Pelvic Structure

The pelvis (: pelves or pelvises) is the lower part of an anatomical trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis or pelvic skeleton).

The pelvic region of the trunk includes the bony pelvis, the pelvic cavity (the space enclosed by the bony pelvis), the pelvic floor, below the pelvic cavity, and the perineum, below the pelvic floor. The pelvic skeleton is formed in the area of the back, by the sacrum and the coccyx and anteriorly and to the left and right sides, by a pair of hip bones.

The two hip bones connect the spine with the lower limbs. They are attached to the sacrum posteriorly, connected to each other anteriorly, and joined with the two femurs at the hip joints. The gap enclosed by the bony pelvis, called the pelvic cavity, is the section of the body underneath the abdomen and mainly consists of the reproductive organs and the rectum, while the pelvic floor at the base of the cavity assists in supporting the organs of the abdomen.

In mammals, the bony pelvis has a gap in the middle, significantly larger in females than in males. Their offspring pass through this gap when they are born.

Structure

The pelvic region of the trunk is the lower part of the trunk, between the abdomen and the thighs. It includes several structures: the bony pelvis, the pelvic cavity, the pelvic floor, and the perineum. The bony pelvis (pelvic skeleton) is the part of the skeleton embedded in the pelvic region of the trunk. It is subdivided into the pelvic girdle and the pelvic spine. The pelvic girdle is composed of the appendicular hip bones ( ilium, ischium, and pubis) oriented in a ring, and connects the pelvic region of the spine to the lower limbs. The pelvic spine consists of the sacrum and coccyx.
* the pelvic cavity, typically defined as a small part of the space enclosed by the bony pelvis, delimited by the pelvic brim above and the pelvic floor below; alternatively, the pelvic cavity is sometimes also defined as the whole space enclosed by the pelvic skeleton, subdivided into:
** the greater (or false) pelvis, above the pelvic brim
** the lesser (or true) pelvis, below the pelvic brim
* the pelvic floor (or pelvic diaphragm), below the pelvic cavity
* the perineum, below the pelvic floor

Pelvic bone

The pelvic skeleton is formed posteriorly (in the area of the back), by the sacrum and the coccyx and laterally and anteriorly (forward and to the sides), by a pair of hip bones.
Each hip bone consists of three sections: ilium, ischium, and pubis. During childhood, these sections are separate bones, joined by the triradiate cartilage. During puberty, they fuse together to form a single bone.

Pelvic cavity

The pelvic cavity is a body cavity that is bounded by the bones of the pelvis and which primarily contains reproductive organs and the rectum.

A distinction is made between the lesser or true pelvis inferior to the terminal line, and the greater or false pelvis above it. The pelvic inlet or superior pelvic aperture, which leads into the lesser pelvis, is bordered by the promontory, the arcuate line of ilium, the iliopubic eminence, the pecten of the pubis, and the upper part of the pubic symphysis. The pelvic outlet or inferior pelvic aperture is the region between the subpubic angle or pubic arch, the ischial tuberosities and the coccyx.

* Ligaments: obturator membrane, inguinal ligament (lacunar ligament, iliopectineal arch)
Alternatively, the pelvis is divided into three planes: the inlet, midplane, and outlet.

Pelvic floor

The pelvic floor has two inherently conflicting functions: One is to close the pelvic and abdominal cavities and bear the load of the visceral organs; the other is to control the openings of the rectum and urogenital organs that pierce the pelvic floor and make it weaker. To achieve both these tasks, the pelvic floor is composed of several overlapping sheets of muscles and connective tissues.''Thieme Atlas of Anatomy'' (2006), p. 137

The pelvic diaphragm is composed of the levator ani and the coccygeus muscle. These arise between the symphysis and the ischial spine and converge on the coccyx and the anococcygeal ligament which spans between the tip of the coccyx and the anal hiatus. This leaves a slit for the anal and urogenital openings. Because of the width of the genital aperture, which is wider in females, a second closing mechanism is required. The urogenital diaphragm consists mainly of the deep transverse perineal which arises from the inferior ischial and pubic rami and extends to the urogenital hiatus. The urogenital diaphragm is reinforced posteriorly by the superficial transverse perineal.Platzer (2004), p. 106

The external anal and urethral sphincters close the anus and the urethra. The former is surrounded by the bulbospongiosus which narrows the vaginal introitus in females and surrounds the corpus spongiosum in males. Ischiocavernosus squeezes blood into the corpora cavernosa penis and clitoridis.''Thieme Atlas of Anatomy'' (2006), p. 136

Variation

Modern humans are to a large extent characterized by bipedal locomotion and large brains. Because the pelvis is vital to both locomotion and childbirth, natural selection has been confronted by two conflicting demands: a wide birth canal and locomotion efficiency, a conflict referred to as the "obstetrical dilemma". The female pelvis, or gynecoid pelvis, has evolved to its maximum width for childbirth—a wider pelvis would make human females unable to walk. In contrast, human male pelvises are not constrained by the need to give birth and therefore are more optimized for bipedal locomotion.

The principal differences between male and female true and false pelvis include:
* The female pelvis is larger and broader than the male pelvis which is taller, narrower, and more compact.''Thieme Atlas of Anatomy'', (2006), p. 113 The female pelvis is lighter and thinner than the male pelvis.
* The female inlet is larger and oval in shape, while the male sacral promontory projects further (i.e. the male inlet is more heart-shaped).
* The sides of the male pelvis converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart.
* The angle between the inferior pubic rami is acute (70 degrees) in males, but obtuse (90–100 degrees) in females. Accordingly, the angle is called subpubic angle in males and pubic arch in females. Additionally, the bones forming the angle/arch are more concave in females but straight in males.
* The distance between the ischia bones is small in males, making the outlet narrow, but large in females, who have a relatively large outlet. The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males. The greater sciatic notch is wider in females.
* The iliac crests are higher and more pronounced in males, making the male false pelvis deeper and more narrow than in females.
* The male sacrum is long, narrow, more straight, and has a pronounced sacral promontory. The female sacrum is shorter, wider, more curved posteriorly, and has a less pronounced promontory.
* The acetabula are wider apart in females than in males. In males, the acetabulum faces more laterally, while it faces more anteriorly in females. Consequently, when males walk the leg can move forwards and backwards in a single plane. In females, the leg must swing forward and inward, from where the pivoting head of the femur moves the leg back in another plane. This change in the angle of the femoral head gives the female gait its characteristic (i.e. swinging of hips).

Development

Each side of the pelvis is formed as cartilage, which ossifies as three main bones which stay separate through childhood: ilium, ischium, pubis. At birth the whole of the hip joint (the acetabulum area and the top of the femur) is still made of cartilage (but there may be a small piece of bone in the great trochanter of the femur); this makes it difficult to detect congenital hip dislocation by X-raying.

There is preliminary evidence that the pelvis continues to widen over the course of a lifetime.

Functions

The skeleton of the pelvis is a basin-shaped ring of bones connecting the vertebral column to the femora. It is then connected to two hip bones.

Its primary functions are to bear the weight of the upper body when sitting and standing, transferring that weight from the axial skeleton to the lower appendicular skeleton when standing and walking, and providing attachments for and withstanding the forces of the powerful muscles of locomotion and posture. Compared to the shoulder girdle, the pelvic girdle is thus strong and rigid.

Its secondary functions are to contain and protect the pelvic and abdominopelvic viscera (inferior parts of the urinary tracts, internal reproductive organs), providing attachment for external reproductive organs and associated muscles and membranes.

As a mechanical structure

The pelvic girdle consists of the two hip bones. The hip bones are connected to each other anteriorly at the pubic symphysis, and posteriorly to the sacrum at the sacroiliac joints to form the pelvic ring. The ring is very stable and allows very little mobility, a prerequisite for transmitting loads from the trunk to the lower limbs.''Thieme Atlas of Anatomy'' (2006), p. 112

As a mechanical structure the pelvis may be thought of as four roughly triangular and twisted rings. Each superior ring is formed by the iliac bone; the anterior side stretches from the acetabulum up to the anterior superior iliac spine; the posterior side reaches from the top of the acetabulum to the sacroiliac joint; and the third side is formed by the palpable, and the pubis, and at the time of sexual maturity these bones become fused together, though there is never any movement between them. In humans, the ventral joint of the pubic bones is closed.

Larger apes, such as '' Pongo'' (orangutans), ''Gorilla'' (gorillas), ''Australopithecus afarensis'', and '' Pan troglodytes'' (chimpanzees), have longer three-pelvic planes with a maximum diameter in the sagittal plane.

Evolution

The present-day morphology of the pelvis is inherited from the pelvis of our quadrupedal ancestors. The most striking feature of evolution of the pelvis in primates is the widening and the shortening of the blade called the ilium. Because of the stresses involved in bipedal locomotion, the muscles of the thigh move the thigh forward and backward, providing the power for bi-pedal and quadrupedal locomotion.

The drying of the environment of East Africa in the period since the creation of the Red Sea and the African Rift Valley saw open woodlands replace the previous closed canopy forest. The apes in this environment were compelled to travel from one clump of trees to another across open country. This led to a number of complementary changes to the human pelvis. It is suggested that bipedalism was the result.

Additional images

File:Slide1ADAA.JPG, Diameters of pelvic inlet
File:Gray235.png, Right hip bone. External surface.
File:Pelvic Girdle Anatomy by Jason Christian.webm, Pelvic girdle anatomy

See also

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Notes

References

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External links

{{DEFAULTSORT:Pelvis, human

Flat bones