_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. THE MUSCLES OF THE THORAX These consist of seven muscles or sets of muscles, which are attached to the thoracic vertebræ, to the ribs and their cartilages, and to the sternum. Functionally, they are muscles of respiration. 1. =Levatores costarum.=—These constitute a series of small muscles which occupy and overlie the upper ends of the intercostal spaces. _Origin._—The transverse processes of the thoracic vertebræ. _Insertion._—The external surfaces of the upper ends of the ribs posterior to the vertebral origin. _Action._—To draw the ribs forward in inspiration. _Structure._—Arising by tendinous fibers, each muscle passes backward and outward and expands at its insertion. Some fibers pass over one rib and are inserted on a succeeding one. At the first and last spaces the muscle cannot be distinguished from the external intercostal, of which it is in reality only a specially developed part. _Relations._—Superficially, the longissimus dorsi; deeply, the ribs, internal intercostal muscles, and the intercostal vessels and nerves. _Blood-supply._—Intercostal arteries. _Nerve-supply._—Intercostal nerves. 2. =External intercostals= (Mm. intercostales externi).—Each of these occupies an intercostal space, from the levatores to the sternal extremity of the rib. They do not occupy the intercartilaginous spaces. _Origin._—The posterior borders of the ribs. _Insertion._—The anterior borders and external surfaces of the succeeding ribs. _Action._—To draw the ribs forward in inspiration. _Structure._—The fibers are directed downward and backward. There is a considerable admixture of tendinous tissue. The thickness of the muscles gradually diminishes toward the lower ends of the spaces. [Illustration: FIG. 184.—DORSAL AND LUMBAR REGIONS OF HORSE, DORSAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] _Relations._—Superficially, the serratus magnus, latissimus dorsi, serratus anticus and posticus, longissimus dorsi, transversalis costarum, rectus thoracis, deep pectorals, obliquus abdominis externus, and panniculus; deeply, the internal intercostals and (in the upper part of the spaces) the intercostal vessels and nerves. _Blood-supply._—Intercostal and internal thoracic arteries.