ARTICUL VTIONS OF THE PELVIS 13
but much depressed about its middle. Ossification takes place
much later'than in the mare, and extends forwards from the
ischial arch. A prominent crest is often formed along its pelvic
aspect, which may greatly fatigue the hand of the obstetrist
by- pressure in manipulating the foetus within the pelvis.
In the sheep, goat, and pig, the ischio-pubic symphysis is
rectilinear, and ossification commences late in life, and is
often absent in animals which have had many young.
The Sacro-Sciatic Ligament completes the lateral wall of the
pelvic cavity behind between the sacrum and innominate
bone. It is a wide four-sided fibrous membrane in which
the white fibres are interwoven. Its upper border is straight,
and fixed to the lateral border of the sacrum and the first few
coccygeal bones. Its lower border is attached to the superior
ischiatic spine and tuber ischii. Between these two points it
forms, with the ischium, a slit-like opening, the lesser sacro-
sciatic foramen, through which the obturator internus
muscle passes. The anterior border between the superior
ischiatic spine and the sacrum bounds, with the ischiatic edge
of the ilium, the greater sacro-sciatic foramen, through which
the gluteal vessels and nerves and the sciatic nerve leave
the pelvis for the thigh. Compression of these nerves against
the bones of the pelvis during parturition may account for
cramp of the hind limbs, or even post-partum paralysis. The
posterior border is not well defined, and is continuous with
the semimembranosus muscle, and above with the fascia over
the muscles of the tail. The inner surface is covered with
peritoneum in its anterior third. In its posterior two-thirds
it is connected with the pelvic organs by means of loose
fibrous tissue. Its outer face is crossed by the sciatic and
gluteal nerves, and the middle gluteus, biceps femoris, and
semitendinosus muscles. In the bitch the ligament has the
form of a strong band, passing from the lateral border of the
sacrum to the tuber ischii.
There are no marked differences in this ligament in other
animals.