Courses Human Skeletal System Classification of Human Skeletal System

Based on the position of the skeletal structures in the body the endoskeleton is divided into two main categories (Figure 7): Axial and Appendicular.

Figure 7. Classification of human skeleton system
Figure 7. Classification of human skeleton system

Axial Skeleton

Axial skeleton forms the longitudinal axis of human body. It consists of skull, ossicles of the middle ear, hyoid bone of throat, vertebral column, and sternum, and rib cage.

Skull

Skeleton of head is called skull, which rests on the upper end of the vertebral column. The bones of the skull support the face and protect the brain. The skull consists of 22 bones, which are divided into two categories: cranial bones and facial bones.

All 22 bones that are fused together except for the mandible or jaw bone (Figure 8A and 8B). Mandible is the only flexible bone in the skull. One frontal bone, two parietal bones, two temporal bones, one occipital bone, one sphenoid bone, and one ethmoid bone form cranium.

Figure 8A. Front aspect of human skull
Figure 8A. Front aspect of human skull
Figure 8B. Lateral aspect of human skull
Figure 8B. Lateral aspect of human skull

The skull is a bone structure that has a main task is the protection of the most important organ in the human body: the brain. Apart from protection it has the following functions:

  1. It provides the rigid walls of a respiratory passage that helps in breathing.
  2. Jaws supported by skull helps in masticating food.
  3. It also protects and supports the sense organs.

Ossicles

There are six ear ossicles called auditory ossicles (two malleus, two incus and two stapes) present that form the axial skeleton (Figure 9). They transmit and amplify sound from the tympanic membrane (eardrum) to the inner ear.

Figure 9. Structure of an ear showing ear ossicles.
Figure 9. Structure of an ear showing ear ossicles.

Hyoid bone

It is a U-shaped bone in the wall of upper part of the throat (Figure 10). It serves as an attachment point for the muscles of the tongue and floor of the mouth. Hyoid is the only bone in the body that does not form a joint with any other bone and hence it is sometimes called floating bone.

Figure 10. Structure of larynx showing hydoid bone
Figure 10. Structure of larynx showing hydoid bone

Vertebral Column

The vertebral column, or spinal column consists of 33 vertebrae (Figure 11A and 11B). It is divided into five regions: Cervical, Thoracic, Lumbar, Sacral, and Coccygeal. The names of the spinal curves correspond to the region of the spine in which they occur. These vertebrae are discussed below.

  1. The seven cervical vertebrae are present in the neck. The first cervical vertebra is called atlas, while the second cervical vertebra is known as axis.
  2. The twelve thoracic Vertebrae are present in the chest and articulate with ribs.
  3. The five lumbar vertebrae are present in the abdomen. They are the largest and strongest in the vertebral column.
  4. The five sacral vertebrae are fused in the adult, forming one structure called sacrum. The sacrum in females is shorter, wider than males.
  5. The four coccygeal vertebrae are fused to form a curved triangular bone, called coccyx. It is considered as a vestigial tail (remnant with no function). In the sixth week of gestation, the human embryo possesses a tail, along with other vertebrae. In the next couple weeks of development, however, the tail disappears, and then later it fuses to form the coccyx, or tailbone, in the adult.

The vertebral column performs several functions. It protects the spinal cord, carries the weight of the body, the intervertebral discs present in between vertebrae give flexibility to the body, and the cervical vertebrae allow free movement of the head.

Figure 11A. 3D illustration of vertebral column (spinal cord/spine)
Figure 11A. 3D illustration of vertebral column (spinal cord/spine)
Figure 11B. Spinal cord schematic diagram showing all vertebrae - cervical, thoracic, lumber, sacrum, and coccyx
Figure 11B. Spinal cord schematic diagram showing all vertebrae

The vertebral formula of human body is C7T12T (5) C (4).

Intervertebral discs are present between the adjacent vertebrae from the second cervical vertebra to the sacrum (Figure 12). Each disc consists of an outer fibrous ring (fibrocartilage) and an inner soft, pulpy, elastic substance. These discs form strong joints, permitting movement of vertebral column, and absorb shock.

Figure 12. Lumbar spine anatomy segment showing intervertebral disc (white)
Figure 12. Lumbar spine anatomy segment showing intervertebral disc (white)

Appendicular Skeleton

The appendicular skeleton is composed of the bones of the upper limbs and the lower limbs. It also includes pectoral girdle, that attaches the upper limbs to the body, and pelvic girdle that attaches the lower limbs to the body.

Pectoral girdle

Pectoral provides points of attachment of the upper limbs to the axial skeleton (Figure 13). It consists of clavicle (collarbone), and scapula (shoulder blades).

Figure 13. A human pectoral girdle
Figure 13. A human pectoral girdle

Pelvic girdle

The pelvis or pelvic girdle is formed by two innominate bones called hip bones (Figure 14). Unlike the pectoral girdles, they are jointed with each other at symphysis pubis. It consists of three separate bones, ilium, ischium, and pubis. On its outer surface it has a deep depression called acetabulum, to which head of femur articulates forming the hip joint.

Figure 14. A human Pelvic girdle
Figure 14. A human Pelvic girdle

Upper limb

The upper limb contains thirty bones in three regions (Figure 15): the arm, the forearm (ulna and radius), the wrist, and hand. Humerus is the largest and longest bone of the upper limb and the only bone of the arm. The hand includes eight bones of carpus (wrist), five bones of metacarpus (palm), and fourteen bones of the phalanges (digits).

Figure 15. Anatomy of upper limbs (anterior view)
Figure 15. Anatomy of upper limbs (anterior view)

Lower Limb

The lower limb consists of thigh, leg, and foot (Figure 16). Each leg consists of one femur (thigh bone), one patella (kneecap), one tibia and one fibula (leg), seven tarsals (ankle), five metatarsals and fourteen phalanges (foot). Femur, or thighbone, is the longest and strongest bone in the body.

Figure 16. Anatomy of lower limbs
Figure 16. Anatomy of lower limbs

Functions of the skeletal system

The most important function skeletal system is to provide support to the body. As beams of a building provide strength to support its weight, the bones, cartilage, tendons, and fibrous connective tissue provides strength to supports human body.It performs the following functions mentioned below.

  1. The skeletal system forms a solid framework that supports and protects the organs of the body and anchors the skeletal muscles. The bones of the axial skeleton act to protect brain and heart, whereas the bones of appendicular skeleton provide support and flexibility at the joints and anchor the muscles.
  2. The bones of the skeletal system act as attachment points for the skeletal muscles of the body, thus helping in movement.
  3. Red bone marrow is found in the hollow space inside of bones. It produces red and white blood cells by a process called hematopoiesis.
  4. The skeletal system acts as a storage house of the body, facilitating growth and repair of body.
  5. Symptoms of pain in the joints often helps in diagnoses of other underlying health problems.
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