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Pelvis

Pelvis

The pelvis is the area of transition between the trunk and the lower limbs. It is located inferoposterior to the abdomen, and is associated with various structures including the pelvic bones and the lower parts of the vertebral column. Externally, it is covered by the inferior portion of the abdominal wall anteriorly, the gluteal region of the lower limb posteriorly, and the perineum inferiorly. The pelvis is subdivided into two parts: the greater (false) pelvis and the lesser (true) pelvis.

Divisions

The greater pelvis is considered part of the abdomen. It is the superior portion associated with the upper parts of the pelvic bone and lower lumbar vertebrae. The lesser pelvis is the inferior portion associated with the lower parts of the pelvic bone, the sacrum, and the coccyx. It has both an inlet and an outlet. The pelvic cavity is comprised of the pelvic inlet, walls and floor. It is a bowl shaped structure that is surrounded by the lesser pelvis, and is continuous with the abdominal cavity superiorly. The pelvic cavity contains the bladder, the sigmoid colon, the rectum, the anal canal, and most of the reproductive organs of both men and woman. The perineum lies inferior to the floor of the pelvic cavity, and forms the pelvic outlet with its boundaries. It contains the external genitalia and the external openings of the genitourinary and gastrointestinal systems.

Bones

The bony framework of the pelvis is composed of the right and left pelvic (hip) bones, the sacrum, and the coccyx. The pelvic bone is irregular in shape and formed by three bones: the ilium, the pubis, and the ischium. These bones are interconnected at birth via cartilage surrounding the area of the acetabulum. The acetabulum is a large articular socket located at the lateral surface of the pelvic bone. Together with the head of the femur, it forms the hip joint. Later on, between the ages of 16 and 18, the ilium, pubis, and ischium fuse together into a single bone. The pelvic bones articulate with the sacrum posteriorly at the sacro-iliac joints, and anteriorly with each other at the pubic symphysis. The sacrum is formed by the fusion of the five sacral vertebrae, and has the shape of an inverted triangle. Its base articulates with the fifth lumbar vertebra (L5), while its apex articulates with the coccyx. The coccyx is the small terminal part of the vertebral column. It formed by the fusion of the four coccygeal vertebrae, and like the sacrum, has the shape of an inverted triangle.

Muscles

The pelvis is associated with various muscles. The flat shaped obturator internus and the triangular shaped piriformis contribute to the lateral walls of the pelvic cavity. The pelvic floor muscles on the other hand, include the pelvic diaphragm and the layers of skeletal muscles present in the deep perineal pouch. The pelvic diaphragm consists of the levator ani and the coccygeus muscles. The pelvic floor separates the pelvic cavity above, from the perineum below.

Neurovasculature

The pelvis and perineum are supplied by the internal iliac artery present on each side. It originates at the bifurcation of the common iliac artery into its internal and external branches. The gonadal, median sacral, and superior rectal arteries also contribute to the arterial supply of the pelvis. The venous drainage mostly follows the course of its corresponding pelvic arteries. The sacral and coccygeal nervous plexuses provide innervation for the muscles of the pelvis.

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