The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates
including mammals and birds, but not bony fish.
All reptiles have an ilium except snakes, although some snake species have a tiny bone
which is considered to be an ilium.
The name comes from the Latin, meaning "groin" or "flank."
The ilium of the human is divisible into two parts, the body and the ala; the separation
is indicated on the top surface by a curved line, the arcuate line, and on the external
surface by the margin of the acetabulum.
Structure Body
The body enters into the formation of the acetabulum, of which it forms rather less
than two-fifths.
Its external surface is partly articular, partly non-articular; the articular segment
forms part of the lunate surface of the acetabulum, the non-articular portion contributes to the
acetabular fossa.
The internal surface of the body is part of the wall of the lesser pelvis and gives origin
to some fibers of the obturator internus.
Below, it is continuous with the pelvic surfaces of the ischium and pubis, with only a faint
line indicating the place of union.
The body of ilium together with the wing forms the ilium.
Ala
The wing of ilium is the large expanded portion which bounds the greater pelvis laterally.
It presents for examination two surfaces—an external and an internal—a crest, and two
borders—an anterior and a posterior.
Development In other vertebrates
Dinosaurs The clade Dinosauria is divided into the Saurischia
and Ornithischia based on hip structure, including importantly that of the ilium.
In both Saurischians and Ornithischians, the ilium extends laterally to both sides from
the axis of the body.
The other two hip bones, the ischium and the pubis, extend ventrally down from the ilium
towards the belly of the animal.
The acetabulum, which can be thought of as a "hip-socket", is an opening on each side
of the pelvic girdle formed where the ischium, ilium, and pubis all meet, and into which
the head of the femur inserts.
The orientation and position of the acetabulum is one of the main morphological traits that
caused dinosaurs to walk in an upright posture with their legs directly underneath their
bodies.
The brevis shelf is a bony ridge running along the internal face of the rear part of the
ilium, which functions as an attachment area for a tail muscle, the caudofemoralis brevis.
The brevis fossa is a depressed area on the brevis shelf.
Clinical significance Biiliac width
In humans, biiliac width is an anatomical term referring to the widest measure of the
pelvis between the outer edges of the upper iliac bones.
Biiliac width has the following common synonyms: pelvic bone width, biiliac breadth, intercristal
breadth/width, bi-iliac breadth/width and biiliocristal breadth/width.
It is best measured by anthropometric calipers.
Attempting to measure biiliac width with a tape measure along a curved surface is inaccurate.
The biiliac width measure is helpful in obstetrics because a pelvis that is significantly too
small or too large can have obstetrical complications.
For example, a large baby or a small pelvis often lead to death unless a caesarean section
is performed.
It is also used by anthropologists to estimate body mass.
History Etymology
Os ilium The 'English' name ilium as bone of the pelvis
can be traced back to the writings of anatomists Andreas Vesalius, whom coined the expression
os ilium.
In this expression ilium can be considered as the genitive plural of the nominative singular
of the noun ile.
Ile in classical Latin can refer to the flank of the body, or to the groin, or the part
of the abdomen from the lowest ribs to the pubes.
Ile is usually encountered as plural in classical Latin.
The os ilium can literally be translated as bone of the flanks.
Origin in Ancient Greek More than a millennium earlier the ossa ilium
were described by the Greek physician Galen, and referred to as, with a quite similar expression,
τά πλατέα λαγόνων ὀστᾶ, the flat bones of the flanks, with λαγών
for flank.
In anatomic Latin, the expression os lagonicum can also be found, based on Ancient Greek
λαγών.
In modern Greek the nominalized adjective λαγόνιο is used to refer to the os
ilium.
Ilium In Latin and Greek it is not uncommon to nominalize
adjectives, e.g. stimulantia from remedia stimulantia or ὁ ἐγκέφαλος from
ὁ ἐγκέφαλος μυελός.
The name ilium as used in English can not be considered as nominalized adjective derived
from the full Latin expression os ilium, as ilium in this expression is a genitive plural
of a noun and not a nominative singular of an adjective.
The form ilium in English is however thought to be derived from the Latin word ilium, an
orthographic variant in Latin of ile, flank or groin.
Whereas the expression of Andreas Vesalius os ilium appropriately expresses bone of the
flanks, the sole term ilium as used in English, lacks this precision and has to be literally
translated as groin or flank.
There exists however in classical Latin an adjective iliusilium.
This adjective however means not with respect to the flanks, but Trojan.
Troy is referred to in classical Latin as Ilium, Ilion or Ilios and in ancient Greek
as Ἴλιον or Ἴλιος.
Usage in official Latin nomenclature The first editions of the official Latin nomenclature,
Nomina Anatomica of the first 80 years used the Vesalian expression os ilium.
In the subsequent editions from 1983 and 1989 the expression os ilium was altered to os
ilii.
This latter expression supposes a genitive singular of the alternate noun ilium instead
of a genitive plural of the noun ile.
Quite inconsistently, in the 1983 edition of the Nomina Anatomica the genitive plural
of ile is still being used in such expressions as vena circumflexa ilium superficialis.
In the current 1998 edition of the Nomina Anatomica, rebaptized as Terminologia Anatomica
the expression os ilium is reintroduced and os ilii deleted.
Additional images
See also This article uses anatomical terminology;
for an overview, see anatomical terminology.
Iliac crest References
External links Anatomy photo:44:st-0701 at the SUNY Downstate
Medical Center pelvis at The Anatomy Lesson by Wesley Norman
This article incorporates text from a public domain edition of Gray's Anatomy.