The ischium () forms the lower and back region of the

hip bone The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, isch ...

(''os coxae''). Situated below the ilium and behind the pubis, it is one of three regions whose fusion creates the

coxal bone The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, ischi ...

. The superior portion of this region forms approximately one-third of the acetabulum.

Structure

The ischium is made up of three parts–the body, the superior ramus and the inferior ramus. The body contains a prominent spine, which serves as the origin for the

superior gemellus muscle The gemelli muscles are the inferior gemellus muscle and the superior gemellus muscle, two small accessory fasciculi to the tendon of the internal obturator muscle. The gemelli muscles belong to the lateral rotator group of six muscles of the h ...

. The indentation inferior to the spine is the

lesser sciatic notch Below the ischial spine is a small notch, the lesser sciatic notch; it is smooth, coated in the recent state with cartilage, the surface of which presents two or three ridges corresponding to the subdivisions of the tendon of the Obturator internu ...

. Continuing down the posterior side, the

ischial tuberosity The ischial tuberosity (or tuberosity of the ischium, tuber ischiadicum), also known colloquially as the sit bones or sitz bones, or as a pair the sitting bones, is a large swelling posteriorly on the superior ramus of the ischium. It marks th ...

is a thick, rough-surfaced prominence below the lesser sciatic notch. This is the portion that supports weight while sitting (especially noticeable on a hard surface) and can be felt simply by sitting on the fingers. It serves as the origin for the

inferior gemellus muscle The gemelli muscles are the inferior gemellus muscle and the superior gemellus muscle, two small accessory fasciculi to the tendon of the internal obturator muscle. The gemelli muscles belong to the lateral rotator group of six muscles of the h ...

and the

hamstring In human anatomy, a hamstring () is any one of the three posterior thigh muscles in between the hip and the knee (from medial to lateral: semimembranosus, semitendinosus and biceps femoris). The hamstrings are susceptible to injury. In quadrupe ...

s. The superior ramus is a partial origin for the internal obturator and the

external obturator muscle The external obturator muscle, obturator externus muscle (; OE) is a flat, triangular muscle, which covers the outer surface of the anterior wall of the pelvis. It is sometimes considered part of the medial compartment of thigh, and sometimes ...

s. The inferior ramus serves partially as origin for part of the

adductor magnus muscle Adductor may refer to: * One of the anatomical terms of motion * Adductor muscle (disambiguation) * Adductor canal The adductor canal, also known as the subsartorial canal or Hunter’s canal, is an aponeurotic tunnel in the middle third of the t ...

and the

gracilis muscle The gracilis muscle (; Latin for "slender") is the most superficial muscle on the medial side of the thigh. It is thin and flattened, broad above, narrow and tapering below. Structure It arises by a thin aponeurosis from the anterior margins o ...

. The inferior ischial ramus joins the inferior ramus of the pubis anteriorly and is the strongest of the hip (coxal) bones.

Body

The body enters into and constitutes a little more than two-fifths of the acetabulum. Its external surface forms part of the lunate surface of the acetabulum and a portion of the

acetabular fossa The acetabular fossa is a fossa located at the centre of the acetabulum. It is occupied by the ligament of head of femur. In contrast to the thick and smooth surrounding lunate surface, the pelvis' articulation with the head of the femur, the ac ...

. Its internal surface is part of the wall of the

lesser pelvis The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor. The pelvic cavity primarily contains the reprod ...

; it gives origin to some fibers of the internal obturator. No muscles

insert Insert may refer to: *Insert (advertising) *Insert (composites) *Insert (effects processing) *Insert (filmmaking) *Insert key on a computer keyboard, used to switch between insert mode and overtype mode *Insert (molecular biology) *Insert (SQL) *Fi ...

on the body. Its anterior border projects as the posterior obturator tubercle. From its posterior border there extends backward a thin and pointed triangular eminence, more or less elongated in different subjects, the

ischial spine The ischial spine is part of the posterior border of the body of the ischium bone of the pelvis. It is a thin and pointed triangular eminence, more or less elongated in different subjects. Structure The pudendal nerve travels close to the ischia ...

, origin of the

gemellus superior muscle The gemelli muscles are the inferior gemellus muscle and the superior gemellus muscle, two small accessory fasciculi to the tendon of the internal obturator muscle. The gemelli muscles belong to the lateral rotator group of six muscles of the hip ...

. Above the spine is a large notch, the

greater sciatic notch The greater sciatic notch is a notch in the ilium, one of the bones that make up the human pelvis. It lies between the posterior inferior iliac spine (above), and the ischial spine (below). The sacrospinous ligament changes this notch into an open ...

; Below the spine is a smaller notch, the

Superior ramus

The superior ramus of the ischium (descending ramus) projects downward and backward from the body and presents for examination three surfaces: external, internal, and posterior. The external surface is quadrilateral in shape. It is bounded above by a groove that lodges the tendon of the external obturator; below, it is continuous with the inferior ramus; in front it is limited by the posterior margin of the

obturator foramen The obturator foramen (Latin foramen obturatum) is the large opening created by the ischium and pubis bones of the pelvis through which nerves and blood vessels pass. Structure It is bounded by a thin, uneven margin, to which a strong membran ...

; behind, a prominent margin separates it from the posterior surface. In front of this margin the surface gives origin to the

quadratus femoris The quadratus femoris is a flat, quadrilateral skeletal muscle. Located on the posterior side of the hip joint, it is a strong external rotator and adductor of the thigh, but also acts to stabilize the femoral head in the acetabulum. Quadratu ...

, and anterior to this to some of the fibers of origin of the external obturator; the lower part of the surface gives origin to part of the

adductor magnus Adductor may refer to: * One of the anatomical terms of motion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminolo ...

. The internal surface forms part of the bony wall of the lesser pelvis. In front it is limited by the posterior margin of the

. Below, it is bounded by a sharp ridge that provides attachment to a

falciform The ''falx'' was a weapon with a curved blade that was sharp on the inside edge used by the Thracians and Dacians. The name was later applied to a siege hook used by the Romans. Etymology ''Falx'' is a Latin word originally meaning 'sickle' b ...

prolongation of the sacrotuberous ligament, and, more anteriorly, gives origin to the transverse perineal and

ischiocavernosus The ischiocavernosus muscle (erectores penis ''or'' erector clitoridis in older texts) is a muscle just below the surface of the perineum, present in both men and women. Structure It arises by tendinous and fleshy fibers from the inner surface of ...

muscles. Posteriorly the ramus forms a large swelling, the

tuberosity of the ischium The ischial tuberosity (or tuberosity of the ischium, tuber ischiadicum), also known colloquially as the sit bones or sitz bones, or as a pair the sitting bones, is a large swelling posteriorly on the superior ramus of the ischium. It marks ...

, where the

s originate.

Inferior ramus

The inferior ramus of the ischium (ascending ramus) is the thin, flattened part of the ischium, which ascends from the superior ramus, and joins the inferior ramus of the pubis—the junction being indicated in the adult by a raised line. The outer surface is uneven for the origin of the

obturator externus The external obturator muscle, obturator externus muscle (; OE) is a flat, triangular muscle, which covers the outer surface of the anterior wall of the pelvis. It is sometimes considered part of the medial compartment of thigh, and sometimes c ...

and some of the fibers of the

; its inner surface forms part of the anterior wall of the pelvis. Its medial border is thick, rough, slightly everted, forms part of the outlet of the pelvis, and presents two ridges and an intervening space. The ridges are continuous with similar ones on the inferior ramus of the pubis: to the outer is attached the deep layer of the superficial perineal fascia (fascia of Colles), and to the inner the inferior fascia of the urogenital diaphragm. Tracing these two ridges downward, they join with each other just behind the point of origin of the

transverse perineal muscles The transverse perineal muscles (transversus perinei) are the superficial and the deep transverse perineal muscles. Superficial transverse perineal ...

. Here the two layers of fascia are continuous behind the posterior border of the muscle. To the intervening space, just in front of the point of junction of the ridges, the transverse perineal attaches, and in front of this is a portion of the

, and the

crus penis For their anterior three-fourths the corpora cavernosa penis lie in intimate apposition with one another, but behind they diverge in the form of two tapering processes, known as the crura, which are firmly connected to the ischial rami. Traced ...

in the male, or the

crus clitoridis The clitoral crura (singular: clitoral crux) are two erectile tissue structures, which together form a V-shape. ''Crus'' is a Latin word that means "leg". Each "leg" of the ''V'' converges on the clitoral body. At each divergent point is a corp ...

in the female. Its lateral border is thin and sharp, and forms part of the medial margin of the

Clinical significance

Clinically, an

avulsion fracture An avulsion fracture is a bone fracture which occurs when a fragment of bone tears away from the main mass of bone as a result of physical trauma. This can occur at the ligament by the application of forces external to the body (such as a fa ...

of the

may occur. Avulsion fractures of the hip bone (avulsion or tearing away of the ischial tuberosity) may occur in adolescents and young adults during sports that require sudden acceleration or deceleration forces, such as sprinting or kicking in football, soccer, jumping hurdles, basketball, and martial arts. These fractures occur at

tubercles In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projection ...

(bony projections that lack secondary

ossification center An ossification center is a point where ossification of the cartilage begins. The first step in ossification is that the cartilage cells at this point enlarge and arrange themselves in rows.Gray and Spitzka (1910), page 44. The matrix in which t ...

s). Avulsion fractures occur where muscles are attached: anterior superior and inferior iliac spines, ischial tuberosities, and ischiopubic rami. A small part of bone with a piece of a tendon or ligament attached is avulsed (torn away).

Ischial bursitis Ischial bursitis (also known as weaver's bottom) is inflammation of the synovial bursa located between gluteus maximus muscle and ischial tuberosity. Signs and symptoms Ischial bursitis causes pain down the posterior aspect of the upper thigh. ...

(also known as ''weaver's bottom'') is inflammation of the

synovial bursa Synovial () may refer to: * Synovial fluid * Synovial joint * Synovial membrane The synovial membrane (also known as the synovial stratum, synovium or stratum synoviale) is a specialized connective tissue that lines the inner surface of capsule ...

located between the

gluteus maximus muscle The gluteal muscles, often called glutes are a group of three muscles which make up the gluteal region commonly known as the buttocks: the gluteus maximus, gluteus medius and gluteus minimus. The three muscles originate from the ilium and sacr ...

and the ischial tuberosity, and is usually caused by prolonged sitting on a hard surface.

History

Adoption of ''ischium'' into English-language medical literature dates back to c. 1640; the Latin term derives from Greek ἰσχίον ''iskhion'' meaning "hip joint". The division of the ''acetabulum'' into ''ischium'' (ἰσχίον) and ''ilium'' (λαγών, ''os lagonicum'') is due to

Galen Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be one of ...

, ''De ossibus''. Galen, however, omits mention of the ''pubis'' as a separate bone.

Other animals

Dinosaurs

The

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

ia is divided into the

Saurischia Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithis ...

and Ornithischia based on hip structure, including importantly that of the ischium. In the majority of dinosaurs, the ischium extends down from the ilium and towards the tail of the animal. The acetabulum, which can be thought of as a "hip-socket", is a cup-shaped 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.Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. . Ornithischia pelvis structure.svg, Ornithischian pelvic structure (left side) Saurischia pelvis structure.svg, Saurischian pelvic structure (left side).

Additional images

Gray235.png, Right hip bone. External surface. Gray236.png, Right hip bone. Internal surface. Gray237.png, Plan of ossification of the hip bone. Gray241.png Gray436.png, The Obturator externus. Skeletal pelvis-pubis.svg, Pelvis

References

* Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. 5th ed. McGraw-Hill Science Engineering, 2009. Print.

External links

* - "The Male Peniel: Hip Bone" * {{Authority control Bones of the pelvis