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Human Skeleton and Body Parts

Humans require support to bear their body weights as well as enable them to move about in their environment. The skeletal system in humans provides them with support and the ability to move about. Three types of skeleton found among these organisms are the hydrostatic skeleton, the exoskeleton and the endoskeleton.

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Axial Skeleton

Axial skeleton consists of the skull, cervical vertebrae, thoracic vertebrae, lumbar vertebrae, sacrum, coccyx, sternum and ribs.

a)     Skull – The skull is made up of the cranial bones (8 pieces). The cranial bones are joined together by the suture and are immovable. The skull protects the brain. The facial bones protect the sensory organs like the eye, the ear and the nose.

b)    Vertebrae – The vertebrae which numbers 33 are connected together by ligaments and cartilaginous discs to form the vertebral column which encloses and protects the spinal cord.

c)     Sternum – The sternum is a flat piece of structure made of cartilage. Three bones are joined together to form the sternum.

d)    Ribs – 12 pairs of ribs together form the rib cage which plays an important role in the breathing mechanisms. The first seven pairs of ribs known as the true ribs are connected to the sternum via cartilage at the anterior end.

The following three ribs known as the false ribs are connected to neither the seventh rib while the last two ribs (11th, 12th ribs) are connected to the sternum nor other ribs. These are referred to as the floating ribs.

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The Structure of a Vertebra

The neural spine and the transverse process serve as a surface for muscle attachments. The neural spine extends backwards while the transverse process is a lateral extension of the vertebra. The neural canal of the entire vertebra together forms a long canal which is occupied by the spinal cord. The centrum is the solid portion of the vertebra that provides support and resists compression.

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Each type of vertebra possesses specific shape and characteristics depending on the respective functions carried out and their position in the vertebral column.

a)     Atlas is the name given to the first cervical vertebra. The atlas vertebra supports the skull and allows the nodding movement of the head. Atlas hardly has a centrum, has a big neural canal, small neural spine and broad, flat transverse processes. A pair of canals present on both sides below the transverse process allows the passage of the vertebral artery.

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b)    Axis is the second cervical vertebra. The centrum is modified to form the odontoid process at the anterior end. This process projects into the neural canal of the atlas allowing the rotating movement between the axis and the atlas. The neural spine is big while the transverse processes are small. Vertebrarterial canals are present.

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c)     The other cervical vertebra (3rd to the 7th) supports the skull and the neck. All the cervical vertebrae coordinate to allow the movement of the head and neck, including nodding and rotating movements. The cervical vertebra has Vertebrarterial canal, flat centrum, small neural spine and transverse processes.

d)    The thoracic vertebra has a long neural spine that extends backwards. The centrum is short but thicker and bigger than that of the cervical vertebra. There are facets at the centrum and the short transverse processes to articulate with the ribs.

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e)     Lumbar vertebra is the biggest vertebra with big, short and flat neural spine which extends posterior. The transverse processes are broad but flat. Both these serve as surfaces for muscle attachments. The centrum is big and thick.

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f)      Sacrum 5 sacral vertebrae are fused to form the triangular shaped sacrum. All the neural spines are short and joined together. On each side of the sacrum are present facets to allow the joint between the sacrum and the ilium. There are no neural canals.

g)     Coccyx (The tail vertebrae) 4 vertebrae are fused together to form the coccyx. It is triangular in shape. Coccyx is small and do not have any specific function.

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The appendicular skeleton consists of the scapula, clavicle, humerus, ulna, radius, pelvic girdle, femur, tibia and fibula.

a)     i. The pectoral girdle as shown in the diagram above consists of the triangular shaped flat scapular and the cylindrical clavicle (collar bone). The scapula also known as the shoulder blade, has at its one end a depression known as the glenoid socket to form a ball and socket joint with the humerus.

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ii. The forearm consist of the humerus, ulna, radius carpals (wrist bones) metacarpals (hand bones) and phalanges (fingers). The radius and ulna of the lower arm form a hinge joint with the distal end of humerus.

b)    i. The pelvic girdle is made up of two parts that are similar. Each part consists of three pieces of boned fused together. These are the ilium, the ischium and the pubis. The two ilium bone is a fuse with sacrum. On the posterior end of the ilium is big socket named as the acetabulum. This socket forms the ball and socket joint with the head of the femur.

ii. The bones that are found in the legs are the femur, tibia, fibula tarsals, metatarsals and the phalanges. The distal end of the femur forms a hinge joint with the tibia. The joint is protected by the knee cap or patella. Tibia is bigger than fibula. The tarsals which form the ankle bone are jointed to the metatarsals forming the sole of the feet. All these bones are arranged in such a way to form and arch to give more support.

Arrangement of Bones, Muscles and Tendons in the Arm to Allow Movement

 

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1.     Skeletal muscles are muscles that are attached to the skeletal bones of the body. The force required to enable the movement of the bones at the joint is provided by the contraction of these muscles.

2.     Muscles can only contract and relax, they cannot elongate. Contraction of voluntary muscles (skeletal muscles) is caused by nerve impulses stimulating the muscle. Voluntary muscles consist of fibers which are grouped into bundles and enclosed in connective tissues.

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3.     Tendons are made up of tough strong fibers which are not elastic but flexible. They are present at the two ends of the skeletal muscle. Tendons connect the muscle to the bone.

4.     A contracting muscle pulls on a bone to produce movement. The tendon transfers the force of a contracting muscle to the bone so as to produce the movement.

5.     A muscle that has contracted will returns to its original length when it relaxes. Two sets of muscles which act antagonistically enable movement of the limbs. When one of a pair muscles contract the other relaxes.

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6.     The biceps muscle of the arm which flexes (bends) the arm is larger and more powerful than the triceps muscle which straightens the arm (extensor). Contraction of the biceps raises the forearm. The triceps is in a relaxed state. When the triceps contacts the forearm is straightened or extended. The biceps is in a relaxed state.

7.     The tendon from one end of the biceps is attached to the scapula and the humerus while at the elbow the tendon at the end of the biceps is attached to the radius. When flexing the forearm, the force of the contraction of the biceps is transferred to the bone via the tendon. The radius is pulled upwards causing the arm to flex or bend.

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8.     The tendon from one end of the triceps is attached to the scapula and the humerus while the tendon at the other end of the triceps is attached to the ulna. Contraction of the triceps causes the ulna to be pulled downward extending or straightening the arm.

Source of Energy for Muscle Contraction

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9.     Muscular contraction is an active process requiring energy. Aerobic respiration occurs in the muscles to generate chemical energy which is used to produce ATP molecules (Adenosine triphosphate) one molecule of glucose when oxidized completely in aerobic conditions can produce 36 molecules of ATP.

This process occurs in the mitochondrion. Muscles are rich in this organelle. So, energy required for muscles contraction, essential to produce the mitochondria.

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10.   The function of cartilage and synovial fluid at joints: a) where two bones meet a point is formed. Synovial joints allow free movements. Example are the ball and socket joints of the humerus and the scapula which allow movement in three planes and the hinge joints of the elbow and knee which allow movement in one plane b) the surfaces of the bones which are in contact are covered with a tough cartilage which is slippery and smooth. The joint is enclosed in a capsule filled with the synovial fluid. The cartilage and the synovial fluid allow movement without friction. The fluid also serves as a shock absorber. Ligaments which are tough, elastic connective tissues hold together the two bones at the joint preventing dislocation during movement.