DEVELOPMENT AND AGE FEATURES OF THE BONES OF THE LOWER LIMB The bones of the lower limbs develop as secondary bones (see Figs 3, 179a). T h e hip bone develops from three primary centres (or points) of ossification and several (up to eight) secondary (ac cessory) centres. The primary centres give rise to the ilium (which appears on the third month of the intrauterine period), the ischium (on the fourth month) and the pubis (on the fifth month ol intrauterine life); the secondary centres give origin to emi nences, depressions, and borders of individual bones. In the region of the acelabulum the three bones are at first joined to one another by layers of cartilage in which secondary ossification centres appear later (by the age of 16-18 years). All centres fuse at 2 0 - 2 5 years of age. T h e pelvis as a whole undergoes changes in size and shape for the most part. However, sexual features characteristic of male and female adults begin differentiation from the age of 8-10 years. T h e pelvis is higher in boys but wider in girls. T h e femur develops from five ossification centres one of which is primary (diaphyseal) and four are secondary. T h e primary cen tre (which appears at the beginning of the second intrauterine month) gives rise to the shaft of the bone. T h e secondary centres of ossification appear in different periods: the centre for the distal femoral epiphysis appears at the end of the intrauterine period; an ossification centre appears in the cartilaginous femoral head at the end of the first or beginning of the second year; the third centre ai ises in the cartilage of the greater trochanter at the age of 3 , and the fourth appears in the cartilage of the lesser trochanter at the age of 8 years. All these bone structures fuse with the shaft of the bone between the ages of 16 and 20. The patella forms in cartilage from a single ossification centre in the second year in girls and in the fourth year in boys and ossifi cation is completed by the age of 16-20. The tibia develops from four ossification centres. O n e is dia physeal and appears in the second month of intrauterine life. Three ossification centres are secondary among which the upper epiphyseal point arises in the ninth month of intrauterine life, the lower epiphyseal centre during the first year of life, and the ossifi cation centre for the tuberosity of the tibia appears at the age of
13. All centres fuse with the shaft at different periods ranging from 16-18 to 20-24 years or age. T h e fibula develops from three ossification centres. The pri mary (diaphyseal) centre appears in the middle of the second month of intrauterine life and gives origin to the shaft and areas of the epiphysis. T h e other two are secondary, epiphyseal, centres; the lower one arises in the first year of life and the upper centre, at the age of 3-5 years. The lower epiphyseal centre of ossification fuses with the shaft at 17-20 and the upper ossification centre at 19-21 years of age. T h e bones of the foot develop in the following manner. The ta lus arises from one ossification centre which appears in the last months of intrauterine life; ossification continues to the age of 8 years. T h e calcaneum forms from two ossification centres: the main, primary, centre appears on the sixth month of intrauterine life and the secondary centre by the age of 9 years and gives rise to the tu ber cakanei. T h e centres fuse by 16-18 years of age. The navicular bone develops from a single ossification centre which appears at the age of 3-5 years. Each cuneiform bone develops from its own ossification centre. T h e lateral bone begins ossification by the end of the first year of life, the intermediate bone at the age of 3, and the medial cunei form bone at the age of 3-4 years. T h e cuboid bone develops from one ossification centre which usually appears before birth, less frequently at 3-6 months of age. Each of the five metatarsal bones develops from two ossifica tion centres: a main, primary, and an accessory (secondary) centre. T h e main centre appears in the second to fifth metatarsals at the beginning of the third month of intrauterine life and in the first bone at the end of this month; the secondary centres form by the age of 4 years and fuse by the age of 17 in females and by the age of 20 in males. Each phalanx develops from two ossification centres. The main centre appears between the third and ninth months of intrauterine life, the secondary centre arises by the age of 4. Fusion occurs be tween the ages of 15 and 20.