Os, (from a cuckoo, whose bill it is said to represent). Also called cauda, coccyx, os-sis sacri acumen. It is situated at the extremity of the os sacrum, and is in some measure an appendix of it; it is bent in a concave form towards the pelvis, to support the rectum and enlarge the cavity of the basin; the fore side is flat, the back part rather convex; it is made up of four or five pieces, like false vertebras, joined together by cartilages, more or less pliable; sometimes all are cemented together. The first piece is the largest; it consists of two parts, betwixt which and the os sacrum is a notch, through which a pair of nerves pass. The other pieces are irregular squares, diminishing as they descend. In children it is almost wholly cartilaginous.
Daventer and some other writers say, that difficult labours are often caused by these bones being anchy-losed: but experience manifests that the difficulty was owing to their impatience; for it is generally found that when the head advances but slowly, or not at all, if we wait, nature generally will perform her office safely and effectually.