Bone Structure and Function: A Comprehensive Guide
Functions of the Skeleton
The skeleton performs several vital functions, including:
- Protecting vital organs: The skull protects the brain, the spine protects the spinal cord, and the rib cage protects the heart and lungs.
- Supporting body shape: Bones provide a framework for the body, and muscles attach to bones to enable movement.
Classification of Bones by Dimension
Bones are classified into three types based on their dimensions:
- Long bones: These bones are longer than they are wide or thick, and are found in the limbs. Examples include the humerus, ulna, femur, and tibia.
- Short bones: These bones have similar length, width, and thickness. Examples include the calcaneus, talus, and vertebrae.
- Flat bones: These bones are thin and wide, with a large surface area. Examples include the frontal, parietal, and scapula bones.
Bone Histology (Study of Tissue)
Bones are composed of two types of bone tissue:
- Spongy bone tissue: This tissue has a network of bony extensions intersecting with spaces containing red bone marrow.
- Compact bone tissue: This tissue is dense and hard, with microscopic spaces.
Both tissue types consist of the same histological structure called bone lamellae. Bone lamellae are a type of tough connective tissue where the substance between cells is calcified. The cells of this tissue are osteocytes, which are elongated and have extensions called canaliculi. These osteocytes communicate with each other. Between the canaliculi are collagen fibers, which are organic substances. This entire structure is impregnated with calcium and phosphate salts (hydroxyapatite), which determine the bone’s shape and hardness.
The arrangement of lamellae determines whether the bone tissue is spongy or compact. In spongy tissue, lamellae are arranged unevenly, forming a lattice and leaving spaces for red bone marrow (which produces red and white blood cells). In compact bone tissue, lamellae are arranged concentrically around a central canal called the Haversian canal, which contains an artery and a nerve. The set of lamellae with a centrally placed Haversian canal is called an osteon or Haversian system. The osteons arranged next to each other form the compact bone tissue.
Bone Anatomy (Study of Macroscopic Structure)
Structure of a Long Bone
Long bones have two ends called epiphyses and a middle shaft. The shaft is mainly composed of compact bone tissue and contains a central canal with yellow bone marrow. The tissue lining the canal is called endosteum, and the tissue covering the outer surface is called periosteum, which has nutritional and ossification functions. The epiphyses are mainly composed of spongy bone covered by a layer of compact bone. In growing bone, a growth plate made of cartilage is located between the epiphysis and diaphysis.
Structure of a Short Bone
Short bones are mainly composed of spongy tissue covered by a thin layer of compact bone.
Structure of a Flat Bone
Flat bones have two sheets of compact tissue (external and internal) with spongy bone in between.
Bones of the Head
The bones of the head are divided into skull bones and facial bones. The skull has eight bones: frontal, parietal, temporal, occipital, ethmoid, and sphenoid. The face has fourteen bones: lacrimal, nasal, zygomatic, vomer, inferior turbinates, palatine, maxilla, and mandible.
Features of the Vertebrae
A vertebra consists of a vertebral body and several processes. Vertebrae are connected by intervertebral discs made of cartilage. They also articulate through their articular processes.
Spinal Column
The spinal column is made up of vertebrae and is divided into five regions:
- Cervical: 7 vertebrae, including the atlas and axis.
- Thoracic: 12 vertebrae.
- Lumbar: 5 vertebrae.
- Sacral: 5 fused vertebrae forming the sacrum.
- Coccyx: 4-5 fused vertebrae.
The spine has natural curves: cervical lordosis, thoracic kyphosis, lumbar lordosis, and sacral kyphosis.
Rib Cage
The rib cage consists of the vertebral column, sternum, and ribs. The sternum has three parts: manubrium, body, and xiphoid process. There are 12 pairs of ribs. The first 7 pairs are true ribs, directly connected to the sternum. The next 3 pairs are false ribs, connected to the sternum through a common cartilage. The last 2 pairs are floating ribs, not connected to the sternum.
Shoulder Girdle
The shoulder girdle consists of the scapula and clavicle. The clavicle is an S-shaped bone that connects the sternum and scapula. The scapula is a triangular bone that articulates with the humerus.
Arm
The arm bone is the humerus. It articulates with the scapula at the shoulder and with the radius and ulna at the elbow.
Forearm
The forearm bones are the radius and ulna.
Hand
The hand bones are divided into carpals, metacarpals, and phalanges.
Pelvic Girdle
The pelvic girdle consists of two hip bones, each formed by the ilium, ischium, and pubis.
Lower Limb
The lower limb consists of the femur (thigh), tibia and fibula (leg), and the foot bones (tarsals, metatarsals, and phalanges).
Joints
Joints connect bones and allow movement. They are classified by their degree of mobility:
- Synovial joints: Highly mobile joints (e.g., knee, elbow, shoulder).
- Amphiarthroses: Semi-mobile joints (e.g., vertebrae, pubic symphysis).
- Synarthroses: Immovable joints (e.g., skull bones).