What Is the Coccyx?
The coccyx, commonly referred to as the tailbone, is the lower end of the spinal column of man and consists of four nodules of bone, like tiny vertebrae, corresponding to the tail, which is found in lower animals. The bones are deeply buried in muscle tissues, but occasionally they jut (protrude) backwards and are surrounded by a fold of skin, so as to form an actual tail. The name coccyx was given by the Greek physician Galen (c.AD. 130—200) and comes from the Greek word for “cuckoo”, as the bone rather resembles a cuckoo’s bill.
The coccyx is the final segment of the vertebral column in humans and apes, and certain other mammals such as horses. In animals with bony tails, it is known as tailhead or dock, in bird anatomy as tailfan. It comprises three to five separate or fused coccygeal vertebrae below the sacrum, attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx.
The coccyx is formed of three, four or five rudimentary vertebrae. It articulates superiorly with the sacrum. In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. The transverse processes are most prominent and noticeable on the first coccygeal segment. All the segments lack pedicles, laminae and spinous processes. The first is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the remaining ones diminish in size from above downward.
Most anatomy books incorrectly state that the coccyx is normally fused in adults. In fact it has been shown that the coccyx may consist of up to five separate bony segments, the most common configuration being two or three segments.
The anterior surface is slightly concave and marked with three transverse grooves that indicate the junctions of the different segments. It gives attachment to the anterior sacrococcygeal ligament and the levatores ani and supports part of the rectum. The posterior surface is convex, marked by transverse grooves similar to those on the anterior surface, and presents on either side a linear row of tubercles–the rudimentary articular processes of the coccygeal vertebrae. Of these, the superior pair are the largest, and are called the coccygeal cornua they project upward, and articulate with the cornua of the sacrum, and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve.
In humans and other tailless primates (e.g., great apes) since Nacholapithecus (a Miocene hominoid), the coccyx is the remnant of a vestigial tail, but still not entirely useless; it is an important attachment for various muscles, tendons and ligaments—which makes it necessary for physicians and patients to pay special attention to these attachments when considering surgical removal of the coccyx. Additionally, it is also a part of the weight-bearing tripod structure which acts as a support for a sitting person. When a person sits leaning forward, the ischial tuberosities and inferior rami of the ischium take most of the weight, but as the sitting person leans backward, more weight is transferred to the coccyx.
The anterior side of the coccyx serves for the attachment of a group of muscles important for many functions of the pelvic floor (i.e., defecation, continence, etc.): the levator ani muscle, which include coccygeus, iliococcygeus, and pubococcygeus. Through the anococcygeal raphe, the coccyx supports the position of the anus. Attached to the posterior side is gluteus maximus which extend the thigh during ambulation.
Many important ligaments attach to the coccyx: the anterior and posterior sacrococcygeal ligaments are the continuations of the anterior and posterior longitudinal ligaments that stretch along the entire spine. Additionally, the lateral sacrococcygeal ligaments complete the foramina for the last sacral nerve. And, lastly, some fibers of the sacrospinous and sacrotuberous ligaments (arising from the spine of the ischium and the ischial tuberosity respectively) also attach to the coccyx. An extension of the pia mater, the filum terminale, extends from the apex of the conus, and inserts on the coccyx.