This joint (Articulatio sacro-iliaca) is a diarthrosis formed between
the auricular surfaces of the sacrum and ilium. These surfaces are not
smooth in the adult, but are marked by eminences and depressions, and
are covered by a thin layer of cartilage. The joint cavity is a mere
cleft, and is often crossed by fibrous bands.
The =capsule= is very close fitting, and is attached around the margins
of the articular surfaces. It is reinforced by the =ventral sacro-iliac
ligament= (Ligamentum sacro-iliacum ventrale), which surrounds the
joint, and is exceedingly strong above.
The =movements= are inappreciable in the adult—stability, not mobility,
being the chief desideratum.
The following ligaments may be regarded as accessory to the joint,
although not directly connected with it.
The =dorsal sacro-iliac ligament= (Ligamentum sacro-iliacum dorsale
breve) is a strong band which is attached to the internal angle (Tuber
sacrale) of the ilium and the summits of the sacral spines.
The =lateral sacro-iliac ligament= (Ligamentum sacro-iliacum dorsale
longum) is a triangular, thick sheet which is attached in front to the
internal angle and border of the ilium above the great sciatic notch,
and below to the lateral border of the sacrum. It blends above with the
dorsal sacro-iliac ligament, below with the sacro-sciatic ligament, and
behind with the coccygeal fascia.
The =sacro-sciatic ligament= (Ligamentum sacrospinosum et tuberosum) is
a quadrilateral sheet which completes the lateral pelvic wall. Its upper
border is attached to the border of the sacrum and the transverse
processes of the first and second coccygeal vertebræ. Its lower border
is attached to the superior ischiatic spine and tuber ischii. Between
these it bridges over the external border of the ischium and completes
the lesser sciatic foramen. The anterior border is concave, and
completes the greater sciatic foramen. The posterior border is fused
with the vertebral head of the semimembranosus muscle.
The =ilio-lumbar ligament= (Ligamentum ilio-lumbale) is a triangular
sheet which attaches the ends of the lumbar transverse processes to the
ventral surface of the ilium below the attachment of the longissimus
muscle (Fig. 184).
[Illustration:
FIG. 153.—LEFT OS COXÆ AND ADJACENT PARTS OF SACRUM AND FEMUR OF
HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]