Pelvic Girdle

PELVIC GIRDLE in most fish is developed very weakly and has the appearance of a small paired plate lying in the abdominal wall of the body. Only in cartilaginous fish are these plates more massive, connected to each other along the midline, and sometimes possess a small ascending process (processus iliacus). In terrestrial vertebrates, the pelvic girdle develops quite significantly. Its dorsal section (processus iliacus) elongates and gains support on the ribs of the spine. The sacral ribs that provide this support shorten and fuse with the lateral processes, and thus the support becomes increasingly solid. On the ventral side, the elements of the right and left sides are connected to each other by a strong and wide symphysis. Already in the most ancient terrestrial vertebrates, three ossification centers develop in the pelvic girdle, converging in the region of the acetabulum: in the dorsal section—the ilium (os ilium); in the ventral section anteriorly (cranially)—the pubis (os pubis); and posteriorly (caudally)—the ischium (os ischium). Between the latter two bones, a more or less large window develops in reptiles, covered by a connective tissue membrane (fenestra ischio-pubica). In the region of the pubic bones in amphibians and most reptiles, there is also a small separate opening for the passage of a nerve (foramen obturatorium). In some fossil reptiles, this opening was located on the border between the pubic and ischial bones, and in mammals, it expands into a large window (foramen obturatum), similar in appearance to the aforementioned window in the girdle of reptiles. In birds, the pelvic girdle is developed quite uniquely, which is connected, on the one hand, with the method of movement—birds walk on only two hind limbs—and, on the other hand, with the size of the eggs laid. Therefore, we have, first of all, a very solid connection and even fusion of the wide iliac bones with a large and complex sacrum over a large extent of the spine, as well as the fusion of all pelvic bones into a single bone (os innominatum). In addition to this, not only the ischial but also the pubic bones are strongly elongated and directed backward, and neither the former nor the latter form a ventral symphysis. In mammals, the structure of the pelvis is less altered. In addition to the three typical bones, a special small bone (os acetabulare) develops in the anterior part of the acetabulum. In the adult animal, all these bones fuse together into a single bone (os coxae) on each side. The general configuration and position of the mammalian pelvis are quite characteristic. The iliac bones usually elongate in length, the entire pelvis becomes lighter and rotates its ventral part in a caudal direction, so that the connection with the sacrum ends up in front of the acetabulum. However, in mammals with a massive body (elephant, hippopotamus), the iliac bones widen again, and the acetabulum is positioned directly vertically under the sacrum. Such shortening and widening of the iliac bones in anthropoid apes and in humans is associated with the transition to bipedalism.