DEVELOPMENT OF SACRUM. 



53 



Fig. 17. Development of Sacrum. 



Formed ly union of Vertelra. 

 2 characteristic points. 



1 A.<LlitioneLl centres 

 for the first 3 pieces * 



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; in front of each articular process is an intervertebral notch, which 

 forms the lower half of the last intervertebral foramen. 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 flattened from before backwards below. 

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

 laminas 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 exter- 

 nally 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 increases the size of 

 the pelvic cavity, and forms a more pro- 

 minent sac ro- vertebral angle. In the 

 male, the curvature is more evenly dis- 

 tributed 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 

 pieces; occasionally the number is 



six 



reduced to four. Sometimes the bodies 

 of the first and second segments are 

 not joined, or the laminae and spinous 

 processes have not coalesced. Occa- 

 sionally, 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 develop- 

 ment of the laminae and spinous pro- 

 cesses. The sacrum, also, varies con- 

 siderably 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 



Fig. 18. 



yrs 



Fig. 19. 



/ 



. nr " 



at 25 - 



