The document summarizes the human endoskeleton. It is divided into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton includes the skull, vertebral column, ribs, and sternum, and forms the central axis of the body. The appendicular skeleton includes the shoulder and pelvic girdles, and the bones of the upper and lower limbs that attach to these girdles. Together, the skeleton provides structure and protection for the body.

1. The Human Endoskeleton

2. Human endoskeleton The skeleton in man refers to the hard, supportive connective tissue around which the organism is built. The skeleton includes all the bones of the body, the joints formed by the attachment of the bones to one another, connective tissues and cartilage which surround the bones and ligaments that connect bone to bone.

3. The Human Skeleton

4. As in all other vertebrates, humans have an internal skeleton which is surrounded by muscles and skin. Such an internal skeleton is called an endoskeleton. In human beings the skeleton consists of more than 200 different kinds of bone which are joined together in various ways to form a rigid framework. The skeleton can be divided into two main parts, viz. the axial skeleton and the appendicular skeleton. The major components of each are represented in the following table:

5. Skeleton Pectoral (shoulder) girdle and upper limbs Skull Pelvic (hip) girdle and lower limbs Vertebral column, ribs and breastbone APPENDICULAR SKELETON AXIAL SKELETON

6. I.The Axial Skeleton The axial skeleton forms the central axis of the body. It consists of the skull, the vertebral column, the ribs and the sternum or breastbone .

7. 1.The Skull The skull consists of 28 different bones (including the ossicles of the ear). The bones of the skull can be divided into two main groups: the cranium which encloses and protects the brain and the facial bones .

8. The Cranium The cranium consists of eight flat bones which are rigidly attached to each other with dentate sutures (joints with teeth-like protrusions) . They envelop and protect the brain. The frontal bone forms the forehead and portions of the eye sockets (or orbits) . The occipital bone , at the base of the skull contains a large opening, called the foramen magnum , through which the spinal cord passes. On each side of the opening is the occipital condyle , - two round protuberances, - by means of which the skull articulates with the first neck (or cervical) vertebra (the atlas). The organs of hearing are situated in the temporal bone, one on each side. The openings leading into these organs can also be seen on each side.

9. The Facial Bones The facial skeleton consists of fourteen irregular bones , which are all (with the exception of the lower jawbone) firmly attached to the cranium by means of sutures. They include the nasal bones, the two jawbones and the cheek bones . The lower jaw articulates with the temporal bone part of the cheek bone, just in front of the ear. This allows for the necessary movement of the lower jaw when food is bitten off and chewed. Both upper and lower jaws have alveolar pockets into which teeth fit.

10. The teeth are embedded in sockets in the ridges of the upper and lower jaw bones. Three regions can be distinguished in a tooth: The root which are embedded in the alveolar pocket of the jaw. The root is firmly attached to the jaw by a surrounding layer of cement and strong connective tissue. The neck is the area where the root(s) and crown meet. The crown projects above the gum. It is covered with a hard, white layer of enamel. The largest part of the tooth consists of dentine which is a harder substance than ordinary bone. The dentine surrounds the cavity which extends from the root to the crown. Blood capillaries and nerves enter the cavity at a small opening in the tip of the root.

11. 2.The Vertebral Column The vertebral column forms the central part of the skeleton. It supports the skull and protects the spinal cord . It also serves as attachment for the ribs, the pectoral and pelvic girdles . The vertebral column consists of separate bones, the vertebrae . The different vertebrae are arranged above each other. Because the separate vertebrae are attached to each other by means of fibrous cartilaginous discs they form a flexible column. Each vertebra has articular surfaces above and below , which allow articulation movement between them.

12. The vertebral column of 33 vertebrae is divided into five regions according to their position and structure. The five regions consist of: Seven cervical (neck) vertebrae, Twelve thoracic (chest) vertebrae, Five lumbar vertebrae, Five fused sacral vertebrae, and Four fused vertebrae.

13. 3.The Ribs Twelve pairs of ribs articulate with the 12 vertebrae of the thoracic region. The ribs are flat, narrow bones with a distinctive bow-shaped curve . Each rib consists of a head or capitulum , a small tubercle (which is a short distance back from the head) and the shaft . The head of the rib articulates with the semi-circular articulating facets formed by the centra of two successive thoracic vertebrae. The tubercle fits into and articulates with the articulating facets on the transverse process. The first seven ribs on each side are joined to the breastbone by bars of hyaline cartilage (called costal cartilage in this region).

14. The first seven pairs of ribs are referred to as true ribs . The cartilages of the 8th, 9th and 10th ribs are joined to the costal cartilage of the rib immediately above (i.e. to the costal cartilage of the 7th rib). These three pairs of ribs are known as vertebrochondral ribs . The last two pairs of ribs have free ends which are not attached to the sternum at all. They are floating ribs . The vertebrochondral ribs and the floating ribs are collectively known as false ribs . The ribs (together with their muscles) play an important role in the breathing mechanism of a mammal. Diagram to illustrate the attachment of the ribs to the thoracic vertebrae and sternum.

15. 4.The Sternum (Breastbone) The sternum is a long, flat, dagger-shaped bone . It is about 15 - 18 cm long and is found in the center of the chest region. The broad upper end supports the collar bones. The first seven pairs of ribs are attached to the articulating facets on the sides of the sternum. The 12 thoracic vertebrae, the 12 pair of ribs and the sternum forms the thorax which protects the delicate and vital organs of the thorax, viz. the heart and lungs.

16. II.THE APPENDICULAR SKELETON The appendicular skeleton consists of the girdles and the skeleton of the limbs . The upper (anterior) limbs are attached to the pectoral (shoulder) girdle and the lower (posterior) limbs are attached to the pelvic (hip) girdle .

17. 1.The Pectoral (Shoulder) Girdle The Pectoral girdle consists of two shoulder blades (scapulae) and two collar bones (clavicles). These bones articulate with one another, allowing some degree of movement.

18. 2.Shoulder Blades (Scapulae) The shoulder blade is a flat triangular bone which stretches from the shoulder to the vertebral column at the back. On the back side it has a bony ridge for the attachment of the muscles. The bony ridge forms a prominent projection, the acromion , above the shoulder joint. Beneath the collar bone and just on the inside of the shoulder joint, is another bony projection of the shoulder blade, the coracoid process , which also serves for the attachment of muscles . The upper outer corner of the shoulder blade ends in the glenoid cavity into which fits the head of the upper arm bone, forming a ball and socket joint.

19. 3.Collar Bones (Clavicles) Each collar bone is rod-shaped and roughly S-shaped . It lies horizontally and articulates with the upper end of the breastbone, right in the middle and front, just above the first rib. The lateral end articulates with the acromium. The Pectoral Girdle.

20. Collar bones serve as a support for the shoulder blades in front and keep the shoulder blades back so that the arms can hang freely at the sides of the body. They prevent the pectoral girdles from getting out of joint easily and ample movement of the shoulders.

21. 4.The Upper Limbs The skeleton of the upper limbs or arm may be divided into five main regions: an upper arm bone, the forearm (radius and ulna), the wrist, the palm of the hand and the fingers.

22. 5.The Pelvic (Hip) Girdle The pelvic girdle consists of two large, sturdy hip bones . Each hip bone consists of three fused bones namely the ilium, ischium and the pubis . The ilium is the largest of the three and forms the upper part of the hip bones. The sacrum fits like a wedge posteriorly between the two hip bones. The sacrum has a large, flat articular surface on each side for articulation with the ilia. The ischium forms the inferior part of the hip bone and the pubis the central in front. The two pubic bones are attached in the middle, on the front side by a symphysis which consists of fibrocartilage and ligaments, the pubic symphysis . The two hip bones and the sacrum form a complete bony ring, the pelvis . On the outer side of the point where the fused bones meet, there is a deep hip socket into which the head of the femur fits.

23. The pelvic girdle forms a strong support for the attachment of the limbs. Strong muscles of the back, the legs and the buttocks are attached to it. It protects some of the internal organs. In females it forms a strong basin-like structure for supporting and protecting the developing foetus during child-bearing. The Pelvic Girdle.

24. 6.The Lower Limbs or Legs The skeleton of the lower limb may be divided into five main regions : the upper leg (thigh), the lower leg, the ankle, the arch of the foot and the toes .

25. The Upper Leg or Thigh The upper leg has a single long bone, the femur and is the longest bone in the body. The head of the femur is turned slightly inwards and has a large, rounded portion which articulates in the acetubulum , forming a ball-and-socket joint . At its distal end, the femur widens to form two large knobs (condyles) which form the hinged knee joint with the main long bone (tibia) of the lower leg. On the anterior side of these two condyles, there is an articular surface against which the kneecap (patella) slides. The patella is a small, triangular, flat bone which develops on the tendon of the thigh muscle and is attached by ligaments to the tibia. This enables movement in the knee joint.

26. The Lower Leg The two bones of the lower leg are the tibia (shinbone) in front and the fibula behind. The tibia is the larger of the two and extends from the knee to the ankle. The upper end of the tibia has two articulating facets into which the condyles of the femur fit to form the knee joint . The lower end of the tibia articulates with one of the tarsals to form the ankle joint. The fibula is smaller than the tibia and is situated on the outside and slightly behind it. The upper end articulates with the tibia but does not form part of the knee joint. The lower end forms part of the ankle joint.

27. The Ankle There are seven short, thick tarsal bones , the largest of which is the heel bone (calcaneum) , which presses firmly onto the ground when one stands, walks or runs. The calf muscles are attached to the calcenum, allowing the heel to be lifted during locomotion. The Arch of the Foot The arch is formed partly by some of the tarsals but mainly by the five long metatarsals , which extends from the tarsals to the toes. The arch is modified for receiving the weight of the body. The Toes There are fourteen short phalanges in the toes of each foot. The big toe has two phalanges and the other toes have three in each .