The muscles of the tail are inclosed in the strong =coccygeal fascia=,
which is continuous in front with the gluteal fascia and blends with the
lateral sacro-iliac ligament. At the root of the tail it is loosely
attached to the subjacent muscles, but further back it is intimately
adherent to them. From its deep face are detached septa which pass
between the muscles to become attached to the vertebræ.
1. =Coccygeus= (Compressor coccygis; ischio-coccygeus).—This is a flat,
triangular muscle which lies chiefly between the sacro-sciatic ligament
and the rectum.
_Origin._—The pelvic surface of the sacro-sciatic ligament near the
ischiatic spine.
_Insertion._—The first four coccygeal vertebræ and the coccygeal fascia.
_Action._—Acting together, to depress (flex) the tail, compressing it
over the perineum; acting singly, to depress and incline it to the same
side.
_Structure._—The origin of the muscle is aponeurotic. Becoming fleshy,
its fibers pass upward and backward and divide into two layers. The
outer layer is attached to the vertebræ, the inner to the fascia;
included between the two lie the intertransversales. When the tail is
raised, the ventral edges of the muscles produce a distinct ridge at
either side of the anus.
_Relations._—Outwardly, the sacro-sciatic ligament and the
semimembranosus; inwardly, the rectum and the sacro-coccygeus ventralis
muscle. The internal pudic artery crosses the outer face of the origin
of the muscle.
2. =Sacro-coccygeus superior= (M. sacro-coccygeus dorsalis medialis;
erector s. extensor coccygis).—This muscle lies along the dorso-median
aspect of the tail, in contact with its fellow.
_Origin._—The last three sacral spines and some of the coccygeal spines.
_Insertion._—The dorsal surface of the coccygeal vertebræ.
_Action._—Acting together, to elevate (extend) the tail; acting singly,
to elevate and incline it laterally.
_Structure._—The muscle has a strong rounded belly. It is inserted by
means of short tendons which fuse with those of the next muscle.
_Relations._—Superficially, the coccygeal fascia; internally, its
fellow; laterally, the sacro-coccygeus lateralis; deeply, the vertebræ.
3. =Sacro-coccygeus lateralis= (M. sacro-coccygeus dorsalis lateralis;
curvator coccygis).—This muscle lies immediately lateral to the
preceding.
[Illustration: