SACRUM. 



129 



Fig. 84.- 



at 



-Development of the Sacrum. 



Additional centres 

 for the first 3 pieces* 



Fig. 85. 



process of the upper piece of the coccyx, and transmits the anterior branch of 

 the fifth sacral nerve. 



The Base of the sacrum, which is broad and expanded, is directed upwards 

 and forwards. In the middle is seen an oval articular surface, which corres- 

 ponds with the under surface of the body of the last lumbar vertebra, bound- 

 ed behind by the large triangular orifice of the sacral canal. This orifice is 

 formed behind by the spinous process and laminas of the first sacral vertebra, 

 whilst projecting from it on each side are the superior articular processes; they 

 are oval, concave, directed backwards and inwards, like the superior articular 

 processes of a lumbar vertebra ; and in front of each articular process is an in- 

 tervertebral notch, which forms the lower half of the last intervertebral fora- 

 men. Lastly, on each side of the articular 

 surface is a broad and flat triangular 

 surface of bone, which extends outwards, 

 and is continuous on each side with the 

 iliac fossa. 



The Apex, directed downwards and 

 forwards, presents a small oval concave 

 surface for articulation with the coccyx. 



The Sacral Canal runs throughout the 

 greater part of the bone ; it is large and 

 triangular in form above, small and flat- 

 tened from before backwards below. In 

 this situation, its posterior wall is incom- 

 plete, from the non-development of the 

 laminse and spinous processes. It lodges 

 the sacral nerves, and is perforated by 

 the anterior and posterior sacral fora- 

 mina, 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 ; and 

 it is much less curved, the upper half of 

 the bone being nearly straight, the lower 

 half presenting the greatest amount of 

 curvature. The bone is also directed 

 more obliquely backwards; which in- 

 creases the size of the pelvic cavity, and 

 forms a more prominent sacro- vertebral 

 angle. 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 laminae 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 



Fig. 86. 



2 Efiipliysa-l lamina 

 for each littera.1 surface 



at 25 - 



