Skeletal System: Anatomy and Function, Diagram

The Skeletal system provides structural support, protects our internal organs, and facilitates movement. The skeletal system of humans is primarily formed by bones, cartilage, ligaments, and tendons. All these work together to form the structure of the human body. The skeletal system also takes part in the hormone regulation and storage of essential minerals. It contributes to blood cell production and in maintaining mineral balance.

Human Skeletal System

The human skeletal system is a complex framework of bones, cartilage, tendons, and ligaments that work together to support the body, protect internal organs, and enable movement. It can be divided into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of bones that lie along the central axis of the body, while the appendicular skeleton consists of the bones that connect limbs to the axial skeleton and enable movement.

Human Skeletal System

Axial Skeleton

The axial skeleton is primarily responsible for protecting vital organs and supporting the body’s weight. It consists of the skull, vertebral column, and ribs. The skull protects the brain and sensory organs such as the eyes, nose, and ears. The vertebral column provides support, flexibility, and protection for the spinal cord. The rib cage surrounds the thoracic cavity and protects vital organs like the heart and lungs.

Skull

The skull is composed of 22 bones, including the cranial bones, facial bones, ear ossicles, and Hyoid bone. The cranial bones protect the brain, while the facial bones provide structure to the face and house the sensory organs.

Cranial Bones

• Frontal Bone: The frontal bone protects the front side of the brain. It constitutes the forehead and the upper part of the eye orbits. 
• Parietal Bones: There are two parietal bones, one on each side of the skull. They form the roof of the cranial cavity 
• Temporal Bones: The temporal bones are located on the sides of the skull above the ear. They protect the sides of the brain 
• Occipital Bone: The occipital bone is situated at the back of the skull. It contains the largest hole of the human body known as the foramen magnum, through which the spinal cord passes. Two condyles are present in the occipital bone which makes the human skull dicondylic. 
• Sphenoid Bone: The sphenoid bone is present at the base of the skull. It forms the floor of the cranium. It protects the lower part of the brain and houses the pituitary gland in its depression known as Sella Tursica.
• Ethmoid Bone: The ethmoid bone is located between the eye sockets and forms part of the nasal cavity.

Facial Bones

• Maxilla: The maxilla is the upper jawbone. It contains the upper teeth.
• Mandible: The mandible is the lower jawbone. It is the only movable bone in the skull. It contains the lower teeth and plays an important role in chewing and speaking.
• Zygomatic Bones: The zygomatic bones are also known as cheekbones. They are located on the sides of the face. They form the cheeks prominence during smiling and also form part of the eye sockets.
• Nasal Bones: The nasal bones are small, rectangular bones that form the bridge of the nose. 
• Lacrimal Bones: The lacrimal bones are the smallest facial bones. They are present in the eye orbits and house the lacrimal or tear glands. 
• Vomer Bone: The vomer bone is a thin, flat and triangular bone present in the midline of the nasal cavity. It is a part of the nasal septum, which divides the nasal cavity into two halves.
• Palatine Bones: The palatine bones form the roof of the buccal cavity by forming the posterior part of the hard palate.
• Inferior Nasal Conchae: These are the bony ridges in the nasal cavity that are richly supplied with blood vessels. They add moisture to the air and warm the air. 

Ear ossicles

There are three pairs of ear ossicles present in the middle ear. They help in intensifying or amplifying sound waves. These are Malleus (Hammer) , Incus(Anvil) and Stapes(Stirrup). Stapes are the smallest bone of the human body. The Stapidius muscle is the smallest muscle in the human body, also found in the ear and associated with the stapes bone. 

Hyoid Bone

It is a U-shaped bone. It does not form any joint. It provides a surface for the attachment of tongue muscle.

Vertebral Column

The vertebral column is a S-shaped structure essential for bipedal locomotion. It provides support and protects the spinal cord.

It consists of 33 vertebrae in embryos and 26 vertebrae in adults. These vertebrae are classified into five regions:

• 7 Cervical vertebrae are present in the neck region. 
• 12 Thoracic vertebrae are present in the thoracic region. In these vertebrae, two additional attachment sites are present on each vertebra for ribs. 
• 5 Lumbar vertebrae are present in the upper abdominal region. They are the strongest and thickest. 
• Sacrum is formed by the fusion of 5 vertebrae and is present in the lower abdominal region. 
• Coccyx is formed by the fusion of 4 vertebrae and is present in the tail region. It is vestigial as it was the tailbone of ancestors. 

Vertebral formula in adults- C₇ T₁₂ L₅ S₍₅₎ Co₍₄₎

Vertebral formula in embryo- C₇ T₁₂ L₅ S₅ Co₄

Curves of the vertebral column

The vertebral column is S-shaped. Lumbar and Cervical curves develop after birth. These curves develop during the crawling stage of a baby. The lumbar curve is essential for bipedal locomotion. It was first evolved in Australopithecus.

Ribs and Sternum

The ribs and sternum along with the vertebral column form the rib cage, which protects the thoracic cavity. There are 12 pairs of ribs, attached to the thoracic vertebrae at the back and the sternum at the front. Ribs are attached to the sternum using hyaline cartilage. The sternum, or breastbone, is a dagger-shaped, flat bone located in the center of the chest. It is present on the ventral side and protects the trachea. 

12 pairs of ribs can be classified into 3 types:

• True Ribs- They are also known as center ribs as they are attached to the vertebral column dorsally and sternum ventrally. The first to Seventh pairs of ribs belong to this category. 
• False Ribs- They are also known as vertebrochondral ribs as they are attached to the vertebral column dorsally and the cartilage of the seventh pair ventrally. Eighth to Tenth pairs of ribs belong to this category. 
• Floating Ribs- They are also known as vertebral ribs as they are attached to the vertebral column dorsally and are free ventrally. Eleventh and Twelfth pairs of ribs belong to this category. 

Appendicular Skeleton

The appendicular skeleton facilitates movement and locomotion. It comprises the bones of the limbs and the girdles that attach them to the axial skeleton. It includes the pectoral (shoulder) girdle which anchors the upper limbs and the pelvic girdle which supports the lower limbs. The bones of the upper and lower limbs are also included in the appendicular skeleton enabling various movements.

Pectoral Girdle

The pectoral girdle is also called the shoulder girdle. It connects the forelimbs to the axial skeleton and provides mobility to the arms. It consists of four bones- two clavicles and two scapulae.

• Scapula, also known as the shoulder blade is a flat and triangular bone. It consists of the body, spine, acromion process, coracoid process and glenoid cavity. The acromion process is responsible for the formation of the shoulder with clavicle bone, the Coracoid process is responsible for the attachment of muscles and the Glenoid cavity is responsible for the articulation of the head of the humerus by the ball and socket joint
• Clavicle, also known as the collarbone is an F-shaped bone. It has two ends, namely, dorsal and ventral ends. The Dorsal end of the Clavicle is attached to the acromion process of the scapula while the ventral end is attached to the sternum.

Forelimbs

Bones of the Forelimbs allow intricate movements and dexterity.

The forelimbs are composed of –

• Humerus, also known as the funny bone, is present in the upper arm. In the humerus, the deltoid ridge is present. 
• Radius and Ulna are present in the forearm or lower arm. Radius is present on the little finger side while Ulna is present on the side of the thumb. The elbow is formed by the junction of the radius, ulna and humerus. 
• 8 Carpals are present in the wrist. They are very irregular in shape like pebbles and slide over each other. 
• 5 Metacarpals are present in the palm. 
• 14 Phalanges are present in fingers. Two are present in the thumb and three in each finger. The digit formula is 2,3,3,3,3.

Pelvic Girdle

The pelvic girdle, also known as the hip girdle, connects the lower limbs to the axial skeleton and provides support for the body’s weight. It consists of two coxal bones. These two bones are joined by a cartilaginous joint, known as Pubic Symphysis. It opens at the time of delivery resulting in a broader hip region of females after childbirth. Coxal bone is formed by the fusion of three bones during the embryonic stage. These bones are Ilium(largest), Ischium, and Pubis(smallest). In the region of Ischium, Obturator Foramen is present which is formed by the ischium and pubis. It is a hole from where blood vessels, nerves, ducts etc. go from the upper body to the lower body. Acetabulum is formed by all three bones and is responsible for the attachment of the femur. 

Lower Limbs

Bones of lower limbs enable walking, running, and various other movements

The lower limbs consist of-

• Femur is the thickest, strongest, and longest bone of the human body. It is present in the thigh. The head of the femur articulates with the acetabulum of the hip girdle. 
• Tibia and Fibula are bones present in the leg. Tibia, also known as the shin bone, is the second longest and strongest bone of the body. 
• 7 Tarsals are present in the ankle. 
• 5 Metatarsals are present in the foot. 
• 14 Phalanges are present in the toe having a digit formula similar to that of the fingers of the Forelimbs.

Structure of Bone and Cartilage

Features	Bone	Cartilage
Composition	Organic matrix (collagen fibers) + inorganic matrix (minerals)	collagen-rich matrix
Hardness	Hard and rigid	Firm but flexible
Flexibility	Non-flexible	Elastic
Blood vessels	Vascular (contains blood vessels)	Avascular (lacks blood vessels)
Nerves	Nervous (contains nerves)	Aneural (lacks nerves)
Mineral deposition	Abundant mineral deposition (calcium, magnesium)	Rare mineral deposition
Cellular components	Osteoblasts, osteocytes, osteoclasts	Chondroblasts, chondrocytes, chondroclasts

Disorders of the Skeletal System

Several disorders are associated with the skeletal system that can impact its structure, function, and overall health.

Osteoporosis

Osteoporosis is a condition that is characterised by low bone density, decreased bone mass due to pores in bones and weakened bone structure and increased risk of fractures. It occurs primarily in older individuals and postmenopausal women due to a lack of oestrogen because oestrogen has a very big role in the deposition of calcium in bones. It can also occur due to Parathormone-Calcitonin imbalance. Adequate calcium intake, regular exercise, and a healthy lifestyle can prevent osteoporosis. 

Arthritis

Arthritis refers to the inflammation of joints, which can cause pain, stiffness, and limited mobility. It can be treated by medication, physical therapy, and lifestyle modifications.

There are three types of arthritis 

• Osteoarthritis- It occurs due to inflammation in the articular cartilage of long bones.
• Rheumatoid arthritis- It is an auto-immune disorder and occurs due to inflammation of the synovial membrane leading to increased synovial fluid. Also, ‘pannus’, abnormal granules are present in the fluid which causes joint pain
• Gout- It occurs due to the accumulation of uric acid crystals in joints. 

FAQs on Skeletal System

Q: What is the composition of bones and cartilage in the skeletal system?

Answer:

Bones are composed of an organic matrix (collagen fibers) and an inorganic matrix (minerals), while cartilage is primarily made up of a collagen-rich matrix.

Q: What are the components of the axial skeleton?

Answer:

The axial skeleton is composed of the skull, vertebral column, and ribs. These structures provide protection to vital organs, such as the brain, spinal cord, and thoracic organs like the heart and lungs.

Q: How many pairs of ribs are there, and how are they classified?

Answer:

There are 12 pairs of ribs in total. They can be classified into three types: true ribs (1st to 7th pairs), false ribs (8th to 10th pairs), and floating ribs (11th and 12th pairs).

Q: What bones make up the pectoral girdle?

Answer:

The pectoral girdle, also known as the shoulder girdle, is composed of two clavicles (collarbones) and two scapulae (shoulder blades).

Q: What is osteoporosis?

Answer:

Osteoporosis is a condition characterized by low bone density, decreased bone mass, and weakened bone structure, leading to an increased risk of fractures.