42 Anatomy of Skeleton 



formis, which arises from the lateral mass opposite the middle three pieces and outside 

 the first foramen, and extends in on the costal bars between the foramina, as far as 

 the limiting markings on the sides of the bodies. Lateral sacral vessels lie on the 

 fascia and reach the sacrum internal to the foramina, and here also the sacral 

 sympathetic chain touches the bone. Superficial to the structures mentioned, the 

 peritoneum lies in front of the bone in its upper half or more, while in the lower part 

 the rectum rests directly on the bone. All these relations are shown in Fig. 35. 



The ala, or upper surface of the lateral mass, lies deeply under cover of the Psoas 

 for the most part, but its inner and front part carries the lumbo-sacral cord and ilio- 

 lumbar vessels, and these appear from under cover of the inner border of the muscle. 

 The last lumbar nerve emerges between the upper sacral articular processes and the 

 back of the intervertebral disc, and its posterior primary division turns back round the 

 process to reach the post-vertebral muscles, while its anterior part runs forward, 

 outward and downward on the ala, being joined here by the branch from the fourth. 

 Thus the nerve appears to emerge through an osseo-fibrous foramen completed exter- 

 nally by arching fibres of the ib'o- lumbar ligament. All these structures are placed 

 deep to the Psoas. The fibres of the ligament pass to the transverse tubercle of the 

 first sacral vertebra and the bone in front of this and account for some of the roughness 

 generally found here : the remaining markings, extending round the margin of the 

 articular area, are caused by the weak anterior sacro-iliac ligaments. 



The sacral canal narrows from above down ; smaller canals leave it at intervals 

 and run down and out, dividing into two terminal parts which at once open at the 

 anterior and posterior foramina. It thus happens that each foramen in front is directly 

 opposite its corresponding posterior opening, and arteries (from lateral sacrals) entering 

 the anterior foramina to supply the structures in the canal can give branches that in 

 a very short course reach the back openings and emerge there : these correspond with 

 the posterior branches of the intercostal and lumbar vessels, and are the proper 

 continuations of the branches of the lateral sacral. 



Passing through the upper aperture are the sacral and coccygeal nerves (part of 

 cauda equina), the filum terminale, and spinal membranes. In the bone the upper 

 four nerves pass out through their proper foramina, and the loose sac of dura mater, 

 with its enclosed arachnoid, only extends about half-way down the sacrum : here it 

 is pierced by the filum terminale, but a very thin layer from the dura is continued on 

 this structure. 



Therefore are only found, passing through the lower aperture, the diverging 

 last sacral and coccygeal nerves, and the centrally placed filum terminale, running on 

 to end in the ligamentous tissue covering the back of the coccyx. 



This aperture is continuous with the " hiatus " : the hiatus is present over the 

 fifth segment and frequently over the fourth. Where the bone is deficient the canal is 

 covered in by ligamentous bands attached to the bony margin. 



The condition of complete consolidation is attained comparatively late in the sacrum, not 

 being found before the age of twenty-five or later. Possibly the persistence of non-fused segments 

 and parts is associated with the pelvic growth that precedes sexual maturity, and consolidation 

 commences in the lower part of the bone a year or two after the age of puberty ; but it must also 

 be remembered that, while growth continues, the lengthening limbs and changed proportions of the 

 trunk lead to alterations in the position of the centre of gravity and in the lines of transmission of 

 weight, so that the later consolidation of the upper part of the bone, delayed till growth has ceased, 

 may be connected with the fact that this is the part of the bone through which weight is transmitted 

 to the innominate bones. 



Even when the bone is firmly joined there are still remnants of the intervertebral discs to be 

 found on section (Fig. 34) : these can be discovered up to past middle life. 



