occipital, bone forming the back and back part of the base of the cranium, the part of the skull that encloses the brain. It has a large oval opening, the foramen magnum, through which the medulla oblongata passes, linking the spinal cord and brain. The occipital adjoins five of the other seven bones forming the cranium: at the back of the head, the two parietal bones; at the side, the temporal bones; and in front, the sphenoid bone, which also forms part of the base of the cranium. The occipital is concave internally to hold the back of the brain and is marked externally by nuchal (neck) lines where the neck musculature attaches. The occipital forms both in membrane and in cartilage; these parts fuse in early childhood. The seam, or suture, between the occipital and the sphenoid closes between ages 18 and 25, that with the parietals between ages 26 and 40.
In four-footed animals the head hangs from the end of the vertebral column, and the foramen magnum is placed posteriorly. The nuchal musculature is strongly developed to support the head, and the occipital markings are heavy.
In apes, with the assumption of semierect posture, the foramen has moved partially downward and forward. The nuchal muscles are powerful and attached high up on the occipital near the suture with the parietals, where a crest (lambdoidal crest) sometimes forms. In human evolution, the foramen magnum has continued to move forward as an aspect of adaptation to walking on two legs, until the head is now balanced vertically on top of the vertebral column. Concurrently, the line of attachment of the nuchal musculature has moved downward from the lambdoidal suture to a point low on the back of the head. In such precursors of man as Australopithecus and Homo erectus, the nuchal markings, often heavy enough to form a protuberance, or torus, were intermediate in position between those in apes and those in modern man.