130 THE SKELETON. 



spinous processes. The sacrum, also, varies considerably with respect to its 

 degree of curvature. From the examination of a large number of skeletons, 

 it would appear that, in one set of cases, the anterior surface of this bone was 

 nearly straight, the curvature, which was very slight, affecting only its lower 

 end. In another set of cases, the bone was curved throughout its whole length, 

 but especially towards its middle. In a third set the degree of curvature was 

 less marked, and affected especially the lower third of the bone. 



Development (Fig 84). The sacrum, formed by the union of five vertebrae, 

 has thirty-five centres of ossification. 



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

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



The laminse 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 

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

 the anterior sacral foramina (Fig. 84), and are developed into separate segments, 

 which correspond with the anterior transverse processes (Fig. 85) ; they are 

 subsequently 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. 86) ; one for the auricular 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, 

 ossification of the central part of the bodies of the first three vertebrae com- 

 mences ; 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 

 vertebrae make their appearance. The laminae join to form the arch, and are 

 united to the bodies, first, in the lowest vertebrae. This occurs about the second 

 year, the uppermost segment appearing as a single piece about the fifth or sixth 

 year. About the sixteenth 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 process 

 gradually extending upwards, all the pieces become united, and the bone com- 

 pletely 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. In front, the Pyriforrnis and Coccygeus; behind, the 

 Gluteus Maximus and Erector Spinae. 



THE COCCYX. 



The Coccyx ( X 6xxv%, cuckoo\ so called from having been compared to a cuckoo's 

 beak (Fig. 87), is usually formed of four small segments of bone, the most 

 rudimentary parts of the vertebral column. In each of the first three segments 

 may be traced a rudimentary body, articular and transverse processes; the last 

 piece (sometimes the third) is a mere nodule of bone, without distinct processes. 

 All the segments are destitute of laminae and spinous processes; and, conse- 

 quently, of spinal canal and intervertebral foramina. The first segment is the 

 largest ; it resembles the lowermost sacral vertebra, and often exists as a sepa- 

 rate piece ; the last three, diminishing in size from above downwards, are usually 

 blended together so as to form a single bone. The gradual diminution in the 

 size of the pieces gives this bone a triangular form, the base of the triangle 

 joining the end of the sacrum. It presents for examination an anterior and 

 posterior surface, two borders, a base, and an apex. The anterior surface is 

 slightly concave, and marked with three transverse grooves, indicating the 



