36 A MANUAL OF ANATOMY 



but on the other side it may remain separate from the second, and present the 

 characters of a fifth lumbar. (4) The number of sacral spines may be reduced 

 from four to three, two, or one, or they may be entirely absent. As a con- 

 sequence of this, the sacral canal, which usually opens on the back of the fifth 

 sacral vertebra, may do so on the back of the fourth, third, second, or first, 

 so that in some cases it may be entirely open posteriorly. (5) The sacrum is 

 liable to much variety as regards the extent of its vertical curve. 



Characters of the Female Sacrum. — In the female the sacrum is 

 smoother, shorter, broader, less curved, and is set more backwards 

 than in the male. 



Ossification. — The sacrum ossifies in cartilage from thirty-five centres. 

 Each segment has three primary centres, one for the body and two for the 

 neural arch. The centre for the body appears in the third month of intra- 

 uterine life in the case of the first three, and after the fifth month in the last 

 two. The centres for the neural arches appear about the sixth month. The 

 neural arches join the bodies, in order from below upwards, from the second 

 to the sixth year. The unidn of the laminae takes place from the eighth to 

 the twelfth year. It, however, fails in the lowest, and sometimes in those higher 

 up. The anterior parts of the lateral masses of the first three vertebrae, which 

 represent the costal elements, have separate centres, which appear about the 

 sixth month. These join the neural arches before uniting with the bodies, 

 the latter union taking place rather later than the union between the neural 

 arches and the bodies. Each vertebra has two annular circumferential 

 epiphysial plates, superior and inferior, which begin to ossify about the sixteenth 

 year. On each side of the sacrum there are two epiphyses, an upper for the 

 auricular surface, and a lower for the sharp edge below, which appear about the 

 eighteenth year. Consolidation begins about the eighteenth year, and pro- 

 ceeds from below upwards, union taking place earlier between the segments 

 of the lateral masses than between the bodies. In the latter case the ossifica- 

 tion invades the intervertebral discs, but in the former it is direct union. The 

 union is complete about the twenty -fifth year, at which period also the lateral 

 epiphyses join the bone. 



The Coccyx. . 



The coccyx is composed as a rule of four rudimentary vertebrae, 

 and it lies below the apex of the sacrum, which constitutes its only 

 articulation. The direction of the bone is downwards and for- 

 wards, and its elements diminish in size from above downwards. 

 It is triangular. 



The first coccygeal vertebra is compressed from before backwards, 

 broad above, narrow below, concave in front, and convex behind. 

 The superior and inferior surfaces are transversely oval, and the 

 lateral borders are sloped downwards and inwards. Two processes 

 project upwards from the dorsal surface at either side, called the 

 cornua, which articulate with the sacral cornua, usually by ligaments, 

 but sometimes directly. Each lateral border presents, superiorly, 

 a projection, called the transverse process, which inclines towards 

 the inferior lateral angle of the sacrum, and is usually connected 

 to it by a ligament, which is sometimes ossified. 



The second coccygeal vertebra presents traces of transverse 

 processes and cornua, whilst the third and fourth are reduced to 

 mere nodules. 



