It thus comes about that the posterior primary divisions of nerves, coming through the first three foramina, are in the substance of Multifidus spina? and are connected by loops here, on the bone : their outer branches run down and out to get on to the great sciatic ligament deep to Gluteus maximus. Lower down the loops between the third, fourth and fifth posterior divisions lie on the bone.
At the extreme edge below the articular surface is the attachment of the Coccygeus fibres, extending down to the coccyx (see Fig. 35). The back and upper fibres of this sheet are ligamentous and constitute the small sacro-sciatic ligament, so that this ligament separates the muscle from the great sciatic ligament, being in contact to some extent and fused with the latter and being continuous with the former.
The fifth sacral nerve emerges at the side of the lower sacral aperture, and its anterior primary division turns outwards and forwards, below the cornu and behind the last segment of the bone. At the outer border of this it reaches the plane of the Coccygeus and pierces this to appear on its pelvic aspect, where it is joined by the descending filament from the fourth sacral and turns down close to the coccygeal attachment of the muscle. The anterior division of the coccygeal nerve runs a similar course at a lower level, behind the first coccygeal body, piercing the muscle lower down to join the fifth sacral nerve in front of it: the nerve thus composed pierces the Coccygeus again near the tip of the coccyx to supply the skin in that region.
The front surface of the bodies-particularly in the middle segments-may show longitudinal markings, which indicate that some at least of the fibres of the anterior common ligament are involved in the general ossification of cartilage and ligament that consolidates the bone ; but in any case the greater part of this ligament is continued down on the front of the bone, forming the anterior sacro-coccygeal ligament at its lower end : the middle sacral artery runs down on the ligament.
At the side of the bodies the parietal pelvic fascia is attached to the margin of the foramina, so that the issuing nerves he deep to the fascia: they lie on the Pyriformis, 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, hes deeply under cover of the Psoas for the most part, but its inner and front part carries the lumbo-sacral cord and iliolumbar 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 externally by arching fibres of the ilio-lumbar hgament. 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 sacroiliac 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.
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.