_Blood-supply._—Intercostal arteries. _Nerve-supply._—Dorsal branches of the thoracic nerves. 7. =Longissimus.=[46]—This is the largest and longest muscle in the body. It extends from the sacrum and ilium to the neck, filling up the space between the spinous processes medially and the lumbar transverse processes and the upper ends of the ribs below; consequently it has the form of a three-sided prism. _Origin._—(1) The internal angle, crest, and adjacent part of the ventral surface of the ilium; (2) the first three sacral spines; (3) the lumbar and thoracic spines and the supraspinous ligament. _Insertion._—(1) The lumbar transverse and articular processes; (2) the thoracic transverse processes; (3) the spinous and transverse processes of the last four cervical vertebræ; (4) the outer surfaces of the ribs, except the first. _Action._—Acting with its fellow, it is the most powerful extensor of the back and loins; by its cervical attachment it assists in extending the neck. By its costal attachment it may also assist in expiration. Acting singly, it flexes the spine laterally. [Illustration: FIG. 181.—RIGHT PORTION OF CROSS-SECTION OF BACK OF HORSE. SECTION IS CUT THROUGH SEVENTH THORACIC VERTEBRA. _a_, Ligamentum nuchæ; _b_, trapezius muscle; _c_, cartilage of scapula; _d_, latissimus dorsi; _e_, panniculus carnosus; _f_, rhomboideus dorsi; _g_, serratus magnus; _h_, serratus anticus; _h′_, lumbo-dorsal fascia, which divides below into three layers; _i_, transversalis costarum; _k_, levator costæ; _k′_, internal intercostal muscle; _l_, longissimus dorsi; _m_, _m′_, seventh thoracic vertebra; _n_, head of eighth thoracic vertebra; _o_, head of eighth rib; _p_, seventh rib; _r_, intercostal artery and nerve; _s_, skin. The fasciæ are indicated by dotted lines. (After Ellenberger, in Leisering’s Atlas.) ]