=M. iliopsoas= (Fig. 162, 8; Fig. 91, _c_).--The iliopsoas (equivalent to the human psoas and iliacus) is a conical muscle emerging from the abdominal cavity onto the medial surface of the femur. _Origin._--(_a_) The portion corresponding to the human psoas (Fig. 162, 8) arises by ten vertebral heads. The first five of these come from the five cranial tendons of origin of the psoas minor (Fig. 162, 9); the sixth from a tendinous expansion which passes from the tendon of the first head over the ventral longitudinal muscles to the transverse process of the fifth lumbar vertebra. The seventh, eighth, ninth, and tenth heads arise by fleshy fibres from the ventral surfaces of the centra of the last four lumbar vertebræ. (_b_) The portion corresponding to the human iliacus arises by fleshy fibres from the ventral border of the ilium, from opposite the auricular impression to the iliopectineal eminence. The portions of the muscle all converge to form a conical mass which ends in a strong tendon lying on its outer surface. _Insertion_ by tendon and fleshy fibres into the apex of the lesser trochanter of the femur. _Relations._--Dorsal surface with the quadratus lumborum (with which this muscle is partly united), the rectus femoris (Fig. 91, _d_), and, by the iliac head, with the gluteus minimus. Ventral and medial surface with the psoas minor (Fig. 162, 9) and the peritoneum. Lateral edge with the transversus abdominis (Fig. 162, 4). _Action_.--Rotates the thigh so as to carry the foot out; also flexes the thigh. 2. Muscles of the Thigh. =M. biceps femoris= (Fig. 68, _t_, page 117).--A very large flat muscle covering about two-thirds of the lateral side of the thigh. _Origin_ (Fig. 90, 2).--From the ventral surface of the tuberosity of the ischium by tendon- and muscle-fibres. The fibres diverge, and near the knee the mass has spread out, ending in a fascia. The dorsal border of the muscle and the common fascia are continuous dorsad with the superficial fascia of the thigh and with the tendon of the caudofemoralis (Fig. 68, _s_). Ventrad it is continuous with the superficial fascia of the shank. _Insertion_ into rather more than the proximal one-third of the dorsal border of the tibia along its lateral margin and into the lateral margin of the patella. In passing over the knee-joint it is closely united to the underlying ligaments and tendons.