What Is the Pelvic Brim?

The sacrum, the hipbone, in mammals is the large, upwardly extending bone that forms the lateral half of the pelvis. The pelvis is made up of two hip bones and a sacrum (and coccyx). The hip bone is composed of the sacrum, ischium, and pubic bone. The three bones in the adult form a bone.

The sacrum is a large, irregular bone that swells irregularly from the center to above and below. In some vertebrates (including prepubertal humans), it consists of three parts: the sacrum, the ischium, and the pubic bone.

The two hip bones are connected at the pubic symphysis and together with the sacrum and coccyx (the pelvic part of the spine) form the skeletal component of the pelvis, the pelvic band surrounding the pelvic cavity. They are connected to the sacrum at the sacroiliac joint, which is part of the axial skeleton. Each hip bone is connected to the hip of the corresponding femur (thigh bone) (which forms the main connection between the lower extremity bone and the axon skeleton) through a large ball and socket joint. ^[1]

The hip bone is made up of three parts: the sacrum, the ischium, and the pubic bone. At birth, these three parts are separated by hyaline cartilage. They are connected to each other in the Y-shaped portion of cartilage in the acetabulum. By the end of puberty, the three areas will merge together and by the age of 25 they will harden. The two hip bones are fused to each other at the pubic symphysis. ^[2]

The hip bone first appeared in fish, where it consisted of a simple, usually triangular, bone with which the pelvic fins joined. The hip bones on each side are usually connected to each other at the front end, and even fused firmly in lung fish and sharks, but they never attach to the spine.

In early tetrapods, this early hip bone evolved into the ischium and pubic bone, while the sacrum formed a new structure, initially in the shape of a rod, but soon with larger bony blades. The acetabulum already exists where the three bones meet. In these early forms, the connection to the spine was incomplete, and a pair of ribs connected the two structures; nevertheless, the pelvis had formed a complete ring in most subsequent forms.

In practice, modern amphibians and reptiles have substantially changed this ancestral structure based on their different forms and lifestyles. The obturator is usually very small in these animals, although most reptiles do have a large gap between the pubic bone and the ischium, called the thyroid window, which appears similar to the obturator of mammals. In birds, the pubic symphysis is only found in ostriches, and the two hip bones are usually widely separated, making it easier to produce large eggs.

In therapsids, the hip bone rotates counterclockwise, moving the sacrum forward and the pubis and ischium backward relative to its position in the reptile. The same pattern can be seen in all modern mammals, and the thyroid window and closed cells will merge to form a single space. The sacrum is usually a narrow triangle in mammals, but the sacrum is larger in ungulates and humans, where it holds strong gluteal muscles. Monotremes and marsupials also have a fourth pair of bones, prepreg tubes or "marsupial bones" that extend forward from the pubic bone and help support abdominal muscles, and support sachets in marsupials. In placental mammals, the entire pelvis is usually wider in women than in men to allow young people to be born.

Cetacean pelvic bones were previously thought to be degenerate, but they are now known to play a role in sexual selection. ^[1]