Skeleton performs a critical function. It serves to support the body, it is most organs. It provides mechanical protection for the brain, heart, lungs, etc. it forms a solid bone cavity. Bones act as levers in the apparatus of the movement. Great importance is their participation in mineral metabolism. Human and animal bones are very similar. All know what they look like.
Fig. 21. Bone cells (1) and bone (2).
Each bone is a complex organ. The basis of it is the bone. If viewed under a microscope (Fig. 21), it is easy to detect a kind of cell shape, interconnected by numerous appendices. The cells produce the protein intercellular substance ossein.
It intercellular substance, representing a volume greater part of bone tissue, providing its mechanical properties. Bone inhomogeneous structure. The outer layer of her tight, and the interior has a porous, spongy structure. It is surrounded by soft film - the periosteum (periosteum). Through it into the bone penetrate the blood vessels and nerves. In its internal layer, adjacent to dense substance, multiply and grow the cells, which is a growth of bones. Fractures, they form a bond between them - bone spur, which is the fusion of fragments. Thus, due to the periosteum is the food; the growth and regeneration of bone fractures. The shape of the bones (as, however, and other organs) is defined by their role, their function (Fig. 22). In the form of bones, very different, but on the principle of their structure can be divided into two groups: flat and tubular bones. The first serve to protect and attach bodies. For example, the bones of the skull protects the brain pelvic bones, internal organs; tubular bones skeleton of the extremities (hands and feet). They represent the levers of movement and therefore stretched in length and serve to attach bodies (such as muscle). Their ends are thickened and consist entirely of a spongy substance (only outside they are covered with a thin layer of dense matter), and their articular surface is always covered with cartilage, participating in formation of the joint. The middle part of the bone, called a body, is the receiver of cylindrical shape and consists only of dense matter. In the cavity of her adult contains yellow bone marrow, which was established fatty tissue, and cells of the spongy substance in all the bones (and in tubular and flat) is red bone marrow, which is an important body, it does hematopoietic function. Here develop and Mature blood cells. The total number of red bone marrow is 1.3-1.5 kg (!), i.e. on volume and weight, it is only slightly less than the liver. Many blood disorders arise from the violation of its normal activities. Blood loss activate the functions of bone marrow.
Fig. 22. The shape of the bones: 1 - long bone; 2 - flat bone; 3 - short spongy bone.
A bone as the organ bearing often very considerable mechanical load, has a very high strength. Its hardness is surprisingly high, it is 30 times harder than brick and 2.5 times harder than granite (!). On the strength and elasticity it meets the concrete! These properties are provided as chemical composition and the features of the internal structure. In adults (in young and middle age) bones contain 30% organic and 70% of inorganic nutrients (phosphate and carbonate of calcium). From these relations depends on its firmness and flexibility, it is easy to show the following classical experiments. If dissolve calcium salts, putting bone in hydrochloric acid, it becomes so soft that it can tie a knot. If remove organic matter (for example, by calcination), it becomes hard and very fragile. In addition, the strength of bones is ensured by its internal structure, namely the structure and location of bone rungs of the spongy substance in accordance with the direction of the pressure and tension. The greater the load - the stronger the bone. For example, in the human body due to its vertical position of the greatest burden falls on a tibia bone of the lower leg, and it is able to withstand the weight in 1560 kg (!), i.e. this bone is strong, approximately 23 times (!) in excess of the normal load (with a body weight of 70 kg).