Sacrum

(sacrum, so called from saeer, sacred.) Os sacrum. Qs basil are. In anatomy. The os sacrum derives its name from its being offered in sacrifice by the ancients, oir perhaps from its supporting the organs of generation, which they considered as sacred.. young subjects it is composed of ‘five or six pieces united by~cartilage; but in more advanced age it becomes one bone, in which, however, we may still easily distinguish the marks of the former separation. Its shape has sometimes been compared to an irregular triangle; and sometimes, and perhaps more properly, to a py flattened before and behind, with ‘its basis placed towards the lumbar vertebra, and its point terminating in the coccyx. We find it convex behind, and slightly concave before, with its inferior portion bent a little forwards. Its anterior surface is smooth, and affords four, and sometimes five transverse lines, of a colour different from the rest of the ‘hone. These are the remains of the inter me- ‘liate cartilages by which its several pieces were anned in infancy. Its posterior convex surface has several prominences, the most remarkable of which are its spinous proce es; these _are usually three in number, and gradually*be- ‘come shorter, so that the third is not so long as the second, nor the second as the first. ‘This arrangement enables us to sit with ease. transverse proce es are formed into one oblong proce , which becomes gradually smaller as it descends. At the superior part of the bone we observe two oblique proce es, of a cylindrical shape, and somewhat concave, which are articulated with the last of the lumbar vertebrae. At the base of each of these oblique proce es is a notch, which, with such another in the vertebrae above it, for in a pa age for the twenty-fourth spinal nerve. viewing this bone, either before or behind, we observe four, and sometimes five holes on each side, situate at each extremity of the - transverse lines we described as marking the divisions of.the bone.. Of these holes, the anterior unes, and of these again, the uppermost, | are the largest, and afford a pa age to the nerves. ‘The posterior holes are smaller, covered with membranes, and destined for the same purpose as the former. Sometimes at the bottom of the bone there is only a notch, and sometimes there is a hole common to it and the os coccygis. ‘The cavity between the body of this bone and its proce es, for the lodgment of the spinal marrow, is triangular, and becomes smaller~as it descends, till at length it terminates obliquely on each side at the lower part of the bone. Below the third division of the bone, however, the cavity is no longer completely bony, as in the rest of the spine, but is defended posteriorly only by a very strong membrane; hence a wound in this part may be attended with the most dangerous consequences. This bone is articulated above, with the last lumbar vertebrae: laterally, it is firmly united, by a broad irregular surface, to the o a in no min at a, or hip-bone: and below -it is joined to the os coccygis. In women the os sacrum is usually shorter, broader, and more curved, than in men, by which means the cavity of the pelvis is more enlarged.