_Relations._--Lateral surface with the gluteus maximus and abductor caudæ internus (Fig. 68, _w_). Medial surface with the levator ani (Fig. 162, 11), with which this muscle may be partly united. _Action._--The two muscles together flex the tail. =M. flexor caudæ longus= (Fig. 162, 12; Fig. 113, _c_, page 270). _Origin_ on the ventral surface of the last lumbar vertebra, of the sacrum, and of the transverse processes of the caudal vertebræ. Caudad the muscle forms long, strong tendons which are _inserted_ on the ventral surface of the tail. _Relations._--Lateral surface with the ilium, the gluteus maximus, the abductor caudæ internus, the iliocaudalis, and the skin of the tail. Dorsally it touches in the caudal region the abductor caudæ externus (Fig. 68, _w_), medially the flexor caudæ brevis (Fig. 162, 13). _Action._--Flexes the tail. =M. flexor caudæ brevis= (Fig. 162, 13). _Origin_ on the ventral surface of the caudal vertebræ, from the first to the seventh or eighth. The principal head comes from the first vertebra. The bundles thus formed pass caudad and are inserted each into the ventral surface of a vertebra some distance caudad of the origin. The muscle extends to about the tenth caudal vertebra, and its most caudal parts are closely interconnected with the flexor caudæ longus. _Relations._--Lateral edge with the flexor caudæ longus. Medial edge in the sacral region with the levator ani (Fig. 162, 11) and the iliocaudalis (Fig. 162, 11′); in the caudal region with the muscle of the opposite side. _Action._--Flexes the tail. 2. =Muscles on the Ventral Side of the Vertebral Column.= A. LUMBAR AND THORACIC REGIONS.--The iliopsoas (Fig. 162, 8) belongs in this group, but since it moves the leg it is described with the muscles of the pelvic limbs. =M. psoas minor= (Fig. 162, 9, page 398).--This muscle lies along the ventral surface of the vertebral column, next to the peritoneum, and extends from the thoracic vertebræ to the ilium. _Origin_ usually by five heads, from the caudal border of the centra of the last two (or one) thoracic and first three (or four) lumbar vertebræ. These heads also serve as origin for a part of the iliopsoas, and are closely united at their origin with the quadratus lumborum. They unite to form an, at first, rather large flat muscle, which rapidly becomes smaller caudad, finally forming a slender tendon which is _inserted_ on the iliopectineal line, just craniad of the acetabulum. _Relations._--Dorsolateral surface with the quadratus lumborum and iliopsoas (Fig. 162, 8). Ventral surface with the pleura and diaphragm near the origin; with the peritoneum caudad. _Action._--Flexes the back in the lumbar region.