The sesamoid bones are more or less rounded nodules of bone imbedded in certain tendons and usually related to articular surfaces. Their functions probably are to modify pressure, to diminish friction, and occasionally to alter the direction of the pull of a muscle. The fact that they are present as cartilaginous nodules in the fetus and in greater numbers than in the adult shows that they are not developed to meet certain physical requirements in the adult. They must be regarded as integral parts of the skeleton phylogenetically inherited. Physical necessities probably come into play in selecting and in regulating the degree of development of the original cartilaginous nodules.
Sesamoid bones are invested by the fibrous tissue of the tendons, except on the surfaces in contact with the parts over which they glide, where they present smooth articular facets.
In the upper limb the sesamoid bones of the joints are found only on the palmar surface of the hand. Two, of which the medial is the larger, are present at the metatarpophalangeal joint of the thumb, imbedded in the tendons of the adductor pollicis.and the flexor pollicis brevis ; one is frequently present in the corresponding joint of the index finger, and one (or two) in the same joint of the little finger. Sesamoid bones are found occasionally at the metacarpophalangeal joints of the middle and ring fingers, at the interphalangeal joint of the thumb, and at the distal interphalangeal joint of the index finger.
In the lower limb the largest sesamoid bone of the joints is the patella, developed in the tendon of the quadriceps femoris. On the plantar aspect of the foot, two, of which the medial is the larger, are always present at the metatarsophalangeal joint of the great toe, imbedded in the tendons of insertion of the flexor hallucis brevis ; one sometimes at the metatarsophalangeal joints of the second and fifth toes, one occasionally at the corresponding joints of the third and fourth toes, and one at the interphalangeal joint of the great toe.
Sesamoid bones apart from joints are seldom found in the tendons of the upper limb ; one is sometimes seen in the tendon of the biceps opposite the radial tuberosity. Sesamoid bones or cartilages are, however, present in several of the tendons of the lower limb- viz. one in the tendon of the peroneus longus, where it glides on the cuboid bone ; one, appearing late in life, in the tendon of the tibialis anterior, opposite the smooth facet of the medial cuneiform bone ; one in the tendon of the tibialis posterior, opposite the medial side of the head of the talus ; one in the lateral head of the gastrocnemius, behind the lateral condyle of the femur; and one in the tendon of the psoas major, where it glides over the pubis. Sesamoid bones are found occasionally in the tendons which wind round the medial and lateral malleoli, and one is sometimes present in the tendon of the gluteus Maximus where it passes over the greater trochanter of the femur.