Insertion into the proximal portion of the bottom of the trochanteric fossa.

Relations.—Ventral surface with the adductor femoris (Fig. 92, g). Dorsal surface with the pubis and ischium. Caudal border with the quadratus femoris (Fig. 90, f).

Action.—Assists the iliopsoas.

B. Muscles on the Medial Surface of the Hip.

—M. obturator internus (Fig. 90, e).—The obturator internus appears as a triangular muscle caudad of the gemellus superior.

Origin by numerous small separate heads from the dorsal surface of the ramus of the ischium along its symphysis and following its medial border from the symphysis nearly to the tuberosity. The fibres form a flat muscle which narrows and passes through the lesser sciatic notch, turns ventrad and ends in a strong flat tendon whose

Insertion is into the bottom of the trochanteric fossa of the femur. The tendon is continued proximad on the inner surface of the muscle and forms a smooth firm surface by which the muscle glides over the dorsal border of the ischium. Into the inner surface of this tendon near its insertion the tendon of the gemellus inferior is inserted.

Relations.—Medial surface within the pelvis with a mass of fat separating it from the pelvic organs. Dorsal or outer surface with the biceps femoris, the tenuissimus, the caudofemoralis, and the great sciatic nerve. Inner surface with the ischium and the great sciatic nerve. Caudal (or ventral) border with the quadratus femoris.

Action.—Abductor of the thigh.

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.

Relations.—Outer surface with the superficial fascia and with a few of the most caudal fibres of the cutaneus maximus (Fig. 62, b, page 94). Cranial (or dorsal) edge with the caudofemoralis (Fig. 68, s) and the vastus lateralis (Fig. 90, d). Caudal border with the semitendinosus (Fig. 68, u) and a mass of fat. Inner surface with the caudofemoralis (Fig. 68, s), the tenuissimus (Fig. 90, g), the obturator internus (Fig. 90, e), the quadratus femoris (Fig. 90, f), the semitendinosus (Fig. 68, u), the semimembranosus (Fig. 90, i), the adductor femoris (Fig. 90, h), the great sciatic nerve (Fig. 163, a), and distad with the following muscles of the lower leg: the tibialis anterior (Fig. 90, n), the extensor longus digitorum (Fig. 90, p), the peroneus longus (Fig. 90, q), and the lateral head of the gastrocnemius (Fig. 90, m).

Action.—Abductor of the thigh, and flexor of the shank.

M. tenuissimus or M. abductor cruris (Fig. 90, g).—A very slender muscle, only three or four millimeters wide.

Origin from the tip of the transverse process of the second caudal vertebra, in common with the caudofemoralis (Fig. 68, s) or gluteus maximus. It passes obliquely beneath the biceps femoris (Fig. 68, t) distad and ventrad, to the distal end of the ventral border of that muscle. Here it becomes continuous with the ventral border of the biceps, ending in a continuation of the same fascia into which the biceps is inserted.

Relations.—Outer surface with the caudofemoralis (Fig. 68, s) and the biceps femoris (Fig. 68, t); distad with the integument. Inner surface with the obturator internus (Fig. 90, e), quadratus femoris (f), semitendinosus (j), adductor femoris (h), semimembranosus (i), and distad with the muscles of the lower leg.

M. caudofemoralis (parameralis, Strauss-Durckheim) (Fig. 68, s, page 117).

Origin by a flat tendon from the transverse processes of the second and third caudal vertebræ. The muscle forms a flat band which passes distad along the middle of the lateral side of the thigh over the pelvis and caudad of the great trochanter. At the middle of the thigh it ends in a very thin tendon. The tendon passes distad along the medial surface of the biceps femoris (Fig. 68, t), pierces the fascia lata near the knee, and passes to its

Insertion into the middle of the lateral border of the patella.

Relations.—Outer surface with the superficial fascia and a few fibres of the cutaneus maximus; distad with the biceps femoris (Fig. 68, t). Cranial border with the gluteus maximus; caudal border with the biceps femoris. Inner surface with the tenuissimus (Fig. 90, g), the gluteus maximus, the abductor caudæ internus, and with the obturator internus (Fig. 90, e); distad with the vastus lateralis.

Action.—Abducts the thigh and helps to extend the shank.

M. semitendinosus (Fig. 90, j).—A long slender muscle on the ventral (caudal) border of the thigh, between the semimembranosus (i) and the biceps femoris (Fig. 68, t).

Origin from the apex of the tuberosity of the ischium beneath the origin of the biceps femoris. The muscle passes to the medial side of the shank and ends in a thin but strong tendon (Fig. 92, i′) about five millimeters broad. The tendon curves proximad and passes beneath the gracilis tendon (Fig. 91, b) to its

Insertion into the crest (dorsal border) of the tibia one or two centimeters from its proximal end.

Relations.—Lateral surface with the biceps femoris (Fig. 68, t), the integument, a mass of fat, and distad with the gastrocnemius (Fig. 92, j) and popliteus (Fig. 92, k). Caudal surface with the integument. Medial surface with the semimembranosus (Fig. 90, i) and distad with the integument of the lower leg.

Action.—Flexor of the shank.

M. semimembranosus (Figs. 90 and 91, i; Fig. 92, h).—A thick prismatic muscle lying along the ventral (or caudal) side of the thigh between the semitendinosus (j) and the gracilis (Fig. 91, b).

Origin by short tendon-fibres from the caudal border of the tuberosity and the ramus of the ischium. The muscle mass is divided throughout most of its length into two portions (Fig. 92, h and h′), one of which (h′) lies laterad and caudad of the other. The two portions pass to the medial side of the knee. The caudal portion (h′) ends in a strong flat tendon five millimeters broad, the

Insertion of which is into the medial surface of the femur on the medial epicondyle, at the middle of its distal border and into the adjacent medial surface of the tibia behind the lateral ligament.

The dorsal part has its insertion into the distal one and one-half to two centimeters of the ridge which is continued from the medial epicondyle of the femur onto the shaft; and into the sesamoid bone of the medial epicondyle.

Relations.—Lateral surface with the semitendinosus (Fig. 90, j), the biceps femoris (Fig. 68, t), and distad with a small part of the adductor femoris (Fig. 92, g), and with the gastrocnemius (Fig. 92, j). Medial surface with the gracilis (Fig. 91, b) and distad with the sartorius (Fig. 91, a). Cranial (or dorsal) border at the origin with the quadratus femoris (Fig. 90, f); for the remainder of its length with the adductor femoris (Fig. 90, h). Caudal border with the integument proximad.

Action.—Extensor of the thigh.

M. sartorius (Fig. 91, a; Fig. 68, q).—A large flat muscle along the medial side of the thigh near its cranial (dorsal) border.

Origin from the ventral half of the crest of the ilium and from the medial half of its ventral border craniad of the auricular impression. The caudal third of the origin is by a thin tendon, the rest by fleshy fibres.

Insertion on a long S-shaped line which begins on the dorsal border of the tibial shaft about two centimeters from its proximal end, passes thence to the middle of the proximal end of the medial surface of the tibia, thence across the ligaments of the knee-joint and the medial surface of the medial epicondyle to the patella, crosses the patella obliquely to the middle of its proximal end, and may be continued for some distance proximad along the middle line on the fascia.

Relations.—Medial surface with the integument and proximad with the internal oblique muscle (Fig. 68, p). Lateral (inner) surface with the tensor fasciæ latæ (Fig. 68, r), the vastus lateralis (Fig. 90, d), the superficial fascia, the vastus medialis (Fig. 92, c), the rectus femoris (Fig. 92, b), and the distal end of the semimembranosus (Fig. 92, h′).

Action.—Adducts and rotates the femur and extends the tibia.

M. gracilis (Fig. 91, b).—The gracilis is a flat, thin, subcutaneous muscle occupying the ventral half of the medial surface of the thigh.

Origin by a strong tendon from the caudal three-fourths of the symphysis of the ischium and pubis. The tendon may extend caudad of the symphysis in the middle line to the level of the tuberosity of the ischium. For about its first centimeter the tendon is common to the two muscles of opposite sides and gives origin on two sides to the fibres of the adductor femoris. The common tendon divides into two which pass laterad, each ending in a convex border from which the muscle-fibres spring.

Insertion.—It ends in a thin aponeurosis, part of which is continuous with that of the sartorius. The aponeurosis is finally inserted into the medial surface of the tibia near its proximal end, while distally it is continuous with the fascia.

Relations.—Outer (medial) surface with the integument. Inner (lateral) surface with the semimembranosus (Fig. 91, i) and the adductor femoris (Fig. 91, h).

Action.—Adducts the leg and draws it caudad.

M. adductor femoris (magnus et brevis) (Fig. 92, g; Fig. 90, h).—The adductor femoris lies between the semimembranosus (Fig. 92, h) and the femur. Its proximal part is covered by the semimembranosus, while its distal part lies outside of it.

Origin by muscle-fibres from the rami of the pubis and the ischium along the whole of the length of the symphysis; from the ramus of the ischium between the symphysis and the tuberosity, and from the tendon of origin common to the two gracilis muscles.

Insertion into nearly the whole of the ventral surface of the shaft of the femur. The area begins at the base of the great trochanter opposite the proximal end and the insertion area of the gluteus maximus. It extends thence distad along the caudal border of the surface as an area about one millimeter wide. At the junction of the first and second thirds of the shaft the area expands and occupies the whole of the ventral surface of the bone, ending at the intercondyloid fossa.

Relations.—Lateral surface with the biceps (Fig. 68, t), the great sciatic nerve (Fig. 163, a), the tenuissimus (Fig. 90, g), and the semimembranosus (Fig. 92, h); dorsolateral border in contact with the vastus lateralis (Fig. 92, c). Dorsal surface with the quadratus femoris (Fig. 90, f) and obturator externus (Fig. 90, e). Caudal surface with the semimembranosus (Fig. 92, h). Medial surface with the gracilis (Fig. 91, b), adductor longus (Fig. 92, f), pectineus (Fig. 92, e), vastus medialis (Fig. 92, c), and distad with the medial head of the gastrocnemius (Fig. 92, j).

Action.—An extensor of the thigh.

M. adductor longus (Fig. 91, g; Fig. 92, f).—A thin muscle which covers the dorsal half of the medial surface of the adductor femoris (Fig. 92, g).

Origin by muscle-fibres from the median three-fourths of the cranial border of the pubis, the line of origin forming a medial continuation of that of the pectineus (Fig. 92, e).

Insertion by a thin aponeurosis into the external linea aspera of the femur along the second and third fifths of the bone.

Relations.—With its medial or cranial surface the adductor longus forms part of the boundary of a depression among the muscles at the proximal end of the medial side of the leg. This depression is called the iliopectineal fossa; it contains the femoral vein and artery and saphenous nerve imbedded in fat (Fig. 127). The medial edge of the adductor longus is in relation with the integument; the lateral edge with the pectineus (Fig. 92, e). Inner or caudal surface with the adductor femoris (Fig. 92, g).

Action.—Adductor of the thigh.

M. pectineus (Fig. 92, e).—A flat band of fibres closely united with the adductor longus (f), of which it appears to be a lateral continuation.

Origin by fleshy fibres from the lateral one-fourth of the cranial border of the pubis. The muscle passes over the smooth outer surface of the pubis between its origin area and the acetabulum, crosses the iliopsoas (d) obliquely and has its

Insertion by muscle-fibres into an elongated area (five millimeters in length) on the shaft of the femur just distad of the lesser trochanter and between the insertion of the adductor femoris (g) and that of the vastus medialis (c).

Relations.—The cranial edge forms part of the floor of the iliopectineal fossa. Lateral surface in relation with the iliopsoas (d) and vastus medialis (c); medial surface with the adductor longus (f) and adductor femoris (g).

Action.—Adductor of the thigh.

M. quadriceps femoris.—The quadriceps femoris is a very powerful extensor muscle on the front of the thigh. It consists of four muscles which unite to form the great extensor of the shank. It is inserted into the patella and through it, by the ligamentum patellæ, into the tibia. It may be compared to the triceps brachii. The parts are:

(1) M. rectus femoris (Fig. 92, b; Fig. 90, c).

Origin by strong tendon from an elongated triangular area which has its base at the acetabulum and its apex about five to seven millimeters craniad of the acetabulum, along the ventral border of the ilium. The area is between that for the capsularis and that for the iliopsoas. The muscle is flat near its origin, but soon becomes a triangular prismatic mass which is united to the dorsal border of the vastus lateralis (Fig. 90, d) at the junction of the middle and last thirds of the thigh.

Insertion into the oblique area on the outer surface of the patella near its proximal border in connection with the vastus lateralis.

Relations.—Outer or lateral surface at the origin with the gluteus minimus, the edge of the capsularis, the gluteus medius (Fig. 90, b), and farther distad with the vastus lateralis (Fig. 90, d). Medial surface with the tensor fasciæ latæ (Fig. 92, a), sartorius (Fig. 90, a), and vastus medialis (Fig. 92, c). Inner (caudal) surface with the vastus intermedius.

(2) M. vastus lateralis (Fig. 90, d).—A flat triangular prismatic mass joined distally to the preceding. It covers the dorsal part of the lateral surface of the thigh.

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.

Origin from nearly the whole of the dorsal surface of the shaft of the femur between the areas for the vastus medialis and vastus lateralis. The area of origin extends distad to an oblique line the middle of which is about one and one-half centimeters from the patellar surface, the lateral border of the area being longer than the medial border.

Insertion by muscle-fibres into the capsule of the joint. The central fibres are inserted about one centimeter proximad of the patella, while the lateral mass passes to the level of the proximal end.

Relations.—Outer surface with the rectus femoris (Fig. 92, b); borders with the vastus medialis (Fig. 92, c) and vastus lateralis (Fig. 90, d); inner surface with the femur.

Action.—The quadriceps extensor is an extensor of the shank. The vastus intermedius acting separately is a tensor of the capsule of the knee-joint.

3. Muscles of the Lower Leg (Crus).

A. On the Ventral Side.

—M. gastrocnemius.—This is the great muscular mass of the calf. It arises by two heads, the caput laterale or lateral head (Fig. 90, m), and the caput mediale or medial head (Fig. 91, k), of the gastrocnemius.

Caput laterale (Fig. 90, m, m′, m″).—The origin of the lateral head is in four portions:

(1) From the tendon of origin of the plantaris which comes from the middle of the lateral border of the patella. The tendon ends in a small conical mass of fibres (m′) which joins the ventral part of the common head.

(2) By a slender flat tendon, distad of the first, from the superficial fascia of the shank (m″). The tendon ends in a conical mass which joins the dorsal border of the common head.

(3) By a strong tendon and by muscle-fibres from the distal border of the lateral sesamoid bone (Fig. 61, 3) of the femur. These fibres (m) form the middle and outer part of the common head.

(4) By muscle-fibres from the middle of the outer surface of the aponeurosis covering the plantaris (Fig. 90, l) along rather more than the proximal half of the tibia. The lateral head thus formed is flat and fusiform in section.

Caput mediale (Fig. 91, k; Fig. 92, j).—The medial head resembles the lateral head closely.

Origin by a strong tendon from the medial sesamoid bone of the femur (Fig. 61, 4) and by muscle-fibres from the surface of the shaft for a short distance proximad of this.

The two heads unite by their contiguous borders at the junction of the first and second thirds of the tibia. The common muscle narrows rapidly and ends in a flat tendon which joins the tendon of the soleus to form the so-called tendon of Achilles (tendo calcaneus) (Fig. 90, t). This has its

Insertion into the proximal end of the calcaneus (Fig. 90, 4) near its ventral border. This tendon together with that of the soleus (Fig. 90, o) and the fascia of the shank forms a tubular sheath for the tendon of the plantaris.

Relations.—(1) Of the lateral head. Lateral (outer) surface with the biceps femoris (Fig. 68, t) and the superficial fascia. Medial (inner) surface with the popliteus (Fig. 92, k), the plantaris (Fig. 90, l) (to which it is closely united), the medial head of the gastrocnemius (Fig. 91, k), the soleus (Fig. 90, o), and the peroneus longus (Fig. 90, q). (2) Of the medial head. Medial (outer) surface with the superficial fascia and the tendons of the gracilis (Fig. 91, b), semimembranosus (Fig. 92, h′), and semitendinosus (Fig. 92, i). Lateral (inner) surface at the origin with the adductor femoris (Fig. 92, g), then with the plantaris (Fig. 91, l), the popliteus (Fig. 92, k), flexor longus digitorum (Fig. 91, n), and the lateral head of the gastrocnemius (Fig. 90, m).

Action.—Extensor of the foot. (It is to be noted that what is called extension in the foot is analogous to what is called flexion in the hand, so that the extensors of the foot are most readily to be compared with the flexors of the hand.)

M. plantaris (Fig. 90, l; Fig. 91, l).—A fusiform muscle covered distally by the gastrocnemius, but appearing between the two heads of the gastrocnemius at their proximal ends (Fig. 90, l).

Origin by a strong tendon from the middle of the lateral border of the patella and by fleshy fibres from the ventral border of the lateral sesamoid.

The patellar tendon ends in a flattened head which passes ventrad over the lateral epicondyle of the femur and then curves distad; it is closely united with the lateral head of the gastrocnemius. The muscle ends in a thick tendon which passes through a sheath formed by the tendons of the gastrocnemius and soleus, and the fascia of the shank. The tendon passes over the grooved proximal end of the calcaneus onto its ventral surface. It is held in place against the groove by two sheets of aponeurosis, which leave the tendon on either side for a distance of more than a centimeter and are attached to the lateral surfaces of the calcaneus at its proximal end. Lateral motion is thus hindered, while longitudinal motion is permitted.

Between this tendon and the tendon of Achilles is a synovial bursa which aids the gliding movement of the tendon. In the sole of the foot opposite the distal end of the calcaneus the tendon broadens and ends in the flexor brevis digitorum (pedis perforatus) muscle, which might also be considered as a second part of the plantaris. In man the plantaris is inserted into the calcaneus along with the gastrocnemius.

Relations.—The muscle is covered throughout by the two heads of the gastrocnemius (Fig. 90, m; Fig. 91, k), except at the proximal end (Fig. 90, l), where it is in contact with the integument. Inner surface in relation with the popliteus (Fig. 92, k), the soleus (Fig. 90, o), the flexor longus digitorum (Fig. 91, n), and flexor longus hallucis (Fig. 91, m).

M. soleus (Fig. 90, o).—A flat fusiform muscle lying beneath the plantaris.

Origin by muscle-fibres from the lateral surface of the head of the fibula, and by tendon-fibres from the proximal two-fifths of its ventral border.

Insertion.—The muscle ends in a slender tendon which joins the lateral border of the gastrocnemius tendon to form the tendon of Achilles (Fig. 90, t), which forms the sheath of the plantaris tendon.

Relations.—Outer surface with the gastrocnemius (Fig. 90, m) and plantaris (Fig. 90, l). Inner surface with the peroneus longus (Fig. 90, q), peroneus tertius (Fig. 90, r), and flexor hallucis (Fig. 91, m).

Action.—Assists the gastrocnemius to extend the foot.

M. triceps suræ.—The gastrocnemius and the soleus are sometimes considered as forming a single muscle, with three heads, under the name triceps suræ.

M. popliteus (Fig. 92, k).—A triangular muscle passing from the femur obliquely toward the medial side over the ventral surface of the proximal end of the tibia.

Origin by a strong tendon from the popliteal groove on the surface of the lateral epicondyle of the femur. In the tendon is a sesamoid bone, the popliteal bone (Fig. 61, 5, page 89), which glides over the ventral part of the lateral articular facet on the proximal end of the tibia. As the muscle passes over the joint it is closely attached to the joint capsule. From the popliteal bone, which with the tendon lies within the capsule of the knee-joint, the muscle-fibres diverge to their

Insertion into the proximal end of the ventral surface of the tibial shaft on the medial side of the medial oblique ridge. The area of insertion extends slightly onto the medial surface of the bone.

Relations.—Outer surface with the gastrocnemius (Fig. 92, j) and plantaris (Fig. 91, l), and with the tendon of the semitendinosus (Fig. 92, i). Distal border with the origins of the soleus and flexor longus digitorum. Inner surface with the capsule of the knee-joint and the tibia.

Action.—Rotates the thigh so as to turn the toes inward.

The three following deep muscles on the ventral surface of the shank are covered by a deep fascia (the deep crural fascia) which separates them from the overlying muscles. This fascia stretches from the dorsal border of the tibia about the ventral side of the shank to the medial border of the tibia. It sends a slip beneath the border of the fibula.

M. flexor longus digitorum (Fig. 91, m and n).—Owing to the reduction of the first digit in the cat the tendon of the flexor longus hallucis (m) has become united to that of the flexor longus digitorum (n), so that the two might be considered separate heads of a single muscle. The head corresponding to the flexor longus hallucis (m) is much larger than that corresponding to the flexor longus digitorum (n). The two heads will be described separately.

(1) M. flexor longus hallucis (Fig. 91, m).—This lies against the ventral surface of the tibia and fibula beneath the popliteus (Fig. 92, k) and soleus (Fig. 90, o).

Origin by fleshy fibres, (1) from the sheet of deep crural fascia which dips between it and the tibialis posterior (Fig. 91, o) and the flexor longus digitorum (n). (2) From the ventral surface of the tibia distad of the lateral oblique line to within one to three centimeters of the distal end of the shaft. (3) From the medial surface of the shaft and head of the fibula and from the whole interosseous ligament. On the fibular side the origin may extend onto the tendon of the popliteus and the fascia of the peroneus longus. The fibres form a flat fusiform mass ending one centimeter from the heel in a strong flat tendon (m′) which passes over the groove on the distal surface of the astragalus and over the groove on the sustentaculum tali. The two grooves are converted into a continuous canal by strong transverse ligaments, and the canal is lined by an extension of the synovial bursa of the ankle-joint. Emerging from the canal the tendon broadens and receives on its medial side the insertion of the tendon of the flexor longus digitorum (n). The tendon gives origin on its outer surface to the lumbricales and to the common plantar ligament. The tendon continues to broaden until it reaches the middle of the length of the metacarpals; here it divides into four tendons which pass to the terminal phalanges of the digits.

Insertion.—The relations of the four tendons and their insertions are identical with those of the flexor profundus digitorum of the hand.

Relations.—Outer surface with the soleus (Fig. 90, o) and plantaris (Fig. 91, l). Lateral border with the soleus, the peroneus tertius (Fig. 90, r), and peroneus brevis (Fig. 90, s). Medial border with the tibialis posterior (Fig. 91, o) and flexor longus digitorum (Fig. 91, n).

Action.—Flexor of the phalanges.

(2) M. flexor longus digitorum (Fig. 91, n).

Origin by muscle- and tendon-fibres from the ventral surface of the tibia over its proximal half between the oblique ridges; by fleshy fibres from an aponeurosis between it and the tibialis posterior (Fig. 91, o), and by tendon from the medial surface of the head of the fibula. At the junction of the middle and distal thirds of the bone the muscle ends in a slender tendon which passes through the ventral groove on the medial surface of the tibia and, curving onto the sole of the foot, becomes attached to the medial border of the common tendon described under the last.

Relations.—Outer surface with the medial head of the gastrocnemius (Fig. 91, k) and with the superficial fascia. Inner surface with the tibialis posterior (Fig. 91, o). Medial border at the proximal end with the popliteus (Fig. 92, k), lateral border with the tibialis posterior (Fig. 91, o), and flexor longus hallucis (Fig. 91, m).

Action.—Flexor of the phalanges.

M. tibialis posterior (Fig. 91, o).—A slender flat fusiform muscle beneath the flexor longus digitorum and between it and the flexor longus hallucis.

Origin by fleshy fibres from nearly the whole medial surface of the head of the fibula, from the inner surface of the aponeurosis between it and the flexor longus digitorum (n), and by a few fleshy fibres from the ventral surface of the tibia between the oblique ridges. Some fibres may also arise from the outer surface of the aponeurosis covering the flexor longus hallucis (m). The muscle ends at about the middle of the tibia in a slender flat tendon which passes parallel to the tendon of the flexor longus digitorum (n) through the dorsal groove on the medial surface of the distal end of the tibia. This groove is converted into a canal by a transverse ligament. Beyond the canal the tendon turns onto the plantar surface of the foot and passes through a groove on the ventral surface of the scaphoid bone. It then divides.

Insertion into the outer tuberosity on the surface of the scaphoid, and onto the proximal end of the ventral surface of the medial cuneiform.

Relations.—Outer and medial surface with the flexor longus digitorum (n). Lateral surface with the flexor longus hallucis (m).

Action.—Extensor of the foot.

B. Muscles on the Dorsal and Lateral Surfaces of the Lower Leg.

—M. peroneus longus (Fig. 90, q).—A slender, fusiform muscle lying superficially on the lateral side of the leg, dorsad of the soleus.

Origin by tendon-fibres from the lateral surface of the head of the fibula and from the proximal half of the lateral surface of its shaft.

At the junction of the middle and distal thirds of the lower leg the muscle ends in a slender tendon which passes through the groove on the lateral surface of the lateral malleolus. This groove is converted into a canal by a transverse ligament. The tendon passes through the groove on the peroneal tubercle of the calcaneus, then turns onto the sole of the foot and passes through the peroneal groove on the cuboid bone. It then turns mediad and passes through the groove between the ventral processes of the lateral cuneiform and the metatarsal until it reaches the first metatarsal. The entire groove is converted into a canal by the overlying ligaments.

Insertion into (1) the base of the fifth metatarsal, and (2) the outer side of the base of the first metatarsal, and (3) by slender branches into the bases of the other metatarsals.

Relations.—Outer (lateral) surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the peroneus tertius (r) and peroneus brevis (s), and with the extensor longus digitorum (p). At the proximal end the dorsal border touches the tibialis anterior (n).

Action.—Flexor of the foot.

M. peroneus tertius (Fig. 90, r).—A slender fusiform muscle beneath the peroneus longus (q).

Origin by fleshy fibres from about the second quarter of the lateral surface of the fibula. The muscle ends in a slender tendon (r′), which passes with that of the peroneus brevis (s) through the groove on the ventral border of the lateral malleolus. The groove is converted into a canal by a transverse ligament. Emerging from it, the tendon turns and passes along the outer margin of the foot. In passing the sesamoid at the base of the first phalanx of the fifth digit it is united to it by a band which passes from the sesamoid to the tendon.

The insertion is finally into the lateral border of the extensor tendon of the fifth digit as it passes from the first to the second phalanx.

Relations.—Outer surface with the peroneus longus (q), the soleus (o), and the superficial fascia. Ventral border with the soleus (o) and flexor longus hallucis (Fig. 91, m). Inner surface with the peroneus brevis (Fig. 90, s).

Action.—Extensor and abductor of the fifth digit and flexor of the foot.

M. peroneus brevis (Fig. 90, s) lies beneath the other peronei.

Origin by fleshy fibres from the distal half of the surface of the fibula, which it embraces. At the lateral malleolus the muscle ends in a thick tendon which passes through the canal on the ventral surface of the malleolus. In the canal it is enlarged and surrounded by a synovial bursa. It then turns onto the foot and passes over the dorsal surface of the calcaneus on its lateral side and has its

Insertion into the tubercle on the lateral side of the base of the fifth metatarsal.

Relations.—Outer surface with the peronei longus (q) and tertius (r). Inner surface with the bone.

Action.—Extensor of the foot.

M. extensor digitorum longus (Fig. 90, p).—A fusiform muscle which lies beneath the tibialis anterior (n), against the interosseous membrane, and between the tibialis anterior and the peronei.

Origin by a thin flat tendon from the lateral surface of the lateral epicondyle of the femur just dorsad of the origin of the popliteus. The tendon becomes narrower and thicker and passes through the capsule of the knee-joint and over a slight groove on the tibia just dorsad of the head of the fibula. The belly of the muscle extends the entire length of the tibia, passing beneath the transverse ligament (5) along with the tendon of the tibialis anterior (n). At the ankle it runs through a fibrous loop which is attached to the dorsal surface of the calcaneus (the annular ligament of the calcaneus); it then turns and passes onto the dorsum of the foot. The muscle ends in four slender tendons, the two middle of which begin at the transverse ligament, while the others begin at the fibrous loop. The tendons diverge to the four digits. Each tendon is connected with the fibrous sheath surrounding the base of the first phalanx, and each has a synovial bursa beneath it at this point. Near the distal end of the first phalanx the tendon is united on the lateral side to the conjoined tendon of the extensor brevis digitorum and an interosseus muscle, and on the medial side to the tendon of the interosseus. The lateral side of the most lateral tendon is not thus united, but is joined on the outer side by the tendon of the peroneus tertius.

Insertion.—The tendon thus formed on the dorsum of each phalanx by the junction of the two or three tendons above mentioned is firmly attached to the base of the second phalanx at its proximal end in the middle of its dorsal surface. In crossing the joint between the first and second phalanges the tendon forms a fibrous pad which protects the dorsum of the joint and glides over it. It then continues to be inserted into the base of the terminal phalanx.

Relations.—Outer surface with the tibialis anterior (n) and the superficial fascia. Inner surface with a part of the tibialis anterior, with the bones, the interosseous membrane, and the peroneus brevis (s).

Action.—Extensor of the phalanges.

M. tibialis anterior (Fig. 90, n).—This is the superficial muscle covering the lateral side of the tibia.

Origin by fleshy fibres from the proximal one-sixth of the lateral surface of the shaft of the tibia, from the proximal third of the medial border of the shaft and head of the fibula, and from the intervening interosseous ligament. The triangular body of the muscle covers the lateral surface of the tibia and the outer surface of the extensor longus digitorum (p). Near the malleolus it ends in a strong tendon which passes beneath the transverse ligament along with the tendon of the extensor longus (p). The tendon crosses the dorsal surface of the foot obliquely toward its medial side.

Insertion into the outer surface of the first metatarsal.

Relations.—Outer surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the extensor longus (p), the tibia, and the interosseous membrane.

Action.—Flexor of the foot.

4. Muscles of the Foot.

A. Muscles on the Dorsum of the Foot.

—M. extensor brevis digitorum (Fig. 90, u).—A broad thin muscle covering the tarsus and part of the metatarsus on the lateral side of the dorsum of the foot. It may be divided into three slips.

Origin from the distal border of nearly the whole calcaneal annular ligament (5), and from the proximal end of the dorsal surface of the three lateral metatarsals. At the middle of the metatarsals the muscle ends in three flat tendons which pass into the three interspaces between the four tendons of the extensor longus. Each tendon is divided into two branches.

Insertion.—The lateral branch of each tendon is inserted into the cartilaginous plate which lies in the metatarsophalangeal articulation of the digit on the outer side. The medial branch joins the lateral side of the extensor longus tendon on the dorsum of the first phalanx.

Action.—Extensor of the digits.

B. Muscles of the Sole of the Foot.

—M. flexor brevis digitorum (pedis perforatus).—This is the direct continuation of the tendon of the plantaris. It lies immediately beneath the superficial plantar fascia. The flat belly of the muscle has its medial border attached by an oblique tendinous band to the medial surface of the scaphoid and the medial cuneiform. It divides into four slips which diverge to the four toes, each ending in a flat tendon. The slips decrease in size from the lateral to the medial side. The three lateral ones overlap one another proximally.

Insertion.—Each tendon expands at the distal end of the metatarsal and is wrapped about the tendon of the deep flexor which perforates it. The two halves unite beneath the perforating tendon and pass together with the perforating tendon through fibrous rings, the annular ligaments. One of these is attached to the sesamoids at the base of the phalanx. The other is attached to the head of the first phalanx. The tendons are finally inserted into the bases of the second phalanges.

The two annular ligaments are connected by fascia so as to form a continuous canal for the tendons. This canal is lined by a synovial membrane. Covering this canal is a more superficial layer of fascia. Before passing into the proximal annular ligament the two middle tendons unite each with the tendon of the corresponding lumbrical muscle. Each also gives off a branch, the lateral one on its lateral side and the medial one on its medial side. These branches, like those of the two middle ones, are inserted into the common phalangeal fascia.

Action.—Flexor of the second phalanges of the digits.

M. quadratus plantæ (called also the plantar head of the flexor longus digitorum) is a thin flat muscle which takes origin from the dorsal part of the lateral surface of the calcaneus and the cuboid. Its fibres converge to a flat tendon which passes transversely across the flexor longus digitorum and beneath the flexor brevis digitorum and calcaneometatarsal.

Insertion.—Into the medial part of the outer surface of the tendon of the flexor longus digitorum.

Action.—It holds the flexor longus tendon in place.

Mm. lumbricales.—These are six.

Origin.—The three larger ones have origin from the outer surface of the expanded portion of the flexor longus digitorum on its distal half. Each ends in a slender tendon.

Insertions.—The tendons unite with the divisions of the tendon of the flexor brevis which pass to the three lateral digits, at their entrance to the first annular ligament. The one to the fifth digit is sometimes absent.

The other three have origin from the tendon of the flexor longus digitorum at the point where it divides. Each occupies one of the three intervals between the four divisions, and its fibres originate from the proximal ends of the two tendons which bound its interval.

Insertion by a very slender tendon into the medial side of the first phalanges of the third, fourth, and fifth digits near their bases.

Action.—Move the third, fourth, and fifth digits toward the medial side.

Mm. interossei.—The interosseus of the second digit forms five portions, which cover the ventral surface of the digit.

Origin.—(1) By two tendons from the ventral process of the lateral cuneiform. The muscle divides into four parts, three of which pass to the medial side of the digit and act as abductors. They are called the long, short, and middle abductors (abductor longus, brevis, and intermedius digiti secundi).

Insertions.—The short head into the medial sesamoid of the metatarsophalangeal joint, the middle into the medial side of the base of the first phalanx, and the long into the extensor communis tendon near the distal end of the first phalanx. The names long, short, and middle refer to the lengths of the tendons as determined by their points of insertion.

The fourth portion which comes from the lateral cuneiform has its insertion into the lateral side of the extensor tendon near the distal end of the first phalanx. It is therefore an adductor longus digiti secundi. In some cases there is an adductor brevis from this same head.

(2) The fifth part of the interosseus of the second digit has origin from the middle of the ligament covering the peroneal canal, along with the middle adductor of the fifth digit. The two muscles diverge. Insertion into the outer side of the base of the first phalanx of the second digit. It is an adductor medius digiti secundi.