Skeleton forms the main supporting framework of the body formed by joining of individual bones and is primarily designed for a more efficient production of movements by the attached muscles.
Bone is a connective tissue, impregnated with calcium salts. Connective tissue forms one-third of the bone and calcium salt about two-thirds.
The inorganic calcium salts are mainly calcium phosphate, partly calcium carbonate, and traces of other salts. These make it hard and rigid, which can afford resistance to compressive forces of weight-bearing and impact forces of jumping.
The organic connective tissue makes it tough and resilient, which can afford resistance to tensile forces.
Despite its hardness and high calcium content the bone is very much living tissue. It is highly vascular, with a constant turn-over of its calcium content.
It can mold itself according to changes in stress and strain it bears, it shows disuse atrophy and overuse hypertrophy.
Functions of bones
- Give shape and support to the body and resist all forms of stress.
- They provide the surface for the attachment of muscles, tendons, ligaments, etc.
- They serve as levers for muscular actions.
- The skull, vertebral column, and thoracic cage protect the brain, spinal cord, and thoracic viscera, respectively.
- Bone marrow manufactures blood cells.
- Bones store 97% of the body calcium and phosphorus.
- Bone marrow contains reticuloendothelial cells which are phagocytic in nature and take part in the immune response of the body.
Our body contains 206 bones. [Can go up to 350 bones with accessory bones count, depending on age.]
It is important to understand normal structures and functions to locate abnormality. Only then we can recognize abnormal functions. Our skeleton is our basic structure on which everything else is laid.
Let us familiarize ourselves with bones in our body. We start from the top and move down. The number after the name of bone would correspond to number in the adjoining diagram.
Skull is formed by cranium and mandible. Cranium is further composed of multiple bones that are united to each other. The lines of the union are called sutures. these sutures are lax in children to allow brain growth. Mandible constitutes our lower jaw that we used the whole day for speaking, eating or gesturing.
There are six bones in middle ears, three on either side. These bones are called Malleus, Incus, and Stapes.
The hyoid bone is present in the throat and serves as support to cartilages of trachea or windpipe. It also serves as attachments to various muscles that help in movement of the trachea.
Shoulder Girdle is an area of the shoulder that is formed by the following bones
- Clavicle : It is a bone that connects the thoracic cage to upper limbs and also functions for transmittal of forces.
- Scapula: It is a complexly structured bone that articulates with the clavicle, thorax and takes part in the formation of the shoulder joint.
Thorax is a cage-like structure that functions to protect vital structures that it contains like lungs, heart etc. it is formed by ribs on either side which are connected to the sternum in front and to vertebrae on back.
Spine or Vertebral column
This is formed by multiple vertebrae which are named according to area
- Cervical vertebrae
- Thoracic vertebrae
- Lumbar vertebrae
The bone that connects shoulder to elbow. This bone is normally cylindrical but flares when it comes near the elbow and becomes flat. It flares on either side to form ridges called condyles.
Ulna and Radius
The bone that forms elbow joint by articulating with the humerus. This is the bone that protrudes on back of elbow on flexion. It can be felt on the aspect of the forearm on side of the little finger.
Radius is [depicted red in image] the bone on other side of forearm i.e. on side of the thumb. It forms a joint with ulna on the upper side. The part that takes part in this joint formation is called the head of the radius. This joint allows twisting movements of the elbow called pronation and supination.
Hand & Wrist
- Carpals or Wrist Bones- These bones connect forearm to hand
- Scaphoid [A]
- Triquetrum [C]
- Pisiform [D]
- Trapezium [E]
- Trapezoid [F]
- Capitate [G]
- Hamate [H]
- Phalanges: These include proximal phalanges, intermediate phalanges, and distal phalanges
Pelvis or pelvic girdle is formed by many bones on either side which on one hand are connected to the spine and on other side to lower limbs. Pelvis functions to transmit the weight to lower limbs and also protect the vital structures like rectum and bladder. Ilium, pubis, and ischium are bones that unite to form pelvis.
More on pelvic anatomy
Structure of lower limb is quite similar to upper limbs with modifications done for weight bearing purpose.
Femur, tibia, and fibula are long bones that form lower limb. The femur is bone of thigh whereas other two bones are present in the leg. Another important bone called patella is present on the anterior side of the knee joint and is commonly known as kneecap.
Foot is as complex as hand, so I am just going to list the names.
- Navicular bone
- Medial cuneiform bone
- Intermediate cuneiform bone
- Lateral cuneiform bone
- Cuboidal bone
- Phalanges: proximal phalanges, intermediate phalanges, and distal phalanges
Bones can be classified by many methods. Here we discuss different types of bones one by one. Bone classification can be done by various parameters.
Classification of Bones According to Shape
Each long bone has an elongated shaft or diaphysis and two expanded ends (epiphyses) which are smooth and articular. The shaft typically has 3 surfaces separated by 3 borders, a central medullary cavity, and a nutrient foramen directed away from the growing end. Examples of typical long bones are humerus, radius, ulna, femur, tibia and fibula, metacarpals, metatarsals, and phalanges
Their shape is usually cuboid, cuneiform, trapezoid, or scaphoid. Examples: carpal and tarsal bones.
Flat bones resemble shallow plates and form boundaries of certain body cavities. Examples: bone in the vault of the skull, ribs, sternum, and scapula.
Vertebra, hip bone, and bones in the base of the skull.
Certain irregular bones contain large air spaces lined by epithelium. Pneumatic bones examples are maxilla, sphenoid, ethmoid, etc. they make the skull light in weight, help in the resonance of voice, and act as air conditioning chambers for the inspired air.
These are bony nodules found embedded in the tendons or joint capsules. They have no periosteum and ossify after birth. They are related to an articular or nonarticular bony surface, and the surfaces of contact are covered with hyaline cartilage and lubricated by a bursa or synovial membrane.
Examples: patella, pisiform, fabella, etc.
The function of the sesamoid bones is not definitely known. Their possible functions are –
- To resist pressure
- To minimize friction
- To alter the direction of pull of the muscle; and
- To maintain the local circulation.
Accessory (Supernumerary) Bones
These are not always present. They may occur as ununited epiphyses developed from extra centers of ossification. Examples: sutural bones, os trigonum, os vesalianum, etc. In medicolegal practice, accessory bones may be mistaken for fractures. However, these are often bilateral and have smooth surfaces without any callus.
Developmental Classification of Bones
Membrane (Dermal) Bones
These types of bones ossify in the membrane (intramembranous or mesenchymal ossification) and are thus derived from mesenchymal condensations. Examples: bones of the vault of the skull and facial bones.
A defect in membranous ossification causes a rare syndrome called cleidocranial dysostosis. It is characterized by three cardinal features:
* Varying degrees of aplasia of the clavicles;
* Increase in the transverse diameter of the cranium, and
* Retardation in fontanelle ossification. It may be hereditary or environmental in origin.
These types of bones ossify in cartilage (intracartilaginous or endochondral ossification), and are thus derived from performed cartilaginous models. Examples: bone of limbs, vertebral column, and thoracic cage.
A defect in endochondral ossification causes a common type of dwarfism called achondroplasia, in which the limbs are short, but the trunk is normal. It is transmitted as a Mendelian dominant character.
These ossify partly in the membrane and partly in cartilage. Examples: clavicle, mandible, occipital, temporal, sphenoid.
Types of Bones Based on the Bone structure
Cortical bone is also called compact bone or lamellar bone.
The shaft of bone in a long bone like femur is a typical example of the cortical bone.
Cortical bone forms the cortex, or outer shell, of most bones. It is much denser than cancellous bone, harder, stronger and stiffer. Cortical bone contributes about 80% of the weight of a human skeleton.
At the microscopic level, the structural arrangement of a cortical bone is different than cancellous.
Cortical bone is arranged for facilitating the following functions
- To support the whole body weight
- Protect organs
- Provide levers for movement
- Store and release calcium.
Cancellous bone is also called trabecular bone or spongy bone.
Compared to compact bone, which is the other type of osseous tissue, it has a higher surface area but is less dense, softer, weaker, and less stiff.
It typically occurs at the ends of long bones, proximal to joints and within the interior of vertebrae.
Cancellous bone is highly vascular and frequently contains red bone marrow where hematopoiesis, the production of blood cells, occurs.