DEVELOPEMENT. - The innominate bone is developed by three primitive and five com plementary points of ossification.
The three primitive points commence in the three component parts of the bone, the ilium, ischium, and pubis respectively, from a single piece of cartilage of' the general form of the bone. That of .the ilium is placed in the thick, arched rib above the cotyloid cavity, (fig. 79. A, a), being apparent, according to Ctuveilhier, the first in order, about the fiftieth day of fietal life. Bischoff, however, says that the time of' its appearance varies from the second to the fourth month, in dif ferent individuals. About the fifth month, it acquires somewhat of the form of the com plete bone. That of the irchium is placed in the upper part of the descending minus (b), and appears second in order of time, always later than the ilium, about the end of the third month, or, according to Bischoff; the fifth month. That of the pubis appears in the superior branch, near the ilio-pectineal eminence (c), at the end of the fifth month. Bischoff fixes it later than Cruveilhier, at the sixth or seventh month. At the period of birth, the cotyloid cavity is still principally cartilaginous (e), the ascending branch of the ischium, the descending of the pubis, and nearly the whole circumference of the ilium, still remaining in the same condition (d).
At the age of six or seven years, the branches of the ischium and pubis are united by bone. About the time of puberty, as first pointed out by M. Serres, a distinct conzplementray point of ossification appears in the cartilage dividing the bones in the cotyloid cavity, which soon including the whole of the Y shaPed cartilage at this part, and assuming its shape and serrated margins, finally unites them in the raised line before described. According to Meckel, the pubis and ischiurn join first with each other, and the ilium becomes united to them afterwards. At the same time appear the four remaining com plementary points as epiphyses, in the fol lowing order : namely, 1. On the whole length of the iliac crest; 2. At the anterior inferior spine, not constant, and said to be more fre quent in the male than the female ; 3. Along
the whole extent of the tuberosity of the ischium ; 4.. On the symphysial surface of the pubis, said by Bedard to be more often found in the female. All these are soldered ,to the bone, about the twenty-fourth or twenty-fifth year, the epiphysis of the iliac crest being the last to join.
The sacrum is produced by the soldering together of five vertebrae. Hence they have been called the fidse or sacral vertebra . Oc casionally six pieces have been found, and, more rarely, according to Scemmerring, four pieces only are present. Each of these five pieces, as in the other vertebrm, results frotn three primary points of ossification, viz., one for each body or central portion, and two for the posterior lateral surfaces and laminx of each Nertebra. These appear later than in the true vertebrm, and are first ma nifest in the bodies of the three upper (fig. B, a), at the second or third month of fcetal life, and in the two lower at the fourth or fifth month. The lateral points are de veloped between the sixth and ninth fcetal months, and are united to the bodies (each after joining with its fellow opposite at the spinous tubercle) at from the second to the sixth years of age, beainning, according to Quain and Sharpey, at the lowest or fifth ver tebra, and going upwards. Besides these, there are two characteristic points of ossification found in each of the three first sacral vertebrx, which are placed immediately above the three upper anterior sacral holes, exactly in the line of pressure from the ilia to the median line (b). These appear, from above down wards, at the same time as the posterior la teral centres just described. They unite first with the posterior lateral osseous points of their respective vertebrx, and with them join their respective central masses. Consequently, the three first sacral vertebrae have each five primary ossific centres, and the two last, each only three ; the whole number of primary points of ossification in the sacrum being thus twenty-one.