Skeletal System

The skeletal system is constituted by bones, cartilages and ligaments. This system provides ‘the shape’ to the body. Further, bones remain as regions for the attachment of muscles. It also helps to hold weight. Structureslike skull, protect inner organs. This system is also useful in locomotion. The bones remain as reservoirs of fat and certain minerals. The bone marrow is the site for the production of erythrocytes.
The bones can be long, short, flat or irregular in shape. Hands and legs have long bones. Short bones are broad in shape. Carpals (wrist bones) and tarsals (antkle bones) are shorter. Flat bones are thin and flattened. Skullbones, ribs, sternum and scapula (shoulder blade) are flat bones. Verterbral and facial bones are irregular in shape. Structure of a typical long bone   bone is covered by a double layered sheath called the periosteum.
The outer layer of the periosteum is fibrous in nature. It is a dense collagenous layer having blood vessels and nerves. A growing long bone has three regions. The long bony part is the diaphysis or shaft. It is made up of compact bone. The end of the bone consists of epiphysis. It is made up of spongy bone. The outer surface of epiphysis is formed of compact bone. In between the epiphysis and diaphysis epiphyseal or growth plate is found. It is made up of hyaline cartilage. Growth in length of bone occurs at this plate. The cavity inside the diaphysis is called the medullary cavity. Thiscavity is lined by a membrane called the endosteum. The cavity inside the diaphysis in adults contain yellow marrow. It is mostly adipose tissue. The
medullary cavity of the epiphysis contains red marrow concerned with blood cell formation.

Dried, prepared bones are used to study skeletal anatomy. The bones are named according to their position in the body. The named bones are divided into two categories: (1) the axial skeleton and (2) the appendicular skeleton. The axial skeleton consists of the skull, hyoid bone, vertebral column and thoracic cage. The appendicular skeleton consists of the limbs and
their girdles. In human body, there are 206 bones, of these 80 are in the axial skeleton, 126 in the appendicular skeleton. Among the bones of the axial skeleton 28 bones are in the skull, 26 bones in the vertebral column, 25bones in the thoracic cage and one remains as the hyoid bone. (details as found below)

Axial skeleton :
              It forms the upright axis of the body. It protects the brain, the spinal cord and the vital organs found within the thorax. a Skull - The human cranial capacity is about 1500 cm3 . It consists of 22 bones. It protects the brain. It supports the organs of vision, hearing, smell and taste. The lower jaw or mandible remains specially attached to the skull.
The skull or cranium is covered by eight bones. They are one pair each of parietal and temporal, individual bones as frontal,sphenoid, occipital and ethmoid. These bones are joined by sutures to form a compact box like structure. The sutures are immovable join

Skull - Lateral view :
              In the front there are 14 facial bones. Of these maxilla, zygomatic,palatine, lacrymal, nasal and inferior nasal koncha remain as pairs. Mandible or lower jaw and vomer are unpaired bones .The parietal and occipital bones are major bones on the posteriorside of the skull. The parietal bones are joined to the occipital bone at the back. The side of the head is formed of the parietal and the temporal bones.The large hole in the temporal bone is the external auditory meatus. Thisopening is meant for transmitting sound waves towards the eardrum. On thelateral side immediately anterior to the temporal, the sphenoid bone is seen. Anterior to the sphenoid bone is the zygomatic bone or cheek bone. It is a prominent bone on the face. The upper jaw is formed of the maxilla. Themandible constitutes the lower jaw.The major bones seen from the frontal view are the frontal bone, zygomatic bone the maxillae and the mandible. The most prominent openings in the skull are the orbits and the nasal cavity. The two orbits are meantfor accommodating the eyes. The bones of the orbits provide protection for the eyes and attachment points for the muscles that move the eyes. The bones forming the oribits are the frontal, sphenoid, zygomatic, maxilla, lacrymal, ethmoid and palatine. The head region also contains 6 ear ossicles. They are Maleus (2), incus (2) and stapes (2).