_Structure._—This is quite complex. The posterior part of the muscle is greatly developed and constitutes the common mass of the loins. This is covered by a strong aponeurosis which blends with the supraspinous and sacro-iliac ligaments, and is attached to the crest and inner (sacral) angle of the ilium and the first and second sacral spines; it furnishes origin to the lumbar portion of the middle gluteus. In its course further forward the muscle receives fasciculi from the lumbar and thoracic spines, but diminishes somewhat in volume. At the withers it divides into two parts. The dorsal division (spinalis et semispinalis), reinforced by bundles from the first four thoracic spines, passes forward under the complexus to be inserted into the spines of the last four cervical vertebræ. The ventral division passes forward and downward underneath the serratus magnus to be inserted into the ribs and the transverse processes of the last four cervical vertebræ. Three sets of fasciculi may be distinguished, viz.: (1) spinal, which are superficial and internal; (2) transverse, attached to the transverse and articular processes, which are internal and deep; (3) costal, which are external. _Relations._—Superficially, the middle gluteus, the lumbo-dorsal fascia, the latissimus dorsi, dorsal serrati, serratus magnus, and complexus; deeply, the multifidus, intertransversales, external intercostals, levatores costarum, the ligamentum nuchæ, and its fellow of the opposite side (in the neck). _Blood-supply._—Dorsal, superior cervical, intercostal, and lumbar arteries. _Nerve-supply._—Dorsal branches of the thoracic and lumbar nerves. 8. =Multifidus dorsi= (semispinalis of the back and loins).—This is a long segmental muscle which covers the sides of the spinous processes of the vertebræ from the sacrum to the neck. _Origin._—(1) The lateral portion of the sacrum; (2) the articular processes of the lumbar vertebræ; (3) the transverse processes of the thoracic vertebræ. _Insertion._—The spinous processes of the first two sacral, the lumbar, thoracic, and last cervical vertebræ. _Action._—Acting with its fellow, it is an extensor of the spine; acting singly, it flexes it laterally. _Structure._—It is composed of a series of bundles which are directed obliquely forward and upward. Each fasciculus passes over several vertebræ to its insertion. In the posterior part of the series the bundles cross two or three vertebræ and are inserted into the summits of the spines. Anteriorly, the bundles have a more horizontal direction, cross three to five vertebræ, and are inserted into the sides of the spines considerably below their summits. A further complication consists in the fusion of several bundles into a common insertion. _Relations._—Superficially, the longissimus; deeply, the vertebral spines. _Blood-supply._—Intercostal and lumbar arteries. _Nerve-supply._—Dorsal branches of the thoracic and lumbar nerves.