Change is a fundamental characteristic of all living things. The human body during its life cycle accordingly passes through various phases of form and structure. In the earliest embryonic phases of development, the changes are very rapid, decreasing in rapidity during the later fetal stages, but continuing at a diminishing rate throughout infancy, childhood and youth up to the adult. Following the acme of maturity, changes continue which lead gradually to senescence and final death of the body.
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The spine (vertebral column) consists of thirty-three superimposed bones termed vertebrae. Of these the upper twenty-four remain separate throughout life and form three groups. The first seven are called cervical, the succeeding twelve thoracic (dorsal), and the last five lumbar. In adult life the last nine vertebras ankylose to form two composite bones named the sacrum and the coccyx. The sacrum is formed by the fusion of five vertebrae from the twenty-fifth to the twenty-ninth inclusive; the four terminal are vestigial, and form the coccyx. In order to gain a general notion of the characters of a vertebra, it is desirable to select a bone from the middle of the thoracic series.
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The digestive apparatus includes the actual intestinal tract, taking those words in their widest sense, and according to its development this tract may be divided into four portions: The oral cavity; the foregut; the midgut; the hindgut.
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There may be recognized in the nose a base, which is directed downward, and a root, situated between the two orbits; the rounded anterior surface is directed forward and upward and is termed the dorsum, and it terminates in the tip of the nose, or apex nasi. The margins representing the lower borders of the base pass backward from the tip to form the alee, and they constitute the lateral boundaries of the nostrils (nares), which are separated from each other by the antero-inferior portion of the nasal septum, the membranous septum.
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The osseous structure is peculiarly fitted, by its solidity and hardness, not only to give support to the soft parts, but also to furnish points of attachment to the muscles, by which the different movements are executed. This solid framework of the body is made up of a number of separate pieces, the aggregate of which has been termed " the skeleton" (sceletum, σχελλω, to dry.)
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