_34_, Coccygeal vertebra; _35_, sacro-coccygeus superior; _36_, intertransversalis; _37_, sacro-coccygeus inferior; _38_, coccygeus; _39_, recto-coccygeus; _40_, coccygeal fascia. (After Ellenberger-Baum, Anat. für Künstler.) ] [Illustration: FIG. 183.—MUSCLES OF PERINEUM OF HORSE. _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.