The five vertebras composing the sacrum form a triangular and anteriorly concave piece, perforated on either side, both in front and behind, by four foramina, for the transmission of the anterior and posterior branches of the sacral nerves. The centra become smaller from above downwards. In the three superior vertebras the neural arch is complete; in the two inferior, the neurapophyses are unconnected by a spine, and the canal is open behind.
In front each sacral vertebra presents a rudimentary pleurapophysis, coalesced with the centrum and the diapophysis precisely as has been described in the lumbar vertebras. These pleurapophyses are distinct bones in the foetus, at which time it can be seen that the ilium belongs to one vertebra only, namely, the first sacral, the hasmal arch of which it assists in forming. The serial homology of the ilium and pubis may be explained, according to Professor Owen,# in either of two ways. Either the ilium is the haemapophysis, and the pubis the haemal spine; or the pleurapophysial element of the sacrum with the ilium forms a divided and articulated rib, and the pubis represents the haemapophysis, the haemal spine being reduced to the cartilaginous structure in the pubic symphysis.
The ischium constitutes the haemapophysis of the second sacral vertebra, and may be excluded from the consideration of the haemal arch of the first sacral vertebra.
In the human skeleton the highly-developed condition of the iliac bones obscures, in some degree, their typical connections; but if the pelvis of the lower vertebrata be examined, these points become far more easily ascertained. The pleurapophysial character of the ilium and ischium, with the mode of formation of the obturator foramen, primitively an intercostal space, is well seen in the skeleton of birds, or of chelonian reptiles, in whom the muscles which arise from the pelvis have no need of the broad and extensive surface of attachment common to animals whose inferior extremities are the chief means employed for quick and varied movement.