Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 18 On the Sacral Bone and the Coccyx

Description of the human sacrum

Beneath the lumbar vertebrae in man ten bones are generally found (G, H, I, K in the fig. in ch. 14, M and N in the skeletal figures, all of figs. 1 and 2 in this ch.), of which six make up the human sacrum 32 and four the coccyx. The six upper bones (A to F in fig. 1) are joined together in the same way it has been related the three bones making up the canine sacrum (1, 2, 3 in fig. 4) are united. In the middle of their anterior surface they display a bond of the sort the bodies of the vertebrae would form (R in the fig. for chapter 14) [discus intervertebralis] if they had joined to the extent that no cartilage were seen in the union of these six bones, 33 except perhaps in very young children whose bones have not yet altogether fused. On the sides of the anterior part, where (so to speak) the transverse processes of these six bones are joined together (L, L, R in figs. 1 and 2) a quite obscure kind of joint appears. On the posterior surface, the union is done away with altogether and a single bone made of those six is observed. No line of a union presents itself here, except sometimes at the meeting of the first bone [vertebra sacralis I] with the second [vertebra sacralis II] (Z in figs. 1 and 2). Often, in fact, the first bone is observed to be joined to the second on its posterior surface by the kind of joint [articulationes zygapophysiales] with which the ascending and descending processes of the lumbar vertebrae are regularly articulated. But although those processes are so joined, the transverse processes of this bone fuse everywhere with those of the second bone, and all such joints are so strong that the first bone is unable to be moved at all upon the second. Sometimes, however, sacra found in cemeteries show the first bone broken away, and it is found separately as if it had been a lumbar vertebra. For were it not that this first bone has thick, broad transverse processes, its shape would be hard to distinguish from that of lumbar vertebrae. In children, the joint of these six bones is conspicuous even on the posterior side and extremely prominent on the anterior. In no humans is the joint wholly invisible on the anterior surface; similarly, on the posterior surface of the sacrum the kind of joint always presents itself which is fashioned by the bodies of the bones. For if you examine the foramen [canalis sacralis] incised in the sacrum for the dorsal medulla (f, g, h, i, k in fig. 2), 34 you will see that the bodies of the bones have fused through the course of the foramen as clearly as on the anterior surface. This set of six bones corresponds to the three bones which make up the sacrum of the dog; for just as in the dog the topmost bone is greater than the middle and the middle greater than the lowest, so in man the bones are smaller the lower their position on the sacrum. The second bone, however, is exceptional in that its transverse processes are broader and thicker, and more prominent anteriorly and posteriorly (from Z to slightly below the lower L in fig. 1) than the other bones of the sacrum. 35 The upper part of the first or topmost bone [vertebra sacralis I] (compare the upper part in figs. 1 and 2 with the figs. in ch. 17, or see the joint in the skeletons and the fig. in ch. 14) corresponds in every way to the superior surface of any lumbar vertebra. And the body of the fifth lumbar vertebra is fitted to the first bone of the sacrum in the same way that the lumbar vertebrae receive the tubercles of the processes [processus articularis inferior vertebrae lumbalis] of the vertebra above. The lower part of the sixth bone (a in fig. 1) differs significantly from the inferior surface of the third canine [sacral] bone: the human bone possesses no descending


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processes by which to articulate with a bone beneath it. Instead, it ends in a circular but slightly broader tubercle [apex ossis sacri], which in humans is attached to the first bone of the coccyx in the same fashion as the bodies of the vertebrae (R in the fig. for ch. 14, or b in figs. 1, 2, 3 of this ch.) 36 are articulated with each other, or better yet the bones of the tail [os coccygis]. Moreover, a depression [facies auricularis] (N, N, etc. in fig. 2) is carved out in the transverse processes [pars lateralis] of the human sacrum, on the outside slightly towards the posterior, to which on each side the iliac bone (A, B, C, etc. in fig. 3, ch. 29) [facies sacropelvica, f. auricularis] is very strongly attached. This depression is quite rough and uneven, and has in the middle along its longitude a projecting line (O, O in fig. 2) or tubercle standing out like a rather wide line, which in turn separates the depression into anterior and posterior depressions. The anterior one [facies auricularis] (N, N in fig. 2) is quite simple but rough, and rather unevenly incised; the posterior (P, Q, F in fig. 2; P and Q mark the two depressions, F the swelling [tuberositas sacralis]) is more deeply hollowed, but shows in its middle a process that swells transversely and subdivides the posterior depression into upper and lower parts. The ilia [facies auricularis ossis ilii] correspond exactly to these depressions, being equipped with matching protrusions and depressions. These cavities and swellings are not covered with cartilage like the other cavities. For although other swellings and depressions are coated with smooth, continuous, and flat cartilage, these depressions of the sacrum produce nothing but cartilaginous ligaments [ll. sacro-iliaca interossea] connecting the sacrum very tightly to the ilium. These depressions are particularly associated with the three upper bones of the sacrum, for the lower bones have slender, broad, and blended transverse processes from which originates the muscle [m. piriformis] (F in the 11th table of muscles) that will be considered the fourth muscle of femoral movements. The transverse process of the fifth sacral bone, however, puts forth a tubercle (R in figures 1 and 2) at the point where it merges with the process of the sixth bone; this tubercle is quite prominent and thick, formed so that two ligaments (g and d in the 10th table of muscles) may conveniently be inserted in the sacrum, 37 one of which will be shown to originate from the acute process of the hip (d in fig. 2, ch. 29 [spina ischiadica]), the other from the upper part of the appendix which you will hear the hip has (l in the same fig.) [tuber ischiadicum].



Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 18 On the Sacral Bone and the Coccyx