The lower extremities resemble the upper very closely in the arrangement of the bones, muscles, arteries, and nerves, though modifications occur, due to the difference in function of the lower limbs. There is one long bone in the upper part or thigh, the femur, and two in the lower part or leg, the tibia and fibula, while over the knee-joint is the patella or knee-cap. The ankle has seven bones and the foot nineteen like the hand.

The Femur.—The femur is the longest bone in the body, being about one-fourth the height of the person. It inclines toward its fellow at the knee in order to bring the knee-joints near the center of gravity in walking, the amount of inclination varying with the width of the hips and the height of the person. On account of the greater width of hip the tendency to knock-knee is greater in women than in men.

The shaft of the femur is enlarged at the extremities and is slightly curved forward, the concavity being strengthened at the back by a longitudinal ridge, the linea aspera, along part of which the gluteus maximus muscle is attached. The head, which is covered with cartilage, except for an oval depression for the attachment of the ligamentum teres, one of the ligaments of the hip-joint, and which articulates with the hollow of the acetabulum in the os innominatum, projects considerably upward, inward, and forward from the shaft, the neck varying much in length and angle. It is generally more horizontal in women than in men and in rickets the great weight on the softened bone tends to press the head down, causing the deformity known as “coxa vera”, in which the neck is almost horizontal. Extending upward, outward, and backward from the shaft at the base of the neck, about three-quarters of an inch lower than the head and about on a level with the acetabulum and the spine of the os pubis, is the greater trochanter. This large, irregular prominence and the smaller one of the lesser trochanter, which is at the lower part of the base of the neck posteriorly, are for the attachment of muscles and to assist in rotating the bone. The lower extremity of the femur is larger than the upper and is flat from before backward. Between its two large eminences, the external and internal condyles, is a smooth depression in front, the trochlear surface, for articulation with the patella. The external condyle is more prominent in front, the internal inferiorly, the latter being the longer of the two by about half an inch. The epiphysis at the lower end of the femur is the only one in which ossification has begun at birth. Therefore, if ossification is found there, the child is known to have arrived at full term.

So many large muscles are attached to the femur that the shaft cannot be detected in the living unless the person is very thin and poorly developed. The outer surface of the greater trochanter, however, and the condyles can be felt.

A string stretched from the anterior superior spine of the ilium to the tuberosity of the ischium passes in the middle just over the upper edge of the greater trochanter. The line thus drawn is known as Nélaton’s line and is of considerable importance in many conditions of the hip. Thus, if the hip is dislocated, the trochanter will be thrown above Nélaton’s line, and in osteomalacia the pelvis sinks and the trochanter is again above the line.

Thigh Muscles.—Of the thigh muscles only a few need be mentioned. One large muscle is the psoas magnus, which has its origin on the front of the last dorsal and all the lumbar vertebræ, passes down across the brim of the pelvis and under Poupart’s ligament, gradually diminishing in size, and terminates in a tendon that is inserted into the lesser trochanter. It serves to flex the thigh on the pelvis and to rotate it outward. The psoas parvus rises from the last dorsal and the first lumbar vertebræ and does not go out of the pelvis.

The sartorius or tailor muscle is flat and ribbon-like and is the longest muscle in the body. It rises from the anterior superior spine of the ilium and is inserted into the upper inner surface of the shaft of the tibia. By it the legs are crossed. It also forms the outer side of an important landmark, Scarpa’s triangle, whose base is formed by Poupart’s ligament and the inner side by the adductor magnus muscle, which passes from the ramus of the os pubis and the tuberosity of the ischium to the linea aspera. The femoral artery bisects the triangle and runs into its apex.

The bulk of the anterior portion of the thigh is formed by the quadriceps extensor, which is really made up of four muscles, the rectus femoris, whose origin is on the anterior inferior iliac spine and above the acetabulum; the vastus externus, which comes from the greater trochanter and the upper linea aspera; and the vastus internus and crureus, which rise from the neck of the femur and the linea aspera. It is inserted into the tubercle of the tibia by the ligamentum patellæ, in which the patella lies. Its action is to extend the leg.

At the back and forming the buttocks are the three glutei muscles, the gluteus maximus, medius, and minimus. All these rise from the outer side of the ilium and have their insertion on or about the great trochanter. They serve to hold the trunk erect and to extend, abduct, and rotate the thigh.

Lower down and forming the back of the thigh are the biceps and the semitendinosus and semimembranosus muscles. The biceps rises by two heads from the tuberosity of the ischium and the linea aspera and is inserted into the head of the fibula. It is on the outer side of the thigh and its tendon, which embraces the external lateral ligament of the knee-joint, forms the outer hamstring. On the inner side are the semitendinosus and the semimembranosus muscles. These rise from the tuberosity of the ischium and are inserted, the one into the upper inner surface of the shaft of the tibia and the other into the internal tuberosity of the tibia. Their tendons form the inner hamstring. Like the biceps they serve to extend the thigh and flex the leg on the thigh, but where the biceps rotates the leg out they, being attached to the inner side of the leg bones, rotate it in.

The patella, or small pan, is a flat, somewhat triangular bone developed in the quadriceps extensor tendon. Four muscles are attached to it as well as the ligamentum patellæ, which holds it to the tibia and gives increased leverage by making the quadriceps extensor work at a greater angle. It articulates with the condyles and serves to protect the joint. One bursa, the prepatella bursa, separates it from the skin and another, surrounded by adipose tissue, from the head of the tibia. The external surface can be seen and felt on the front of the knee and the bone can be moved from side to side when the leg is straight.

Joints of the Lower Extremity.—The hip-joint is a ball-and-socket joint but is not so freely movable as the shoulder-joint, the head of the femur being held in the acetabulum by many strong ligaments, of which the most important is the capsular ligament.

The knee-joint is largely a hinge joint, but in some positions it has some rotation. It is formed by the condyles of the femur, the head of the tibia, and the patella, and has fourteen ligaments, including the ligamentum patellæ and the crucial ligaments. Its synovial sac is the largest found in any joint. Two semilunar cartilages, placed on the head of the tibia, serve to deepen the socket for the condyles, changing somewhat in shape and thickness as the joint moves. The interval between the thigh and the leg bones can be felt at the knee. When the leg is extended the juncture of the bones is slightly above the patella, while in flexion a knife passed below the apex of the patella will pass into the joint.

Congenital dislocation of the hip occurs. Separation of the epiphysis of the femur may occur and sometimes the neck, rarely the lower part of the shaft, is fractured. Either condyle may be fractured off or there may be a T-fracture, in which case the popliteal artery may be injured. In dislocation the head may be behind or in front of the acetabulum. Impacted hip, where the neck of the femur has, in a fall, been driven into the head, is common in old people. Sometimes, especially in young children, the bone is infected, osteomyelitis. Sarcoma occurs. Most tubercular disease of the hip originates at the upper extremity of the femur, tuberculosis generally starting in the head and then attacking the capsule and the soft parts of the joint. If neglected, shortening of the leg may result, in which case the bone has to be broken and set at an angle in order to enable the child to walk.

Occasionally a bit of cartilage gets broken off in the knee-joint and wedged between the bones, so that the joint cannot be straightened. This is dislocation of the semilunar cartilage and necessitates an operation for removal of the piece. The cartilage will eventually be replaced by fibrous tissue and in a few months the leg will be all right. Dislocation of the knee is rare, though it may occur in any direction. Often the bursæ of the joint are irritated, as by kneeling to scrub floors, and bursitis or housemaid’s knee results. Fracture of the patella may be caused by muscular traction or by direct violence, and is generally repaired by making an incision and sewing the parts of the bone together. Tumor albus or white swelling is tuberculosis of the knee and is fairly common in children. Specific knee means syphilis of the knee and generally occurs in both knees.

The Tibia.—The tibia or shin bone is next longest to the femur and is on the inner side of the leg, corresponding to the ulna in the arm. The shaft is prismoid and is more slender for the lower quarter, where fracture is consequently most frequent. The anterior border forms the crest or shin and can be felt for its upper two-thirds. The lower extremity, which is smaller than the upper, articulates with the astragalus bone of the ankle and with the fibula. Its head or upper extremity is expanded into two lateral tuberositis for articulation with the femur and for muscular attachment, both of which can easily be felt just below the bend of the knee. Their upper surfaces are smooth and concave, with a vertical bifid spine in the middle and a prominent tubercle for the attachment of the semilunar cartilages on either side. On the anterior surface of the head, below, is a rough eminence or tubercle, which also can be felt. The lower part of this is for the attachment of the ligamentum patellæ, while the upper part, which is smoother, is for the bursa that is placed under the tendon to prevent friction. On the back of the outer tuberosity is a facet for the head of the fibula. At the lower end there projects downward on the inner side, overhanging the arch of the foot, the internal malleolus, the prominent part of the ankle. It is on a higher level and somewhat farther forward than the external malleolus.

The Fibula.—The fibula is the most slender of all the bones in proportion to its length and is on the outer side of the leg. Its head is small and placed toward the back of the tibia below the knee-joint, from which it is excluded. The head articulates with the external tuberosity and has extending upward from it the styloid process. To it is attached the biceps tendon or outer hamstring. At the lower extremity of the shaft is the external malleolus, which articulates with the astragalus and forms the outer ankle. The only parts of the fibula that can be felt, besides the malleolus, which is very prominent, are the head and the lower external surface of the shaft.

In fracture of the leg both bones are usually broken, though either may be broken separately. Pott’s fracture is fracture of the lower fibula, and may be caused by stamping hard when stepping on to the sidewalk. In rickets the tibia becomes bowed outward and forward, causing bow leg, a condition which in very young children may be rectified by manipulation. Later on braces are needed and after five years the bones have to be broken and set straight.

The Ankle.—The ankle or tarsus has but seven bones where the wrist has eight. They are the os calcis or heel bone, which is the largest and strongest and forms the tuberosity of the heel; the astragalus, which is next largest and helps to form the ankle-joint; the cuboid; the navicular (boat-like) or scaphoid; and the internal, middle, and external cuneiform bones. The astragalus is above and partially in front of the os calcis, to which is attached the tendo Achillis. The cuboid is on the outer side of the foot, in front of the os calcis and behind the metatarsals. It is noticeable in congenital club-foot, in which condition the tarsal bones may be distorted in shape and misplaced. The navicular or scaphoid is on the inner side of the foot, between the astragalus and the three cuneiform bones.

The Foot.—There are five metatarsal bones in the foot, corresponding to the five metacarpals in the hand, and the toes have the same number of phalanges as the fingers, though they are shorter and stronger. The big toe corresponds to the thumb.

Fracture of the os calcis and the astragalus are most commonly caused by a fall from a height, while the metatarsals and phalanges are generally broken by something heavy falling upon them. Because of their delicate structure, their distance from the heart, and the differences of temperature to which they are subjected, the tarsal bones are especially liable to become tubercular, amputation of the feet even becoming necessary at times. In diabetes there may be a perforating ulcer on the sole of the foot and the bone may become diseased.

Muscles of the Leg.—The greater part of the calf of the leg is formed by the gastrocnemius, a large bulging muscle, which rises from the condyles of the femur and is inserted along with the soleus, whose origin is on the back of the upper fibula, and the plantaris, which comes from the linea aspera, into the os calcis by a common tendon, the tendo Achillis, the largest and strongest tendon in the body. Its action is to extend the foot and to rotate it slightly inward. Other extensors of the foot, which also evert it, are the peroneus longus and the peroneus brevis at the upper and outer part of the leg, the former rising from the outer tuberosity of the tibia and the upper fibula and being inserted into the first metatarsal and the internal cuneiform, the latter arising from the lower fibula and being inserted into the fifth metatarsal. The foot is flexed, adducted, and rotated inward by means of the tibialis anticus, which rises from the outer tuberosity and the upper two-thirds of the outer surface of the tibia and is inserted into the internal cuneiform bone.

In the foot, and corresponding to the palmar fascia in the hand, is the plantar fascia, the densest of all fibrous membranes. There are also various annular ligaments, and the foot muscles are arranged similarly to those in the hand.

The Blood Supply of the Lower Extremity.—The blood supply of the lower extremity comes from the external iliac artery, a branch of the common iliac, which passes obliquely downward and outward along the border of the psoas muscle to Poupart’s ligament, where it enters the thigh and becomes the femoral artery. Its only important branches are the deep epigastric, which goes up along the internal abdominal ring, and the deep circumflex iliac. As the femoral artery it passes down the inner side of the thigh to the internal condyle of the femur, being very superficial at Scarpa’s triangle, where it can be compressed with the thumb to stop hemorrhage below. If a tourniquet is applied, it should be applied a little lower down. The first and most important branch of the femoral is the profunda femoris.

About two-thirds of the way to the knee the artery takes the name popliteal. It lies superficially in the popliteal space back of the knee, but above and below it is covered with muscles. Its branches supply the knee-joint and nearby muscles and are unimportant. At the lower border of the popliteus muscle, a small muscle at the knee, it divides into the anterior and posterior tibial arteries. The course of the former of these may be marked by a line from the inner side of the head of the fibula to midway between the malleoli at the front of the ankle, where it terminates in the dorsalis pedis artery for the back of the foot. By this last the pulse is sometimes taken and its pulsation is a guide in determining how high up to amputate in gangrene of the foot. The posterior tibial extends obliquely down the back of the leg to the heel, where it divides into the internal and external plantar arteries which go to the sole of the foot. Its most important branch is the peroneal.

Besides the deep veins accompanying the arteries there are the superficial veins, the internal or long saphenous on the inner side of the leg and thigh and the external or short saphenous on the middle of the leg posteriorly and emptying into the popliteal vein. Varicosity often occurs in these veins.

Nerves.—The nerves of the muscles about the hip are branches of the lumbar nerve. The anterior crural supplies the anterior part of the thigh, the gluteal the muscles of the same name, and the great sciatic the large muscles of the back of the thigh. Below the knee the anterior tibial goes to the tibialis anticus and the internal popliteal to the muscles of the calf, while the peroneus muscles are supplied by the musculo-cutaneous.