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Show f WHY WE BEHAVE LIKE HUMAN BEINGS Br GEORGE DORSEY, Pa. D U D. I What Makes Giants and Dwarfs. ON THIS day we are born we bava 'nsed up only 2 per cent of our allotted growth power. We can grow 08 per cent more If we are spared. We double our weight the first six months; a calf does It In fifty daysj a dog, in eight We Increase onr weight 200 per cent In the first year, less than 30 In the second, only B In the fifth. Increase In weight then picks np again sad continues until the tenth yesr. to drop back from tba eleventh to the thirteenth, from the fourteenth to the seventeenth, puberty years. It Increases In-creases again, to 12 per cent That la our last spurt It drops to per cent during the eighteenth year; to 1 per cent during the twenty-second. 8Uture elso Increases by sports. By the time the Infant can walk. It has grown from twenty to thirty-four inches; thereafter, until puberty, It growa between two and three Inches a year. The thirteenth la the rapid growing grow-ing year for girls, the sixteenth for boys. Between fourteen snd sixteen the boy Increases his stature eight Inches. Girls usually attain their full stature by twenty, sometimes by eighteen; eigh-teen; boy a by twenty-five. But both may continue growth three or four years longer. The new-born's brain Is already one-Sfth one-Sfth the destined weight about ten ounces; by the second year two-fifths, or as large as an adult anthropoid ape's. Full brain weight comes before twenty-five; after that It loses weight rapidly In old age, The two elements In growth are weight and height Weight often continues con-tinues beyond maturity, long after the body has taken on Its last cubit The giant can grow no taller; the fat lady knows no limit Stature Is determined almost entirely en-tirely by the skeleton. Only skin and a thin layer of fat cover skull and the bones of the feet; thin cartilage covers cov-ers the ends of the leg bones ; between the vertebrae are thin pads of cartilage. car-tilage. Stature growth, then, is largely a matter of growth of skull, bodies of vertebrae, and especially of the leg bones. Bones grow from centers of ossification. ossifica-tion. Centers for the principal bones of the body appear by the end of the second month of fetal life; centers for the ends, or epiphyses, appear later many not until puberty, when the skeleton begins to assume Its permanent perma-nent form. The number of ossification centers varies In different bones. The long bones of the arms and legs have at least three: one In the shaft Itself and one at each epiphysis. The humerus hum-erus at fifteen years Is still In three parts; shaft, two heads; but the heads are more closely connected with abaft than at birth. By maturity, the beads are so united with the shaft that It la not possible to see where they grew on. In general, facial and skull-dome bones are formed from membrane "skin" bones; the other bones begin in cartilage. Bone-forming cells multiply multi-ply by division, absorb lime salU from the blood, ossify, and so continue until the cartilage Is replaced by bone. Increase In-crease In length ends when the cartilage cartil-age disappears. In the mature skeleton skele-ton there can be no further growth In suture or In length of arms. If final conversion of cartilage to bone is delayed, gigantic stature resuIU; if the process Is reversed, dwarfs. Only the articulating or Joint surfaces of mature bones are covered by cartilage. Bones Increase In girth by additions of bone cells from the surrounding membrane. Long bones are hollow. To preserve their relative proportion of bone wall to cavity, bone cells on the Inside are destroyed as fast as cells are added to the outside. Thus the cavity grows with the bone, the form, and strength of the bones are preserved. pre-served. This process keeps up until late In life. With old age the bones become thin and delicate. Complicated changes take place In acquiring the upright gait A chick can run from IU shell ; a baby cannot even straighten Its legs. Tbey bend In at the knees and are drawn up at the hips, and are only 60 per cent of head-trunk length. By maturity they will be over 100 per cent As the walking walk-ing days approach, the legs grow fast Knee and hip Joints change; the legs can now be straightened out The soles of the feet no longer turn In. The baby at birth can clap Its feet almost as easily as Its hands. The spine also changes. It Is not solid, but consists of twenty-four vertebrae ver-tebrae with pads of cartilage between. At birth a large percentage of the column col-umn Is cartilage. Powerful muscles develop to bold the spine erect; others, oth-ers, acting on the ribs as levers, to balance the trunk and spine. Standing Is a complex act Involving Involv-ing nearly all our big muscles. When we stand "at attention," powerful ligaments liga-ments in the hip joint hold the body. This relieves the muscles from strain, but locks the knee Joint We stand easier If the knees are slightly bent and the knee-caps loose. The feet muscles must bind the many small bones together to give support and from the Instep or arch. A man can stand up asleep, but not if muscles of feet or of legs are "asleep." (ft by Gaoraa A. Doraay.) |