On either side of the body of the base of the sacrum

Base of the sacrum

The base of the sacrum, which is broad and expanded, is directed upward and forward.In the middle is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral...

 is a large triangular surface, which supports the Psoas major and the lumbosacral trunk

Lumbosacral trunk

The lumbosacral trunk is nervous tissue that connects the lumbar plexus with the sacral plexus.-Structure:The lumbosacral trunk comprises the whole of the anterior division of the fifth and a part of that of the fourth lumbar nerve; it appears at the medial margin of the psoas major and runs...

, and in the articulated pelvis is continuous with the iliac fossa

Iliac fossa

The iliac fossa is a large, smooth, concave surface located on the internal surface of the ilium...

. This is called the ala.

It is slightly concave from side to side, convex from before backward, and gives attachment to a few of the fibers of the Iliacus.

The posterior fourth of the ala represents the transverse process, and its anterior three-fourths the costal process of the first sacral segment.

It serves as part of the border of the pelvic brim

Pelvic brim

The pelvic brim is the edge of the pelvic inlet. It is an approximately apple-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis....

.Additionally ala also forms the base of the lumbosacral triangle.

Structures Attached to Ala:

1)Iliolumbar ligament at the lateral portion of tuberosity.

2)Lumbosacral ligament at the medial portion of the rough portion.

3)Origin of a few fibres of the illiacus from the smooth part


Structures in relation:
a)sympathetic trunk

b)lumbosacral trunk

c)iliolumbar artery (branch of posterior division of internal iliac artery)

d)obturator nerve