At the age of sixteen years, the epiphysial or complementary ossific points begin to form, viz.:— On each articulatina surface of the bodies of the sacral vertebarx is developed, as in the true vertebrae, a horizontal plate of bone, which, after coalescing with the bodies to which they respectively belong, finally (ex cept the first and last) become soldered to each other from below upwards, commencing with the two last vertebrx, at from the six teenth to the eighteenth years, and com pleting the formation of the sacral bone by the union of the two first vertebr, at from the tv,enty-fifth to the thirtieth years. Be tween the eighteenth and the twentieth years begins the formation, by scattered granules, of four lateral plates of bone — one on each side, forming the iliac articular surfaces, opposite to the three first vertebrm— and one on each side, opposite the two last. These unite with the sacral bone about the same time that its upper vertebrm coalesce. The number of complementary points of ossification in the sacrum will thus be found to be fourteen, and the total number of sacral ossific centres thirty-five. M. Weber, however, assigns nine
points of ossification to the first, seven to the second, and five to each of the three lower.
The coccyx is ossified by a single centre for each of its four pieces. Occasionally, in one of the upper are two ossific points. That of the highest piece first appears about the time of birth ; that of the second bone is next evident, according to Beclard, at from five to ten years of age ; the third, at from ten to fifteen years; and the fourth, at from fifteen to twenty-. The two upper first unite together, then the two lower, the bone being consoli dated by the union of the two resulting por tions at various periods of life. In advanced lire, and, more frequently, in the male subject, this bone is often found ankylosed to the sacrum. According to M. Weber, each coc cygeal vertebra has two to four points of ossification.