processus falciformis (Fig. 314)-runs upwards and forwards on the medial aspect of the ramus of the ischium.
The ligamentum sacrotuberosum is believed by many to represent the original or proximal end of the long or ischial head of the biceps femoris muscle.
Ligamentum Sacrospinosum. The sacro-spinous ligament (O.T. small sacrosciatic lig.) (Figs. 314 and 313) is situated in front, and in a measure under cover of the sacro-tuberous ligament. Triangular in form, it is attached by its base to the last two segments of the sacrum and the first segment of the coccyx, and by its pointed apex to the tip and superior aspect of the spina ischiadica. This ligament is intimately associated with the coccygeus muscle, and by some it is regarded as being derived from it by fibrous transformation of the muscle fasciculi.
By the sacro-tuberous and sacro-spinous ligaments the two sciatic notches of the hip bone are converted into foramina. Thus the sacro-spinous ligament (lig. sacrospinosum) completes the boundaries of the greater sciatic foramen (foramen ischiadicum majus); while the sacro- tuberous ligament (lig. sacrotuberosum), assisted by the sacro-spinous ligament (lig. sacrospinosum), closes the lesser sciatic foramen (foramen ischiadicum minus).
SYMPHYSIS OSSIUM PUBIS.
The anterior wall of the osseous pelvis is completed by the articulation of the bodies of the two pubic bones, which constitutes the symphysis pubis. This joint conforms in its construction to the general plan of an amphiarthrosis. Thus it is median in position; each pubic bone is covered by a layer of hyaline cartilage, which closely adapts itself to the rough tuberculated surface of the pubic bone; while between these two hyaline plates there is an interposed fibro-cartilage called the lamina fibrocartilaginea interpubica, in the interior of which there is usually a vertical antero-posterior cleft. This cavity, which is placed nearer the posterior than the anterior aspect of the joint, does not appear until between the seventh and tenth years, and as it is not lined by a synovial stratum, it is supposed to result from the breaking down of the interpubic lamina.
Lig. Pubicum Anterius.-The anterior pubic ligament (Fig. 313) is a structure of considerable thickness and strength. Its superficial fibres, which are derived very largely from the tendons and aponeuroses of adjoining muscles, are oblique, and form an interlaced decussation. The deeper fibres are short, and extend transversely from one pubic bone to the other.
Lig. Pubicum Posterius.-The posterior pubic ligament (Fig. 313) is very weak and consists of scattered fibres which extend transversely between contiguous pubic surfaces posterior to the articulation.
Lig. Pubicum Superius.-The superior pubic ligament also is weak; it consists of transverse fibres passing between the two pubic crests.
Lig. Arcuatum Pubis.-The arcuate ligament of the pubis (O.T. inferior or subpubic ligament) occupies the arch of the pubis, and is of considerable strength. It gives roundness to the pubic arch and forms part of the inferior aperture of the pelvis. It has considerable vertical thickness immediately below the interpubic fibro-cartilage, to which it is attached. Laterally it is attached to adjacent sides of the inferior rami of the pubis. Its inferior border is free, and separated from the upper border of the fascia of the urogenital diaphragm by a transverse oval interval, through which the dorsal vein of the penis passes backwards to the interior of the pelvis.
FASCIA DIAPHRAGMATIS UROGENITALIS INFERIOR.
The inferior fascia of the urogenital diaphragm (O.T. superficial layer of the triangular ligament) is a membranous structure which occupies the pubic arch below and distinct from the arcuate ligament of the pubis. It assists in completing the pelvic walls anteriorly in the same manner that the obturator membrane does laterally. Indeed, these two structures occupy the same morphological plane. The fascia presents two surfaces-one superficial or perineal, the other deep or pelvic— and both of these surfaces are associated with muscles. Its lateral borders are attached to the sides of the pubic arch, while its base is somewhat ill-defined, by reason of its fusion with the fascia of Colles in the urethral region of the perineum.
The apex of the fascia is truncated, free, and well defined, constituting the transverse perineal ligament, above which there is the interval for the dorsal vein of the penis. It is pierced by a number of vessels and nerves, but the principal opening is situated in the median plane one inch below the pubic arch, and transmits the urethra.
The obturator membrane (Fig. 316) occupies the obturator foramen. It is attached to the pelvic aspect of the circumference of this foramen. It consists of fibres irregularly arranged and of varying strength, so that sometimes it almost appears fenestrated. At the highest part of the foramen it is incomplete and forms a U-shaped border, between which and the bony circumference of the foramen the obturator canal is formed. In this position the membrane is continuous with the parietal pelvic fascia which clothes the medial side of the obturator internus muscle, above the superior free margin of the muscle. From the lateral or femoral aspect of the membrane some of its fibres are prolonged to the antero-inferior aspect of the capsule of the hip-joint.
Mechanism and Movements of the Pelvis.-The human pelvis presents a mechanism the principal requirement of which is stability and not movement, for, through the pelvis, the weight of the trunk, superimposed upon the sacrum, is transmitted to the lower limbs. Moreover, its stability is largely concerned in the maintenance of the erect attitude. The movements of its various parts are therefore merely such as are consistent with stability, without producing absolute rigidity.
The two hip bones, being bound together by powerful ligaments at the pubic articulation, constitute an inverted arch, of which the convexity is directed downwards and forwards, while its piers are turned upwards and backwards, and considerably expanded in relation to the posterior parts of the iliac bones. Between the piers of this inverted arch the sacrum is situated. This bone is in no sense a key-stone to an arch, because, as may readily be seen in antero-posterior transverse section, the sacrum is wider in front than behind, and the superposed weight naturally tends to make the sacrum fall towards the pelvic cavity, and so fit less closely between the hip bones. The sacrum is in reality an oblique platform, in contact with each hip bone through its articular auricular surfaces, and in this position it is suspended by the interosseous and posterior sacro-iliac ligaments, and kept securely in place by the "grip" due to the irregularity of the opposed surfaces of the two sacro-iliac articulations. Since the weight of the trunk is transmitted to the anterior and superior end of this sacral platform, there is a natural tendency for the sacrum to revolve upon the transverse axis which passes through its sacro-iliac joints. If this were permitted, the promontory of the sacrum would rotate downwards and forwards towards the pelvic cavity, as really does occur in certain deformities. This revolution or tilting downwards of the anterior part of the sacrum is prevented by the action of the sacrotuberous and sacro-spinous ligaments, extending from the ischial tuberosity to the posterior and inferior end of the suspended platform of the sacrum. Not only so, but these ligaments, acting on a rigid sacrum, tend to hold up the weight upon the sacral promontory.
The various ligaments passing between the last lumbar vertebra and the sacrum and ilium retain the weight of the trunk in position upon the anterior end of the sacrum, and resist its tendency to slip forwards and downwards towards the pelvic cavity. The entire weight of the trunk and pelvis is transmitted to the heads of the thigh bones in the most advantageous position, both for effectiveness and the strengthening of the inverted back of the hip bones, for it will be evident that the heads of the femora thrust inwards upon the convex side of the arch, very much at the place where the arches are weakest, viz., at the springing of the arch from its piers. The forces which tend to cause movement of the pelvic bones during parturition act from within the pelvis, and have for their object the increase of the various pelvic diameters, in order that the foetal head may more readily be transmitted. For this purpose the wedge-like dorsal surface of the sacrum is driven backwards, and a certain amount of extra space may thereby be obtained. An important factor, however, in the increase of the pelvic capacity at this period is found in the relaxation of its various ligaments.
ARTICULATIONES EXTREMITATIS INFERIORIS.
The Hip-Joint.-The human body provides no more perfect example of an enarthrodial diarthrosis than the hip-joint. Combined with all that variety of movement which characterises a multi-axial joint, it nevertheless presents great stability, which has been obtained by simple arrangements, for restricting the range of its natural movements. This stability is of paramount importance for the maintenance of the erect attitude, and the mechanical adaptations whereby this result is obtained are such that the erect attitude may be preserved without any great degree of sustained muscular effort.
Articular Surfaces.-The head of the femur is globular in shape, and considerably exceeds a hemisphere. It is clothed with hyaline articular cartilage on those parts which come into direct contact with the acetabulum. There is frequently more or less of extension of the articular cartilage from the head to the adjoining anterior part of the neck, an extension which is accounted for by the close and constant apposition of this portion of the neck with the posterior aspect of the ilio femoral ligament. The limit of the articular cartilage covering the head is indicated by a sinuous border. Further, there is an absence of articular cartilage from the fovea or pit on the head of the femur.
Capsula articularis (reflected)
The acetabulum is a deep cup-shaped cavity which presents a notch on its anteroinferior margin. The interior of the cup is lined with a ribbon-like band of articular cartilage which extends to the brim of the cavity, but does not cover its floor. This articular ribbon-shaped band is widest on its superoposterior aspect, and narrowest at the anterior margin of the acetabular notch.
Lig. Transversum Acetabuli.-The transverse ligament (Fig. 315) bridges the acetabular notch, and consists of strong transverse fibres which are attached to
FIG. 315.-DISSECTION OF THE HIP-JOINT. Bottom of the acetabulum removed, and capsule of the joint thrown laterally towards the trochanters.