The Coccyx, so called from having been compared to a cuckoo's beak, is usually formed of four small segments of bone, the most rudimentary parts of the vertebral column. In each of the first three segments may be traced a rudimentary body, articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone, without distinct processes. All the segments are destitute of pedicles, laminae, and spinous process, and, consequently, of intervertebral foramina and spinal canal. The first segment is the largest; it resembles the lowermost sacral vertebra, and often exists as a separate piece; the last three, diminishing in size from above downward, are usually blended together so as to form a single bone. The gradual diminution in the size of the pieces gives this bone a triangular this bone a triangular form the base of the triangle joining the end of the sacrum. It presents for examination an anterior and posterior surface, two borders, a base, and an apex. The anterior surface is slightly concave, and marked with three transverse grooves, indicating the points of junction of the different pieces. It has attached to it the anterior sacro-coccygeal ligament and Leva-tor ani muscle, and supports the lower end of the rectum. The poster, surface is convex, marked by transverse grooves similar to those on the anterior surface; and presents on each side a lineal row of tubercles, the rudimentary articular processes of the coccygeal vertebras. Of these, the superior pair are large, and are called the cornua of the coccyx; they project upward, and articulate with the cornua of the sacrum, the junction between these two bones completing the fifth posterior sacral foramen for the transmission of the posterior division of the fifth sacral nerve. The lateral borders are thin, and present a series of small eminences, which represent the transverse processes of the coccygeal vertebras. Of these, the first on each side is the largest, flattened from before backward, and often ascends to join the lower part of the thin lateral edge of the sacrum, thus completing the fifth anterior sacral foramen for the transmission of the anterior division of the fifth sacral nerve; the others diminish in size from above downward, and are often wanting. The borders of the coccyx are narrow, and give attachment on each side to the sacro-sciatic ligaments, to the Coccygeus muscles in front of behind them. The base presents an oval surface for articulation with sacrum. The apex is rounded, and has attached to it the tendon of the external Sphincter muscle. It is occasionally bifid, and sometimes defected to one or other side.