Vertebral Column 43 



The sacrum presents sexual distinctions. The female bone is broader than the 

 male in proportion to its length, and shows a different anterior curve : in the male 

 the curve is more or less uniform from above downward, but in the female it is sharply 

 marked at the lower part of the bone, the upper portion being almost flat. 



The difference in breadth between the bones is well expressed by the " sacral index," in 

 which the breadth (at the base) is multiplied by 100 and divided by the anterior length ; this 

 gives an average index for British males of about 112 and for females 116. Indices above 100 

 class the sacra as platyhieric, those with indices below 100 being dolichohieric. The lower races tend 

 to be dolichohieric, although their females are usually platyhieric. The proportionate breadth of 

 the sacrum is a human character ; the gorilla gives an index of 72. 



The sacrum is occasionally found to contain six segments instead of the normal 

 five. This may result from inclusion of the proximal coccygeal element or, less 

 commonly, from the inclusion of the last lumbar ; in the latter case the added bone 

 may present lumbar characters on one side and sacral on the other. In other rarer 

 cases the sacrum may possess only four segments : if this condition is due to failure 

 of fusion with the highest segment there will be twenty-five presacral vertebrae. 



Opposite views are held as to the possibility of the human sacrum being in 

 process of shifting its position along the column : certain observers hold that the 

 sacral consolidation is tending to progress cranially, while others deny this or even 

 maintain that it may be shifting caudally. The first and oldest of these views does 

 not appear to receive support from embryology. 



Ossification. 



The centres from which the bone is formed are those of its constituent vertebras 

 plus costal and epiphysial centres in the lateral mass, but the number of these last is 

 variously estimated by different observers : possibly this bone, like others, is not 

 absolutely constant in the number of its small secondary centres. 



Each segment is laid down in cartilage, including the body and the neural half- 

 arches with their transverse and articular processes. Costal chondrification forms the 

 costal bars, which quickly fuse with the other cartilaginous parts proximally, and with 

 each other distally to make the basis of the outer part of the lateral mass carrying 

 the articular surface. Thus the bone as a whole is preformed in cartilage. Primary 

 centres appear in the cartilage at very variable times, those for the proximal segments 

 generally much earlier than the distal centres. The centres for the bodies appear 

 between the third and seventh or eighth month, those for the neural arches after the 

 mid-fo2tal period, and the costal centres shortly after the neural centres : within these 

 limits there seems to be much individual variation. 



The centres extend very slowly, so that at birth the sacrum consists mainly of 

 cartilage. Secondary centres begin to show about puberty and continue to appear 

 until after twenty-one : they include epiphysial plates for the bodies, epiphysial plates 

 for the lateral masses, and a variable number of centres for the different bony 

 prominences. 



The centres may be classed as follows : 



I. PRIMARY. 



i for each body . .... 5 



i for each half neural arch 10 



i for upper three costal bars . 



21 



(Costal centres may be 2 or 4 on each side.) 



