THE SKELETAL SYSTEM: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
A skeleton is the framework of bones that supports our entire body. It helps us stand upright, walk, run, move our body parts, and even breathe properly. It also gives shape to the body and protects the internal organs. The skeletal system is made up of bones, cartilages, ligaments, and tendons. It is closely connected to the muscular system because bones and muscles work together for movement. Bones and cartilages are both types of connective tissues. Bones contain mineral salts like calcium phosphate, which make them hard and stone-like in appearance.
Dpharmguru’s exam insights:
In my years of teaching anatomy, I have observed that students often get overwhelmed by the number of bones in the human body. Remember this simple trick: “206 bones in an adult skeleton — 80 in the axial and 126 in the appendicular skeleton.” This is a very common exam question, so make sure you memorize these numbers!
STRUCTURE OF HUMAN SKELETON
The human skeleton is mainly divided into two parts:
- Axial Skeleton: Bones along the main axis of the body (head, neck, trunk)
- Appendicular Skeleton: Bones of the limbs and their supporting girdles
| Skeleton | Major Parts | Number of Bones |
|---|---|---|
| Axial Skeleton | Skull, Vertebrae, Thoracic Cage | 80 |
| Appendicular Skeleton | Pectoral Girdle, Pelvic Girdle, Upper and Lower Limbs | 126 |
FUNCTIONS OF HUMAN SKELETON
- Shape and Support: Bones form a strong framework that gives shape to the body and supports it.
- Protection: They protect delicate organs like the brain, heart, and lungs.
- Movement: Bones work with muscles to help the body move.
- Acid–Base Balance: Bones help to maintain blood pH by releasing or absorbing alkaline salts like calcium.
- Mineral Storage: Bones store important minerals like calcium and phosphorus.
- Blood Production: Bones help in forming blood cells (haemopoiesis).
- Detoxification: Bones can absorb and store harmful metals and toxins.
- Sound Transmission: Bones help in the process of hearing.
BONES
A bone is a hard living tissue made of protein (collagen) and minerals (mainly calcium). The study of bones is called Osteology. Bones not only support the body but also give protection and shape.
Types of Bones
- Long Bones: Longer than they are wide. Example: humerus, femur. They have a shaft and help in movement.
- Short Bones: Small and cube-shaped, almost equal in length and width. Example: bones of wrist and ankle.
- Flat Bones: Thin and flat. They protect organs and give surface for muscle attachment. Example: skull, ribs, sternum, scapula.
- Irregular Bones: Have complex shapes that do not fit in other categories. Example: vertebrae, facial bones.
- Sesamoid Bones: Develop inside tendons to reduce friction. Example: patella (kneecap).
- Sutural Bones: Very small bones found between the joints of cranial bones in the skull.
Dpharmguru’s exam insights:
A common exam question is: “Which is the longest bone in the human body?” The answer is the femur. And “Which is the smallest bone?” The answer is the stapes in the ear. Also, remember the patella is a sesamoid bone—this is frequently tested!
Structure of Bones (Macroscopic)
- Compact Bone: The strong, hard outer layer of bone. It has tiny cylindrical structures called Haversian systems or osteons. Each osteon has concentric rings called Haversian lamellae, and a Haversian canal in the center which carries blood vessels and nerves.
- Spongy (Trabecular) Bone: Found inside the bone. It has a mesh-like structure with spaces that contain bone marrow. It makes bones lighter and allows space for blood vessels.
Microscopic (Cellular) Structure of Bones
- Osteoblasts: Bone-forming cells. They make a protein mixture called osteoid, which later hardens to form bone.
- Osteocytes: Mature bone cells found inside small spaces called lacunae. They maintain the bone and help it respond to stress.
- Osteoclasts: Large, multi-nucleated cells that break down old bone tissue. This process is called bone resorption and helps in bone remodeling.
AXIAL SKELETON
The axial skeleton includes all those bones that form the central axis of the human body. It supports and protects the head, neck, and trunk (torso). In a child, the axial skeleton has 87 bones, while in an adult it has 80 bones, because some bones fuse as a person grows.
Structure and Functions of Skull
The bones of the head region form the skull. The skull rests on the vertebral column, which lies along the vertical axis of the body. An adult human skull is made up of 28 bones, most of which are immovable, except for the mandible (lower jaw), which can move.
The skull is divided into two main regions: Cranium and Facial Bones.
The skull and face together form different cavities, such as the cranial cavity, which holds the brain. The skull bones are joined together by immovable joints called sutures. Major Sutures of the Skull include: Coronal Suture, Squamous Suture, and Lambdoid Suture.
The braincase is formed by 8 bones, collectively called the neurocranium, which surrounds and protects the brain and brainstem. The facial portion of the skull, made up of 14 bones, is called the splanchnocranium.
Dpharmguru’s exam insights:
Remember: The only movable bone in the skull is the mandible (lower jaw). This is a very common exam question! Also, the sutures of the skull are synarthrodial joints—meaning they are immovable.
Bones of the Cranium
- Frontal Bone: Forms the front part of the skull (forehead). Contains frontal sinuses.
- Parietal Bones: Two large bones forming the sides and roof of the skull.
- Temporal Bones: Located near the ear region. Contains the carotid canal, jugular foramen, mandibular fossa, TMJ, mastoid process, and styloid process.
- Occipital Bone: Found at the back of the skull. Contains the foramen magnum and occipital condyles.
- Sphenoid Bone: Found in the center of the skull, shaped like a bat. Contains the sella turcica (holds the pituitary gland).
- Ethmoid Bone: Located in the front part of the skull, separating the nasal cavity from the brain. Contains the cribriform plate, labyrinth, and perpendicular plate.
Functions of the Cranium
- Protects the brain from injury.
- Provides surface area for attachment of muscles that move parts of the head.
- Inner surface attaches to meninges (brain membranes).
- Offers attachment points for facial muscles, allowing expressions.
Facial Bones
- Nasal Bones: Small bones that form the bridge of the nose.
- Nasal Septum: Divides the nose into right and left parts.
- Maxillae: Paired bones forming the upper jaw. Hold the upper teeth.
- Palatine Bone: Forms the back part of the roof of the mouth.
- Zygomatic Bones: Commonly called cheekbones.
- Mandible: The largest and strongest facial bone. Forms the lower jaw and is the only movable bone in the skull.
- Vomer: A thin, flat bone that forms part of the nasal septum.
- Lacrimal Bones: Smallest and most delicate facial bones. Contain lacrimal ducts, which produce tears.
- Orbit: The eye socket, a bony cavity that protects the eyeball.
Hyoid Bone
The hyoid bone is a U-shaped (horseshoe-shaped) bone, also called the lingual bone. It lies in the middle of the neck, between the chin and thyroid cartilage. It is the only bone in the human skeleton that does not directly attach to any other bone. Instead, it is held in place by muscles and ligaments.
Structure and Functions of Vertebral Column (Spine)
The vertebral column, commonly called the backbone, forms the central support of the body. It consists of 33 vertebrae in children and 26 vertebrae in adults (because some fuse during growth).
Structure of a Typical Vertebra:
- Body: The thick front portion, which connects to other vertebrae through intervertebral discs.
- Vertebral Arch: The back part of the vertebra that protects the spinal cord.
- Vertebral Foramen: The central opening through which the spinal cord passes.
- Processes: Bony projections for muscle and ligament attachment (transverse, spinous, superior articular).
Dpharmguru’s exam insights:
Remember the regions of the vertebral column: “Cervical = 7, Thoracic = 12, Lumbar = 5, Sacral = 5 (fused), Coccygeal = 4 (fused).” A common exam question is: “How many vertebrae are in the cervical region?” The answer is 7.
Regions of the Vertebral Column
- Cervical Region (7 vertebrae): Found in the neck region. Each has small holes (foramina) in its transverse process.
- Thoracic Region (12 vertebrae): Located in the chest region. Have facets for rib attachment.
- Lumbar Region (5 vertebrae): Present in the lower back. Large and strong since they bear body weight.
- Sacral Region (5 fused vertebrae): Fuse to form the sacrum, which connects the spine to the pelvis.
- Coccygeal Region (4 fused vertebrae): Form the coccyx (tailbone), located at the base of the spine.
Functions of the Vertebral Column
- Maintains upright posture.
- Keeps the body balanced.
- Forms a supporting framework for the trunk along with ribs and sternum.
- Supports head and limbs and allows their movement.
- Serves as a site for muscle, tendon, and ligament attachment.
- Protects the spinal cord and nearby vital organs.
- Helps in equal distribution of body weight.
Structure and Functions of Thoracic Cage (Thorax)
The thoracic cage or chest cavity lies between the neck and abdomen. It consists of ribs, sternum, and thoracic vertebrae. There are 12 pairs of ribs, all attached to the thoracic vertebrae at the back.
Types of Ribs:
- True Ribs (1–7): Directly attached to the sternum by cartilage.
- False Ribs (8–10): Indirectly attached to the sternum.
- Floating Ribs (11–12): Not attached to the sternum at all.
Functions of Rib Cage:
- Allows movement of the chest during breathing.
- Helps in costal breathing by expanding and contracting.
- Protects the heart, lungs, and other organs in the thorax.
Sternum (Breastbone)
The sternum is a long, flat bone located in the center of the chest. It connects with ribs through cartilages and forms the front part of the rib cage.
Parts of the Sternum:
- Manubrium: The upper part of the sternum.
- Body: The middle portion where ribs 2 to 7 attach.
- Xiphoid Process: The lowest part, soft in babies but hardens in adults. Used as a landmark in CPR.
APPENDICULAR SKELETON
The appendicular skeleton forms the bony framework of the upper and lower limbs, along with their supporting girdles. It includes 126 bones.
Structure and Functions of Pectoral Girdle
The pectoral girdle connects the upper limbs to the axial skeleton. It has two halves — left and right — and each half consists of two bones:
- Scapula (shoulder blade): A large, flat, triangular bone located at the back of the upper chest.
- Clavicle (collarbone): An S-shaped, long, and narrow bone that connects the upper limb to the trunk.
Structure and Functions of Upper Extremity (Upper Limb)
Each upper limb has 30 bones, divided into three main parts:
- Arm (Humerus): The longest bone of the upper limb.
- Forearm (Radius and Ulna): The ulna is medial, and the radius is lateral.
- Hand (Carpals, Metacarpals, and Phalanges): 8 carpals, 5 metacarpals, and 14 phalanges.
Structure and Functions of Pelvic Girdle
The pelvic girdle is a ring-shaped, irregular, arch-like structure made up of hip bones (coxal or innominate bones). It connects the lower limbs to the axial skeleton and supports the trunk of the body.
Each half of the pelvis has three bones — ilium, ischium, and pubis — which are separate in children but fuse together in adults.
Functions of Pelvic Girdle:
- Supports the body weight transmitted from the vertebral column.
- Protects pelvic organs like urinary bladder, reproductive organs, and developing foetus in females.
- Provides attachment for lower limb muscles and helps in locomotion.
Structure and Functions of Lower Extremity (Lower Limb)
The lower limb has 30 bones, which allow movement, support, and balance of the body.
- Femur: The longest and strongest bone in the body.
- Patella: A small, flat, sesamoid bone located in front of the knee joint.
- Tibia: The larger and medial bone of the lower leg.
- Fibula: The slender, lateral bone of the leg.
- Tarsal Bones (7): Proximal row: Talus and Calcaneus; Distal row: Medial, Intermediate, and Lateral Cuneiforms, Cuboid, and Navicular.
- Metatarsals (5): Five long bones in the mid-foot.
- Phalanges (14): 2 in the great toe, 3 in each of the other toes.
Dpharmguru’s exam insights:
The skeletal system is the framework of the body. Understanding the bones, their types, and functions is essential for understanding how the body moves and protects itself. In exams, pay special attention to the number of bones in each region and the classification of bones. Remember: “Femur is the longest bone, stapes is the smallest, and patella is a sesamoid bone.” These are almost guaranteed to appear!
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
- Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy (8th ed.). Wolters Kluwer.
- National Institutes of Health (NIH). (2022). Skeletal System Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
