Part 71

THE FASCIA AND MUSCLES OF THE TAIL

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.

FIG. 182.—CROSS-SECTION OF TAIL OF HORSE CLOSE TO ANUS.

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.) ]

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.