_a_, Coccygeus; _b_, retractor ani; _c_, _c′_, sphincter and externus;
_d_, recto-coccygeus; _e_, sacro-coccygeus inferior lateralis; _f_,
retractor penis; _g_, bulbo-cavernosus; _h_, ischio-cavernosus; _i_,
internal pudic artery; _k_, anus; _l_, penis. (After
Ellenberger-Baum, Top. Anat. d. Pferdes.)
]
_Origin._—The sides of the sacral spines, with the multifidus, and the
transverse processes of the sacral and coccygeal vertebræ.
_Insertion._—The lateral surface of the coccygeal vertebræ, except the
first four.
_Action._—Acting with its fellow, to assist the preceding muscle in
elevating the tail; acting singly, to incline it to the same side.
_Structure._—This muscle appears to be a direct continuation of the
multifidus dorsi. The belly is fusiform and receives reinforcing
fasciculi from the transverse processes of the sacrum. This is succeeded
by bundles of tendons, as many as four lying alongside of each other.
_Relations._—Superficially, the lateral sacro-iliac ligament and the
coccygeal fascia; dorsally, the sacro-coccygeus superior; ventrally, the
intertransversales; deeply, the vertebræ and a branch of the lateral
coccygeal artery and accompanying vein and nerve.
4. =Intertransversales caudæ= (Mm. intertransversarii caudæ).—These
consist of muscular bundles which lie on the lateral aspect of the tail,
between the preceding muscle and the sacro-coccygeus inferior. They
begin on the lateral edge of the sacrum and occupy the spaces between
the transverse processes, to which they are attached. They are, however,
not arranged in a strict segmental manner.
_Action._—Acting together, to fix the coccygeal vertebræ; acting singly,
to assist in lateral flexion.
5. =Sacro-coccygeus inferior= (Mm. sacro-coccygei ventrales; depressor
coccygis).—This muscle lies on the ventral aspect of the sacrum and
coccyx. It is composed of two portions, described by Bourgelat and the
German anatomists as separate muscles.
(_a_) The =outer portion= (M. coccygeus ventralis lateralis) is much the
larger of the two. It arises from the outer part of the ventral surface
of the sacrum, about as far forward as the third foramen, and is
inserted into the transverse processes and ventral surface of the
coccygeal vertebræ.
(_b_) The =inner portion= (M. sacro-coccygeus ventralis medialis) arises
from the ventral surface of the sacrum internal to the preceding muscle
and the first eight coccygeal vertebræ, and is inserted into the ventral
surfaces of the coccygeal vertebræ.
_Action._—Acting together, to depress (flex) the tail; acting singly, to
incline it laterally also.
_Structure._—The outer portion has a somewhat compressed belly, and
receives bundles from the transverse processes of the coccygeal
vertebræ. The inner portion is much smaller and shorter, reaching only
about to the middle of the tail.
_Relations._—Ventrally, the pelvic and coccygeal fasciæ; dorsally, the
sacrum, coccygeal vertebræ, and the intertransversales; laterally, the
sacro-sciatic ligament, the coccygeus, and the coccygeal fascia;
medially, its fellow, the recto-coccygeus, and the middle coccygeal
vessels. Branches of the lateral coccygeal vessels and nerves lie
between the outer division of the muscle and the intertransversales.
_Blood-supply._—Middle and lateral coccygeal arteries.
_Nerve-supply._—Coccygeal nerves.