THE SKELETON SYSTEM

The adult human body consists of approximately 206 bones, which are organized into an internal framework called the SKELETON.  Because the human skeleton is an internal structure, biologists refer to it as an ENDOSKELETON.  The variation in size and shape among the bones that make up the skeleton reflects their different roles in the body.

THE SKELETON

1. In order to retain their SHAPE and FORM, living things need some type of support.

2. In single-celled organisms, this support is provided by the cell membrane.

3. In multicellular animals, some form of a SKELETON provides the support.

4. There are TWO TYPES of animal skeletons:

    A. EXOSKELETON - AN OUTSIDE SKELETON, ARTHROPODS (SPIDERS, CRUSTACEANS, INSECTS, AND CRABS).

B. ENDOSKELETON - AN INSIDE SKELETON, VERTEBRATES.

5. The skeleton of humans is composed of a special CONNECTIVE TISSUE (TISSUE THAT JOINS OTHER TISSUES TOGETHER) CALLED BONE.

6.  BONES AND THEIR ASSOCIATED TISSUES - CARTILAGE, TENDONS, AND LIGAMENTS - MAKE UP THE SKELETON SYSTEM.

7. THE HUMAN SKELETON SYSTEM CONSISTS OF 206 BONES.

8. THE SKELETON SYSTEM CONSISTS OF BONES AND OTHER STRUCTURES THAT MAKE UP THE JOINTS OF THE SKELETON.

9. THE TYPES OF TISSUE PRESENT ARE BONE TISSUE, CARTILAGE, AND FIBEROUS CONNECTIVE TISSUE, WHICH FORM THE LIGAMENTS THAT CONNECT BONE TO BONE.

FUNCTIONS OF THE SKELETON SYSTEM

1. THE BONES THAT MAKES UP THE SKELETON SYSTEM SERVES FOUR IMPORTANT FUNCTIONS:

    A. Provides a framework that supports the body; the muscles that are attached to the bones move the skeleton.

    B. Protects some internal organs from mechanical injury; the rib cage protects the heart and lungs OR SKULL AROUND THE BRAIN, for examples.

    C. Contains and protects the Red Bone Marrow, Hematopoiesis (blood-forming) tissues (RBC or Erythrocytes).  Some White Blood Cells (Leukocytes) are also produced in bones.

    D. Provides a storage site of inorganic salts, such as CALCIUM.  Calcium may be removed from bone to maintain a normal blood calcium level, which is essentially for BLOOD CLOTTING and PROPER FUNCTIONING OF THE MUSCLES AND NERVES.

2. Bones also provide a system of levers (rigid rods that can be moved about a fixed point) on which a group of specialized tissues (Muscles) act to produce motion.

STRUCTURE OF BONES

1. Bones are a solid network of MOIST, LIVING CELLS (OSTEOCYTES), LIVING TISSUE. AND FIBERS (COLLAGEN) THAT ARE SUPPORTED BY A MATRIX (DEPOSITS) OF CALCIUM SALTS.

2. The calcium salts give bones the STRENGTH and PROTECTIVE functions.

3. The Function of OSTEOCYTES is to REGULATE the amount of calcium that is deposited in, or removed from, the Bone Matrix.

4. Each bone is surrounded by a tough membrane called the PERIOSTEUM, A FIBEROUS CONNECTIVE TISSUE MEMBRANE WHOSE COLLAGEN FIBERS MERGE WITH THOSE OF THE TENDONS AND LIGAMENTS THAT ARE ATTACHED TO THE BONE.

5. The Periosteum contains a network of blood vessels, which supply oxygen, nerves and nutrients to the bone.

6. The jointed surfaces of bones are covered with ARTICULAR CARTILAGE, WHICH PROVIDES A SMOOTH SURFACE FOR MOVEMENT.

7. Beneath the Periosteum is a thick layer of COMPACT BONE (One of two types of bone tissues)

8. Compact bone is dense and similar in texture to ivory; it is far from being SOLID.  A thick layer of compact bone enables the shaft of long bones (called the Diaphysis) to endure the large amount of stress it receives upon impact with a solid object.

9. Compact bone is composed of cylinders or tubes of mineral crystals and protein fibers called LAMELLAE.

10. In the center of each cylinder is a narrow channel called HAVERSIAN CANALS THAT CONTAIN BLOOD VESSELS AND NERVES.

11. Blood vessels run through interconnected Haversian Canals, creating a network that carries nourishment to the living bone tissue

12. The second type of bone tissue, SPONGY BONE, it is the inside layer of compact bone.

13. Spongy bone IS NOT soft and spongy. But actually quite strong.  Near the ends of bones (EPIPHYSIS) where force is applied, spongy bone is organized into structures that resemble the supporting girders of a bridge.

14. THE STRUCTURE OF SPONGY BONE HELPS ADD STRENGTH TO BONE WITHOUT ADDING MASS. It is arranged along points of pressure or stress, making bones both light and strong.

15. Embedded in COMPACT and SPONGY bone are cells known as OSTEOCYTES that can either deposit the calcium salts in bone or absorb them again.

16. OSTEOCYTES ARE RESPONSIBLE FOR BONE GROWTH AND CHANGES IN THE SHAPE OF BONES.

17. The cavities of bones contain a soft tissue called BONE MARROW.

18. THERE ARE TWO TYPES OF BONE MARROW FOUND IN MOST BONES:

    A. YELLOW BONE MARROW - FOUND IN MOST BONES, BUT PRIMARILY FILLS THE SHAFTS OF LONG BONES AND IS MADE UP OF BLOOD VESSELS, NERVE CELLS, BUT CONSISTS MOSTLY OF FAT CELLS (ADIPOSE TISSUE). IT SERVES AS AN ENERGY RESERVE.  IT CAN ALSO BE CONVERTED TO RED BONE MARROW AND PRODUCE BLOOD CELLS WHEN SEVERE BLOOD LOSS OCCURS.

    B. RED BONE MARROW - FOUND IN SPONGY BONE, THE ENDS OF LONG BONES, RIBS, VERTEBRAE, THE STERNUM, AND THE PELVIS - PRODUCES RED BLOOD CELLS AND SPECIAL WHITE BLOOD CELLS CALLED LYMPHOCYTES, AND OTHER ELEMENTS OF BLOOD (PLATELETS).

 

 

 

 

CLASSIFICATION OF BONES:

BONES CANE BE CLASSIFIED AS ONE OF FOUR TYPES BASED ON THEIR SHAPE:
  


 

1. LONG BONES - THE BONES OF THE ARMS, LEGS, HANDS, AND FEET (BUT NOT THE WRIST OR ANKLES).  The shaft of the long bones is the DIAPHYSIS, and the ends are called EPIPHYSIS.  The Diaphysis is made up of compact bone and is hollow, forming a canal within the shaft.  This marrow canal contains yellow bone marrow, which is mostly adipose tissue. The Epiphyses are made of spongy bone covered by a thin layer of compact bone.

2. SHORT BONES - THE BONES OF THE WRIST AND ANKLES.

3. FLAT BONES - THE RIBS, SHOULDER BLADES, HIPBONES, AND CRANIAL BONES.

4. IRREGULAR BONES - THE VERTEBRAE AND FACIAL BONES.

5. SHORT, FLAT AND IRREGULAR BONES ARE ALL MADE OF SPONGY BONE COVERED WITH A THIN LAYER OF COMPACT BONE.  RED BONE MARROW IS FOUND WITHIN THE SPONGY BONE.

DEVELOPMENT OF BONES

1. Bone growth begins long before birth.  The basic shape of a long bone, such as an arm bone is first formed as CARTILAGE.

2. Cartilage is a tough but flexible CONNECTIVE TISSUE, THAT UNLIKE BONE DOES NOT CONTAIN BLOOD VESSELS.

3. Cartilage cells must rely on the DIFFUSION of nutrients from tiny blood vessels (CAPILLARIES) in surrounding tissue.

4. The cells that make up cartilage are scattered in a network of fibers composed of an ELASTIC PROTEIN called COLLAGEN.

5. CARTILAGE IS DENSE AND FIBEROUS, CAN SUPPORT WEIGHT, BUT IS STILL EXTREMELY FLEXIBLE.

6. MANY BONES IN A NEWBORN BABY ARE COMPOSED ALMOST ENTIRELY OF CARTILAGE.  "SOFT SPOT" OF A BABIES HEAD!

7. Latter the cartilage cells will be replaced by cells that form the bones.  THE CARTILAGE IS REPLACED DURING OSSIFICATION, OR THE PROCESS OF BONE FORMATION.

8. Ossification begins to take place up to seven months before birth as MINERAL (CALCIUM AND PHOSPHORUS) DEPOSITS ARE LAID DOWN NEAR THE CENTER OF THE BONE (CENTER OF OSSIFICATION) IN EACH BONE.

9. BONE TISSUE FORMS AS OSTEOCYTES SECRET MINERAL DEPOSITS THAT REPLACE THE CARTILAGE. OR A BONE MATRIX GRADUALLY REPLACES THE ORIGINAL CARTILAGE.

10. The long bones develop and grow THROUGH OUT CHILDHOOD at centers of ossification in their EPIPHYSIS (ENDS).

11. Growth occurs in the EPIPHYSEAL DISC or PLATE (GROWTH PLATES) AT THE JUNCTION OF THE DIAPHYSIS WITH EACH EPIPHYSIS (AT EACH END OF THE BONE)

12. AN EPIPHYSEAL DISC IS STILL CARTILAGE, AND THE BONE GROWS IN LENGTH AS MORE CARTILAGE IS PRODUCED ON THE EPIPHYSIS SIDE.

13. ON THE DIAPHYSIS SIDE, OSTEOBLAST (CELLS THAT PRODUCE BONE MATRIX, (A BLAST CELL IS A "PRODUCING" CELL, AND "OSTEO" MEANS BONE) TO REPLACE CARTILAGE.

14. Between the ages of 16 and 25 years, ALL of the cartilage of the EPIPHYSEAL DISC IS REPLACED BY BONE.  THIS IS CALLED CLOSURE OF THE EPIPHYSEAL DISC, AND THE BONE LENGTHENING PROCESS STOPS.

15. In adults, cartilage is found in those parts of the body where FLEXIBILITY IS NEEDED.

16. SUCH PLACES INCLUDE THE TIP OF THE NOSE, THE EXTERNAL EAR, THE VOICE BOX (LARYNX), AND THE ENDS OF BONES WHERE JOINTS ARE FORMED. CARTILAGE IS ALSO FOUND WHERE THE RIBS ARE ATTACHED TO THE BREASTBONE (STERNUM), THUS ALLOWING THE RIB CAGE TO MOVE DURING BREATHING.

17. CARTILAGE PROVIDES AN IMPORTANT COMBINATION OF STRENGTH AND FLEXIBILITY.

FRACTURES AND THEIR REPAIR

1. A FRACTURE MEANS THAT A BONE HAS BEEN CRACKED OR BROKEN.  A bone fracture may be a simple crack, or the bone may actually break into two or more pieces.

2. THERE ARE DIFFERENT TYPES OF FRACTURES CLASSIFIED AS TO THE EXTENT OF DAMAGE:

    A. SIMPLE (CLOSED) - The broken parts are still in normal anatomical position; surrounding tissue damage is minimal (skin is not pierced).

    B. COMPOUND (OPEN) - The broken end of a bone has been MOVED, and it pierces the skin; there may be extensive damage to surrounding blood vessels, nerves, and muscles.

    C. GREENSTICK - The bone splits LONGITUDINALLY (BREAKS ALONG THE LONG AXIS OF THE BONE).  The bones of children contain more collagen than do adults bones and tend to splinter rather than break completely.

    D. COMMINUTED - Two or more intersecting breaks create several bone fragments.

    E. IMPACTED - The broken ends of a bone are forced into one another; many bone fragments may be created.

    F. SPONTANEOUS (PATHOLOGIC) - A bone breaks without apparent trauma; may accompany bone disorders such as OSTEOPOROSIS.

THE REPAIR PROCESS

1. Even simple fracture involves significant bone damage that must be repaired if the bone is to resume normal function.

2. Fragments of dead or damage bone must first be removed.  This is accomplished by OSTEOCLAST (A

BONE-DESTROYING CELL; REABSORBS BONE MATRIX AS PART OF GROWTH OR REPAIR OF BONES), WHICH DISSOLVE AND REABSORB THE CALCIUM SALTS OF BONE MATRIX.

3. Imagine a building that has just collapsed; the ruble must be removed before reconstruction can take place.  This is what the Osteoclasts do.

4. Then NEW BONE must be produced.

5. The INNER LAYER of the PERIOSTEUM contains OSTEOBLASTS that are activated when bone is damaged.  The Osteoblasts produce bone matrix to knit the broken ends of bone together. Holding the broken ends close to each other and keeping them completely still speeds the healing of bones.  That is why encasing the fractured limb in a cast often treats a bone fracture.

6. Since most bone has a good blood supply, the repair process is usually relatively rapid, and a simple fracture often heals within 6 weeks.

7. Other factors that influence the repair include the age of the person, general state of health, and nutrition (a diet with sufficient calcium, phosphorus, vitamin D and protein, if any of these nutrients is lacking, bone repair will be a slower process).

OSTEOPOROSIS

1. OSTEOPOROSIS CAUSES BONES TO BECOME BRITTLE.

2. As bones grow longer, they also grow THICKER and DENSER.  In young adults the density of bone usually remains constant as bone tissue is broken down and replaced at a steady rate.

3. During middle age, bone replacement gradually becomes less efficient, and bones may become LESS DENSE.

4. THE LOSS OF BONE DENSITY IS CALLED OSTEOPOROSIS, AND CAN CAUSE BONES TO BECOME LIGHT, BRITTLE, AND EASILY BROKEN.

5. Although both men and women lose bone as the age, women are at a GREATER RISK for Osteoporosis for TWO reasons:

    A. Women's bones are usually smaller and lighter than men's bones.

    B.  The production of female sex hormones declines rapidly during Menopause (Shutdown of menstrual and ovarian cycles).  Sex hormones help to maintain bone density; this decline in hormone production increases the rate of bone loss.

6. BONE DENSITY CAN ONLY BE INCREASED DURING YOUR TEENS AND TWENTIES!  REGULAR EXERCISE AND A HEALTHY DIET WILL MAKE YOU HEALTHIER NOW AND WILL ALSO PAY OFF LATER.

7. THE STRONGER YOUR BONES ARE NOW, THE LESS LIKELY YOU ARE TO AFFECTED BY OSTEOPOROSIS LATER.  In the US, more than 600,000 bone fractures a year result from Osteoporosis.

 

JOINTS:  WHERE TWO BONES MEET

1. JOINTS, OR PLACES WHERE TWO BONES COME TOGETHER, PERMIT THE BONES TO MOVE WITHOUT DAMAGING EACH OTHER.

2. Joints are responsible for keeping bones far enough apart so they do not rub against each other as they move.  At the same time, joints hold the bones in place.

3. THE CLASSIFICATION OF JOINTS IS BASED ON THE AMOUNT OF MOVEMENT POSSIBLE:

    A.  SYNARTHROSIS IS AN IMMOVABLE JOINT; THEY ARE OFTEN CALLED FIXED JOINTS, AND ALLOW NO MOVEMENT BETWEEN BONES.  These joints are interlocked and held together by Connective Tissue, or they are fused together.  The places where the bones of the SKULL meet (SUTURE) meet are examples of immovable joints.

    B. AMPHIARTHROSIS IS A SLIGHTLY MOVABLE JOINT. (SEMI MOVABLE JOINTS)  These joints permit a small amount of movement.  These bones are farther apart from each other than immovable joint bones. The joints between the two bones of the lower leg (TIBIA and FIBULA) and the joints of the vertebrae are examples of slightly movable joints.

    C.  DIARTHROSIS IS A FREELY MOVABLE JOINTMOST OF THE JOINTS OF THE BODY ARE FREELY MOVABLE JOINTS.

4. In freely movable joints, the ends of the bones are covered with a layer of cartilage that provides a smooth surface at the joint.

5. The joints are also surrounded by a fibrous joint capsule that helps hold the bones together and at the same time allows for movement.

6. The Joint Capsule consists of TWO Layers:

    A. One of the layers of the joint capsule may thicken to form strips of tough Connective Tissue called LIGAMENTS.  LIGAMENTS ARE ATTACHED TO THE MEMBRANES THAT SURROUND BONES AND HOLD BONES TOGETHER AND IN PLACE.

    B. THE OUTER LAYER OF THE JOINT CAPSULE PRODUCES SYNOVIAL FLUID, WHICH FORMS A THIN LUBRICATING FILM OVER THE SURFACE OF A JOINT AND PROTECTS THE ENDS OF BONES FROM FRICTION.  This lubricating film enables the cartilage found on the ends of the bones to slip past each other more smoothly as the joint moves.

7. In some freely movable joints, SMALL POCKETS OF SYNOVIAL FLUID CALLED BURSAE FORM.  A BURSAE REDUCES THE FRICTION BETWEEN THE BONES OF A JOINT AND ALSO ACTS AS A

TINY SHOCK ABSORBER.

8. If a joint is injured, too much fluid moves into the bursa, causing it to swell and become painful, a condition called BURSITIS.

9. A more serious disorder that affects the joints is ARTHRITIS or INFLAMMATION OF THE JOINT. There are 2 forms of arthritis that affect joints.

    A. RHEUMATOID ARTHRITIS - It develops when the immune system begins to attack the body.  The joints become inflamed, swollen, stiff, and deformed.

    B. OSTEOARTHRITIS - Is a degenerative joint disease (DJD) in which the cartilage covering the surface of bones becomes thinner and rougher.  As a result, bone surfaces rub against each other, causing severe discomfort.

10. FREELY MOVABLE JOINTS ARE GROUPED ACCORDING TO THE SHAPES OF SURFACES OF THE ADJACENT BONES.  THERE ARE 6 TYPES OF FREELY MOVABLE JOINTS:

    A. BALL AND SOCKET JOINT - Permits circular movement - the widest range of movement.  THE SHOULDER joint, which enables you to move your arm up, down, forward and backward, as well as to rotate it in a complete circle.

    B. HINGED JOINT - Permits a back-and-forth motion.  The knee enables your leg to flex and extend.  The elbow allows you to move your forearm forward and backward.

    C. PIVOT JOINT - Permits rotation of one bone around another.  The elbow enables your hand to turn over. It also allows you to turn your head from side to side.

    D. GLIDING JOINT - Permits a sliding motion of one bone over another.  Found at the ends of the collarbones, between wrist bones, and between anklebones.

    E. SADDLE JOINT - Permits movement in two planes.  This type of joint is found at the base of the thumb.

    F. ELLIPSOID JOINT - Allows for a hinge type movement in two directions.  The joints that connect fingers with the palm and toes with the soles of feet are examples.
 
 


 

 

SKELETAL ORGANIZATION

1. ALL THE BONES IN THE BODY MAKES UP THE SKELETON.  THERE ARE 206 TOTAL BONES IN THE HUMAN BODY.

2. THE SKELETON SUPPORTS THE BODY'S WEIGHT, ENABLES IT TO MOVE, AND PROTECTS MANY OF ITS INTERNAL ORGANS.

3. THE HUMAN SKELETON HAS TWO DIVISIONS: 

    A. THE AXIAL SKELETON - CONSIST OF THE SKULL,

VERTEBRATE COLUMN, AND THE RIB CAGE.

    B. THE APPENDICULAR SKELETON - CONSIST OF THE BONES OF THE ARMS AND LEGS, SHOULDER, AND THE PELVIC GIRDLE.

AXIAL SKELETON BONES

1.  The skull consists of 8 CRANIAL BONES and 13 FACIAL BONES.

2.  The ears consists 6 BONES, AND FLOATING IN THE THROAT 1 BONE THE HYOID. 

3.  The Vertebral Column (Spinal Column or Backbone) consists of 7 CERVICAL (NECK) VERTEBRAE, 12 THORACIC, 5 LUMBAR, AND 5 FUSED VERTEBRAE INTO 1 SACRUM, AND 4 TO 5 SMALL FUSED VERTEBRAE INTO 1 COCCYX (YOUR TAIL BONE).

4.  THE RIB CAGE (THORACIC CAGE) CONSISTS OF THE 12 PAIRS OF RIBS- 24 BONES AND THE STERNUM, OR BREASTBONE.

APPENDICULAR SKELETON BONES

1. THE PECTORAL GIRDLE CONSISTS OF 4 BONES, AND UPPER LIMBS CONSISTS OF 60 BONES.

    A.  THE HANDS AND WRIST CONSISTS OF 54 SEPARATE BONES.

2. THE PELVIC GIRDLE CONSISTS OF 2 BONES AND THE LOWER LIMB CONSISTS OF SIXTY BONES.

    A. THE FEET AND ANKLES CONSISTS OF 54 SEPARATE BONES.