_Origin_ from a triangular area on the dorsal and lateral surfaces of the shaft and the great trochanter of the femur. The base of the area is at the great trochanter, and its apex is at the junction of the second and third fifths of the shaft on the linea aspera. The linea aspera forms the ventral boundary of the triangle, while its dorsal boundary is formed by a line drawn from its apex to the dorsomedial angle of the great trochanter. The mass unites with the rectus femoris (_c_) by its dorsal border at the junction of the middle and distal thirds of the thigh. _Insertion_ into the oblique area on the outer surface of the patella near its lateral border in connection with the rectus femoris. (3) =M. vastus medialis= (Fig. 92, _c_) lies on the medial side of the thigh. _Origin_ by fleshy fibres over a diamond-shaped area on the shaft of the femur lying between the medial branch of the linea aspera and the area for the vastus lateralis. Proximad the area is bounded by the spiral line, and distad it is bounded by a line parallel to the spiral line and having its proximal end at about the junction of the first and second thirds of the bone. The muscle forms a triangular, prismatic mass. It ends in a thin aponeurosis which is continuous dorsad with the border of the rectus femoris (Fig. 92, _b_), and ventrad sometimes with the tendon of the gracilis (Fig. 91, _b_). _Insertion_ into the medial border of the patella and the ligamentum patellæ. The insertion may extend onto the head of the tibia. The adjacent surfaces of the vastus lateralis and the vastus medialis are connected over their distal thirds by an aponeurosis which passes transversely beneath the rectus femoris. The distal end of the aponeurosis is inserted into the proximal border of the patella. Its proximal end receives the insertion of some muscle-fibres which form a part of the vastus medialis as here described. This mass of fibres is described by Strauss-Durckheim as the =crural= muscle. Sometimes the transverse aponeurosis is continuous with only one of the two vasti and sometimes with neither, so that the insertion of the crural of Strauss-Durckheim becomes more or less independent. _Relations_ of the vastus medialis.--Outer (medial) surface with the sartorius (Fig. 91, _a_) and the structures in the iliopectineal fossa. Cranial (or dorsal) surface with the rectus femoris (Fig. 92, _b_) and vastus intermedius. Caudal (ventral) surface with the pectineus (Fig. 92, _e_), adductor longus (Fig. 92, _f_), adductor femoris (Fig. 92, _g_), and semimembranosus (Fig. 92, _h_). (4) =M. vastus intermedius.=--A flat mass of muscle which lies beneath the rectus femoris.