OSTEOLOGY. 



a third set, the degree of curvature was less marked, and affected especially the 

 lower third of the bone. 



Development (fig. 17). The sacrum, formed by the union of five vertebras, has 

 thirty-Jive centres of ossification. 



The bodies of the sacral vertebras have each three ossific centres ; one for the 

 central part, and one for the epiphysal plates on its upper and under surface. 



The laminae of the sacral vertebrae are each developed by two centres ; these 

 meet behind to form the arch, and subsequently join the body. 



The lateral masses have six additional centres, two for each of the first three 

 vertebra. These centres make their appearance above and to the outer side' of 

 the anterior sacral foramina (fig. 17), and are developed into separate segments, 

 which correspond with the anterior transverse processes (fig. 18); they are subse- 

 quently blended with each other, and with the bodies and the posterior transverse 

 processes, to form the lateral mass. 



Lastly, each lateral surface of the sacrum is developed by two epiphysal plates 

 (fig. 19); one for the articular surface, and one for the remaining part of the thin 

 lateral edge of the bone. 



Period of Development. At about the eighth or ninth week of foetal life, ossifi- 

 cation of the central part of the bodies of the first three vertebras commences ; 

 and, at a somewhat later period, that of the last two. Between the sixth and 

 eighth months, ossification of the lamina takes place : and, at about the same 

 period, the characteristic osseous tubercles for the three first sacral vertebra? make 

 their appearance. The laminae join to form the arch, and are united to the bodies, 

 first, in the lowest vertebra. This occurs about the second year, the uppermost 

 segment appearing as a single piece about the fifth or sixth year. About the six- 

 teenth year, the epiphyses for the upper and under surfaces of the bodies are 

 formed : and, between the eighteenth and twentieth years, those for each lateral 

 surface of the sacrum make their appearance. At about this period, the last 



two segments are joined to one another; and this pro- 

 cess gradually extending upwards, all the pieces 

 become united, and the bone completely formed from 

 the twenty-fifth to the thirtieth year of life. 



Articulations. "With four bones; the last lumbar 

 vertebra, coccyx, and the two ossa innominata. 



Attachment of Muscles. The Pyriformis and 

 Coccygeus on either side ; behind, the Gluteus maxi- 

 mus, and Erector spina?. 



THE COCCYX. 



The Coccyx (*oxxt>|, cuckoo), so called from resem- 

 bling a cuckoo's beak (fig. 20), is usually formed 

 of four small segments of bone, the most rudi- 

 mentary parts of the vertebral column. In each 

 of the first three segments may be traced a rudi- 

 mentary body, articular and transverse processes; 

 the last "piece (sometimes the third) being merely 

 a rudimentary nodule of bone, without distinct 

 processes. All the segments are destitute of lami- 

 nce and spinous processes; and, consequently, of 

 spinal canal, and intervertebral foramina. The first 

 segment is the largest, resembles the lowermost 

 sacral vertebra, and often exists as a separate piece ; 

 the last three, diminishing 'in size from above down- 

 wards, are usually blended together so as to form 

 a single bone. The gradual diminution in the 

 of the pieces gives this bone a triangular 



Fig. 20. Coccyx. 

 Cornva 



size 



form, articulating by its base with the end of the 



