158 THE SKELETON. 



and triangular in form above, small and flattened, from before backward, below. 

 In this situation its posterior wall is incomplete, from the non-development of the 

 laminae and spinous processes. It lodges the sacral nerves, and is perforated by 

 the anterior and posterior sacral foramina, through which these pass out. 



Structure. It consists of much loose, spongy tissue within, invested externally 

 by a thin layer of compact tissue. 



Differences in the Sacrum of the Male and Female. The sacrum in the 

 female is usually wider than in the male ; the lower half forms a greater angle with 

 the upper, the upper half of the bone being nearly straight, the lower half pre- 

 senting the greatest amount of curvature. The bone is also directed more obliquely 

 backward, Avhich increases the size of the pelvic cavity ; but the sacro-vertebral 

 angle projects less. In the male the curvature is more evenly distributed over the 

 whole length of the bone, and is altogether greater than in the female. 



Peculiarities of the Sacrum. This bone, in some cases, consists of six pieces ; 

 occasionally, the number is reduced to four. Sometimes the bodies of the first 

 and second segments are not joined or the laminsB and spinous processes have not 

 coalesced. Occasionally the upper pair of transverse tubercles are not joined to 

 the rest of the bone on one or both sides ; and, lastly, the sacral canal may be open 

 for nearly the lower half of the bone, in consequence of the imperfect development 

 of the laminae and 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 toward its middle. In a third set the degree of curvature was less 

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



Development (Fig. 125). The sacrum, formed by the union of five vertebrae, has 

 thirty-five centres of ossification. 



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

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

 Occasionally the primary centres for the bodies of the first and second piece of the 

 sacrum are double. 



The arch of each sacral vertebra is developed by two centres, one for each 

 lamina. These unite with each other behind, 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. 125), and are developed into separate segments 



Two epiphysial laminse 

 for each lateral surface.* 



Additional centres 

 for the first three pieces.* 



At birth. 



V2j3iJ/7 



\'^TV 77- /*>S I I 



At 4% years. 

 FIG. 125. Development of the sacrum. FIG. 126. FIG. 127. 



(Fig. 126) ; they are subsequently blended with each other, and with the bodies 

 and transverse processes to form the lateral mass. 



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

 (Fig. 127) : one for the auricular surface, and one for the remaining part of the 

 thin lateral edge of the bone. 



