The sacrum, much the largest piece of the vertebral column, is placed, when the body is in the erect position, at the superior and posterior part of the pelvis, beneath the last lumbar vertebra, above the coccyx, and between the ossa innominata, between which it is inserted, in some measure like a keystone into an arch.
The true vertebra are divided into three sets, named from the regions they occupy, cervical, dorsal, lumbar. They present, 1. certain general characters by which they may at once be distinguished from bones of any other class; 2. those of each region (cervical, dorsal, lumbar,) exhibit peculiar characters by which they are severally distinguished ; 3. certain vertebras present special or individual characters.
Amongst the lumbar vertebrae, the fifth only is distinguishable by any peculiarity deserving of notice, its body being thicker anteriorly than posteriorly, and its transverse process short, thick, and rounded.
The first dorsal vertebra is marked at each side by a complete articular surface for the first rib, and on its inferior border by a slight excavation, which receives half the head of the second : the upper articular processes are oblique, and the spinous more nearly horizontal than those below it.
The cervical vertebrae (french: vertèbres cervicales) are seven in number; they are smaller than those in the other regions, which results from the size of the body and processes being less than that of the corresponding parts in the dorsal and lumbar classes. The vertebral foramen is of a triangular form, and larger proportionally than in the other classes.
These bones of the coccyx (French: coccyx), when united together, which is usually the case in advanced life, are supposed to resemble a cuckoo's bill and are therefore called coccygeal. Most commonly there are four of them, sometimes but three; in a few instances five have been found. They diminish gradually in size from above downwards, which gives them, when taken together, a pyramidal form. As they are placed in a continuous line with the inferior third of the sacrum, they form a slightly concave surface anteriorly, a convex one posteriorly.
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