Anatomy, physiology and hygiene for high schools . llow centralcavity. This central or medullary (Latin medulla, mar-row ) cavity runs throughout the shaft. It isfilled with soft yellow marrow. In it run bloodvessels, lymphatics, and nerves to supply thebone. The tissue of the articular extremities is com-posed of a mass of spongy substance filled withred marrow and coated by a thin layer of hard,compact bone. This combination of hard, compact exteriorwith a reticulated interior formation and a hol-low center gives the bones great strength withsmall weight. If the bones were solid through-out
Anatomy, physiology and hygiene for high schools . llow centralcavity. This central or medullary (Latin medulla, mar-row ) cavity runs throughout the shaft. It isfilled with soft yellow marrow. In it run bloodvessels, lymphatics, and nerves to supply thebone. The tissue of the articular extremities is com-posed of a mass of spongy substance filled withred marrow and coated by a thin layer of hard,compact bone. This combination of hard, compact exteriorwith a reticulated interior formation and a hol-low center gives the bones great strength withsmall weight. If the bones were solid through-out they would be much heavier and less strong,as a tube will bear more weight than a rod con-taining the same amount of material.^ The flat bones of the body have the same gen-eral structure, but possess no hollow formation keeps the bone from jarring with theshock in falls. In animals like the alligator, whose recum-bent posture protects them from falls, the bones containmuch less of this spongy structure. The traheculce (Latin,. Section of along bone. This spongy 1 This principle is utilized in the construction of tiibular bridges. In birds thehollow of some bones, instead of man-ow, contains air, being connected with the secures greater lightness than that of human bones. 56 PHYSIOLOGY AND HYGIENE smaU beams) of the trellis tissue are arranged in archesto support the weight which the bone has to bear. Histological structure of bone (Greek histos, tissue/ andlogos, ^discourse—study of tissue). Histologically thebone is made up of a collection of cells and their processes,and intercellular substance in which lime salts have beenincorporated. Examined under the microscope even the densest part ofthe bone proves to be simply a fine network of small bony plates and thisnetwork run ca-nals, the Haver-sian canals, car-rying the bloodvessels for thenourishment ofthe tissue. Thesecanals open at theperiphery (Greekperi, ^ about, andplier
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