It is a depressor of the whole caudal appendix.
The Superior Sacro-coccygeal ([Fig. 68], 39; [Fig. 69], 34; [Fig. 70], 43).—The fasciculi which form this muscle arise from the crest of the sacrum, and proceed thence to end successively on the coccygeal vertebræ. It is in contact in the middle line with the corresponding muscle of the opposite side.
It raises the tail and inclines it laterally; if the muscle of one side contracts at the same time as that of the other the tail is elevated directly.
The Lateral Sacro-coccygeal ([Fig. 68], 40; [Fig. 69], 35; [Fig. 70], 44).—Situated on the lateral part of the caudal region, this muscle arises, in the dog, from the internal border of the iliac bone and the external border of the sacrum; in the horse, it arises from the crest of the sacrum. It is inserted into the coccygeal vertebræ.
It produces lateral movement of the tail.
The Inferior Sacro-coccygeal ([Fig. 68], 41; [Fig. 69], 36; [Fig. 70], 43).—This muscle, which is fairly thick, arises from the inferior surface of the sacrum and the corresponding surface of the sacro-sciatic ligament; it is inserted into the coccygeal vertebræ.
It depresses the caudal appendix.
Muscles of the Neck
Mastoido-humeralis ([Fig. 66], 3; [Fig. 68], 9, 9, 10; [Fig. 69], 12; [Fig. 70], 15).—One of the most important muscles of the region of the neck in man is the sterno-cleido mastoid. We recollect that, in its inferior part, it is divided into two bundles, one of which arises from the manubrium of the sternum, and the other from the inner third of the clavicle, whence the denominations of the sternal portion and clavicular portion. The muscle formed by the union of these two portions is then directed obliquely outwards, backwards, and upwards, to be inserted into the mastoid process of the temporal bone and the two external thirds of the superior curved line of the occipital bone.
Now, the animals which we are here considering have but a rudimentary clavicle or are entirely without it. From the absence of this item of the skeleton there necessarily result modifications in the arrangement of the muscles of this region, which we must at the very outset explain, before undertaking the special study of the muscle which is the subject of the present paragraph.