Human Skeleton System

The bones that make up the hand include the Carpal (Wrist) bones, Metacarpal (Palm) bones, and the Phalanges (Fingers), which are categorized as Proximal, Middle, and Distal.

Sesamoid bones are small, round bones embedded within tendons, usually found near joints. They resemble sesame seeds in size and shape.

The epiphysis refers to the ends of a long bone, which are often expanded and involved in joint formation.

The metaphysis is the growth zone located between the diaphysis and epiphysis in growing bones.

The medullary cavity is the central hollow space in the diaphysis that contains bone marrow.

The cortex, or cortical bone, is the dense, compact outer layer of bone.

Cancellous bone, also known as spongy bone, is the inner porous bone found in the epiphyses and flat bones.

The periosteum is the outer fibrous membrane that covers the bone, except at joints.

The endosteum is a thin membrane that lines the medullary cavity of bones.

The components of the lower leg include the Femur (Thigh), Patella (Kneecap), Tibia, and Fibula.

The diaphysis is the shaft or long, central part of a long bone.

Both compact and cancellous (soft) bone types are found in most of the bones.

The human skeletal system is the internal framework of bones, cartilage, ligaments, and joints that provides structure, support, movement, and protection to the body.

The key components of the Human Skeletal System include:

1. Bones - 206 in adults; provide structural support
2. Cartilage - flexible tissue at joints; reduces friction
3. Ligaments - tough bands that connect bone to bone
4. Joints - points where two or more bones meet and allow movement

The skeletal system has several key functions:

1. Protects internal organs - The skeleton provides a protective framework for vital organs.
2. Stores and releases fat - Bones serve as a reservoir for fat storage.
3. Produces blood cells - Bone marrow is responsible for the production of red and white blood cells.
4. Stores and releases minerals - Bones store essential minerals like calcium and phosphorus, which can be released into the bloodstream as needed.
5. Facilitates movement - The skeletal system works with muscles to enable movement.
6. Supports the body - The skeleton provides structure and support to the body.

The main bones of the human skull from the anterior view include the Frontal Bone, Nasal Bone, Temporal Bone, Zygomatic Bone, Maxilla, and Mandible.

The bones that form the pelvic girdle include the Hipbone, which consists of the Pubis and Ischium.

The bones included in the forearm are the Ulna and Radius.

The main components of the human spine from the posterior view include the Cervical Vertebrae (1st and 2nd), Thoracic Vertebrae (1st and 12th), Lumbar Vertebrae (1st and 5th), Sacrum, and Coccyx.

The two main divisions of the skeletal system are the Axial Skeleton and the Appendicular Skeleton.

The human skull consists of 22 bones categorized into:

• Cranial bones: 8
• Facial bones: 14
• Auditory ossicles: 6 (3 in each ear)

The total number of bones in the human skeletal system is 206.

The components of the axial skeleton include:

• Cranium: 8 bones
• Face: 14 bones
• Hyoid: 1 bone
• Auditory ossicles: 6 bones
• Vertebral column: 26 bones
• Sternum: 1 bone
• Ribs: 24 bones

The appendicular skeleton includes:

• Pectoral girdles: 4 bones (2 clavicles, 2 scapulae)
• Upper extremities: 60 bones (2 humeri, 2 ulnae, 2 radii, 16 carpals, 10 metacarpals, 28 phalanges)
• Pelvic girdle: 2 bones (hip bones)
• Lower extremities: 60 bones (2 femurs, 2 fibulae, 2 tibiae, 2 patellae, 14 tarsals, 10 metatarsals, 28 phalanges)

The two main divisions of the human skeleton are the Axial Skeleton and the Appendicular Skeleton.

The Axial Skeleton includes the following bones:

1. Skull (Cranium & face)
2. Hyoid bone
3. Auditory ossicles
4. Vertebral column
5. Thorax (ribs, sternum)

The Appendicular Skeleton includes the following bones:

1. Limbs
2. Shoulder girdle
3. Pelvic girdle

The cranial bones protect the brain and provide structure to the skull.

The skull is composed of 22 bones, including cranial bones, facial bones, ear ossicles, and the hyoid bone.

The occipital bone is located at the back of the skull and contains the foramen magnum, the largest hole in the human body, through which the spinal cord passes. It also has two condyles that make the human skull dicondylic.

The sphenoid bone is located at the base of the skull, forming the floor of the cranium. It protects the lower part of the brain and houses the pituitary gland in the sella turcica.

The parietal bones, of which there are two, form the roof of the cranial cavity and protect the brain.

The ethmoid bone is located between the eye sockets and forms part of the nasal cavity, contributing to the structure of the face and the nasal passages.

The maxilla is the upper jawbone that contains the upper teeth.

The mandible is the only movable bone in the skull and contains the lower teeth, playing an important role in chewing and speaking.

The zygomatic bones are commonly known as cheekbones and are located on the sides of the face, forming the cheeks' prominence during smiling and part of the eye sockets.

The nasal bones are small, rectangular bones that form the bridge of the nose.

The lacrimal bones are the smallest facial bones, located in the eye orbits, and house the lacrimal or tear glands.

The vomer bone is a thin, flat, triangular bone located in the midline of the nasal cavity and is part of the nasal septum, which divides the nasal cavity into two halves.

The palatine bones form the roof of the buccal cavity by creating the posterior part of the hard palate.

The inferior nasal conchae are bony ridges in the nasal cavity that are richly supplied with blood vessels, adding moisture to the air and warming it.

The paired facial bones include:

1. Nasal Bone - located at the top of the nose.
2. Lacrimal Bone - small bone located on the inside of the eye socket near the nose.
3. Zygomatic Bone - forms the cheekbone.
4. Maxilla - forms the upper jaw.
5. Inferior Nasal Conchae - located inside the nasal cavity.

The unpaired facial bones include:

1. Vomer - located at the center of the nasal cavity.
2. Mandible - forms the lower jaw.

The three pairs of ear ossicles are Malleus (Hammer), Incus (Anvil), and Stapes (Stirrup). They help in intensifying or amplifying sound waves.

The Stapes is the smallest bone in the human body.

The Stapedius muscle is the smallest muscle in the human body, located in the ear and associated with the stapes bone.

The hyoid bone is a U-shaped bone that does not form any joints.

The primary function of the hyoid bone is to provide a surface for the attachment of tongue muscles.

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

In adults, the vertebral column consists of 26 vertebrae classified into five regions:

1. Cervical: 7 vertebrae in the neck region.
2. Thoracic: 12 vertebrae in the thoracic region, with two attachment sites for ribs on each.
3. Lumbar: 5 vertebrae in the upper abdominal region, the strongest and thickest.
4. Sacrum: Formed by the fusion of 5 vertebrae in the lower abdominal region.
5. Coccyx: Formed by the fusion of 4 vertebrae in the tail region, vestigial as it was the tailbone of ancestors.

The vertebral formula in adults is: C7 T12 L5 S(5) CO(4), indicating the number of cervical, thoracic, lumbar, sacral, and coccygeal vertebrae.

The three main sections of the human spinal column are:

1. Cervical vertebrae (C1-C7) - located at the top.
2. Thoracic vertebrae (T1-T12) - located in the middle.
3. Lumbar vertebrae (L1-L5) - located at the bottom.

Additionally, the sacrum (S1-S5) and coccyx are at the very bottom of the spine.

The first two cervical vertebrae are:

1. Atlas (C1) - supports the skull.
2. Axis (C2) - allows for the rotation of the head.

The rib cage protects the thoracic cavity and vital organs such as the trachea. It is formed by the ribs, sternum, and vertebral column.

The sternum, or breastbone, is a dagger-shaped, flat bone located in the center of the chest on the ventral side.

The three parts of the sternum from top to bottom are:

1. Jugular notch
2. Manubrium
3. Body of the sternum
4. Xiphoid process

True Ribs are the top seven pairs of ribs that connect directly to the sternum via costal cartilage.
False Ribs are the next three pairs that connect to the sternum indirectly through the costal cartilage of the ribs above them.

Floating Ribs are the bottom two pairs of ribs that are not connected to the sternum at all. They are characterized by their lack of attachment to the sternum, providing more flexibility in the rib cage.

The primary function of the appendicular skeleton is to facilitate movement and locomotion.

The appendicular skeleton includes the bones of the limbs and the girdles that attach them to the axial skeleton, specifically the pectoral girdle (shoulder girdle) and the pelvic girdle (supports the lower limbs).

The pectoral girdle anchors the upper limbs to the axial skeleton, allowing for a range of movements.

The appendicular skeleton enables various movements through the bones of the upper and lower limbs and their attachment to the girdles.

The pectoral girdle connects the forelimbs to the axial skeleton and provides mobility to the arms.

The pectoral girdle consists of four bones: two clavicles and two scapulae.

The scapula, or shoulder blade, is a flat and triangular bone that includes the body, spine, acromion process, coracoid process, and glenoid cavity.

The acromion process is responsible for the formation of the shoulder joint with the clavicle bone.

The coracoid process is responsible for the attachment of muscles to the scapula.

The glenoid cavity is responsible for the articulation of the head of the humerus by the ball and socket joint.

The clavicle, or collarbone, is an F-shaped bone that connects the dorsal end to the acromion process of the scapula and the ventral end to the sternum.

The main bones of the pectoral girdle include the clavicle, scapula, and humerus.

The glenoid cavity serves as the socket for the humerus, allowing for a wide range of motion in the shoulder joint.

Key features of the scapula include the acromion, coracoid process, and the glenoid cavity.

The pelvic girdle connects the lower limbs to the axial skeleton and provides support for the body's weight.

The coxal bone is formed by the fusion of three bones during the embryonic stage: Ilium (largest), Ischium, and Pubis (smallest).

The Pubic Symphysis is a cartilaginous joint that joins the two coxal bones, allowing for slight movement and flexibility, especially during childbirth.

The pelvic girdle opens at the time of delivery, resulting in a broader hip region of females after childbirth.

The Obturator Foramen, present in the region of Ischium, is a hole through which blood vessels, nerves, and ducts pass from the upper body to the lower body.

The Acetabulum is formed by the fusion of the three bones of the coxal bone and is responsible for the attachment of the femur.

The main components of the pelvic girdle include:

1. Ilium - the largest part of the pelvic bone.
2. Iliac crest - the top border of the ilium.
3. Iliac spine - bony projections on the ilium.
4. Acetabulum - the socket for the hip joint.
5. Ischium - the lower part of the pelvic bone.
6. Pubis - the front part of the pelvic girdle.

Pneumatic bones are hollow bones that contain air-filled spaces (sinuses) lined by mucous membrane. They are:

• Light in weight but strong
• Improve respiratory efficiency (especially in birds)
• Assist in voice resonance in humans
• Lined by epithelium continuous with nasal mucosa

Examples include the frontal bone, maxilla, ethmoid bone, sphenoid bone, and temporal bone (mastoid part).

The lower limbs consist of the following bones:

1. Femur: The thickest, strongest, and longest bone in the human body, located in the thigh. It articulates with the acetabulum of the hip girdle.

2. Tibia: Also known as the shin bone, it is the second longest and strongest bone in the body, located in the leg.

3. Fibula: A bone present in the leg alongside the tibia.

4. Tarsals: 7 bones present in the ankle.

5. Metatarsals: 5 bones present in the foot.

6. Phalanges: 14 bones present in the toes, with a digit formula similar to that of the fingers of the forelimbs.

The main bones of the right lower limb include:

1. Os Coxa (hip bone)
2. Femur (thigh bone)
3. Patella (knee cap)
4. Tibia (shin bone)
5. Fibula (calf bone)
6. Foot bones (including tarsals, metatarsals, and phalanges)

The components of the foot include:

• Phalanges (toes)
• Metatarsals (mid-foot bones)
• Tarsals (ankle bones), which consist of:
  • Cuneiforms
  • Cuboid
  • Navicular
  • Talus
  • Calcaneum (heel bone)

The different types of bones are:

1. Long Bones: These are longer than they are wide and are primarily found in the limbs (e.g., femur, humerus).
2. Short Bones: These are approximately as wide as they are long and provide stability and support (e.g., carpals, tarsals).
3. Irregular Bones: These have complex shapes that do not fit into the other categories (e.g., vertebrae, facial bones).
4. Flat Bones: These are thin and flat, providing protection and a surface for muscle attachment (e.g., skull, ribs).
5. Sesamoid Bones: These are small, round bones that form within tendons (e.g., patella).

Long bones develop via endochondral ossification, where the hyaline cartilage plate is gradually replaced by bone.

A long bone consists of:

1. Diaphysis - the shaft of the bone, made of thick compact bone.
2. Epiphyses - the ends of the bone, primarily composed of spongy bone and covered by a thin layer of compact bone.
3. Metaphysis - the region at the neck of the bone, located between the diaphysis and epiphysis, where growth occurs during development.
4. Marrow cavity - enclosed by the diaphysis, containing bone marrow.

Examples of long bones include:

• Humerus
• Fibula
• Tibia
• Metacarpal bones
• Metatarsal bones
• Phalanges
• Radius and ulna

Short bones are usually as long as they are wide and are typically found in the carpus of the hand and tarsus of the foot.

Short bones have a thin external layer of compact bone that covers vast spongy bone and marrow, giving them a cuboid shape.

The main function of short bones is to provide stability and some degree of movement.

Examples of short bones include:

• Carpals - wrist bones (8 per wrist)
• Tarsals - ankle bones (7 per ankle)

Flat bones consist of two layers of compact bone that cover both spongy bone and bone marrow space. They grow by replacing connective tissue and are covered by fibrocartilage on their articular surfaces.

Examples of flat bones include:

1. Skull bones
2. Ribs
3. Sternum
4. Scapulae

The primary function of flat bones is to protect internal organs such as the brain, heart, and pelvic organs. Additionally, their flat shape provides large areas for muscle attachments.

Irregular bones have complex shapes that do not fit into the categories of long, short, flat, or sesamoid bones. Their varied shapes reflect specialized functions.

The primary functions of irregular bones include:

1. Protecting nervous tissue (e.g., vertebrae protect the spinal cord)
2. Some have large air spaces lined by epithelium, known as pneumatic bones
3. Muscle attachment points
4. Contributing to the support and structural integrity of the body

Examples of irregular bones include:

• Vertebrae (cervical, thoracic, lumbar)
• Sacrum and Coccyx
• Pelvic bones
• Facial bones with sinuses such as:
  • Ethmoid
  • Sphenoid
  • Maxilla

Sesamoid bones serve several functions:

1. Protect tendons from wear and tear
2. Increase mechanical efficiency of muscles
3. Alter the direction of muscle pull to improve leverage

Yes, the number of sesamoid bones can vary between individuals. Some people may have extra sesamoid bones, especially in the hands or feet.

The largest and most consistent sesamoid bone is the patella, also known as the kneecap.

In the hands, small sesamoid bones are found in the tendons of the thumb (flexor pollicis brevis). In the feet, they are located under the big toe near the first metatarsal head.

Sesamoid bones are small, round bones that are embedded within tendons. They are commonly found in areas where a tendon passes over a joint, such as the patella (kneecap) in the knee and the sesamoid bones in the foot beneath the big toe. Their primary function is to protect tendons from stress and wear, and to improve the mechanical advantage of the muscles that act on the joint.

Sutural Bones are small bones found within the sutures of the skull. They are highlighted in the back of the skull image, indicating their location within the cranial sutures.

An example of an Irregular Bone is the Vertebra, which is shown in the image as a single vertebra bone.

Short Bones are roughly cube-shaped bones. An example is the Carpal bones, which are displayed in a cluster in the image.

Flat Bones are thin and often curved bones. An example is the Parietal bone, shown in two views: one of the outer surface and another as a cross-section showing the External table, Internal table, and Diploë (spongy bone).

A Long Bone is characterized by its length being greater than its width. An example is the Humerus, which is displayed in the image.

Sesamoid Bones are small, round bones that are embedded within tendons. An example is the Patella (kneecap), which is illustrated in the image.

Bone elevations can be classified as follows:

1. Linear Elevation: May be a line, lip, ridge, or crest.
2. Sharp Elevation: Includes spine, styloid process, cornu (horn), or hamulus.
3. Rounded or Irregular Elevation: Includes tubercle, tuberosity, epicondyle, malleolus, or trochanter. A ramus is a broad arm or process projecting from the main part of the bone.

Depressions in bone features may include:

• Pit
• Impression
• Fovea
• Fossa
• Groove (sulcus)
• Notch (incisura)

Openings in bone features can be classified as:

• Foramen
• Canal
• Hiatus
• Aqueduct

A large cavity within a bone is called a sinus, cell, or antrum.

Smooth articular areas in bone features may include:

• Facet
• Condyle
• Head
• Capitulum
• Trochlea

The main components of the structure of bones include:

1. Bone Matrix: Composed of organic and inorganic materials, providing strength and flexibility.
2. Osteocytes: Mature bone cells that maintain bone tissue.
3. Osteoblasts: Cells responsible for bone formation.
4. Osteoclasts: Cells that break down bone tissue.
5. Periosteum: A dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints.
6. Bone Marrow: The soft tissue inside bones where blood cells are produced.

The diaphysis is the long central part of a long bone, made of compact bone (cortex) that surrounds the medullary cavity. The medullary cavity contains bone marrow, which is yellow in adults and red in children.

The epiphyses are the expanded ends of a long bone, made of spongy bone that contains red marrow. They are covered by articular cartilage to facilitate joint movement.

The metaphysis is the region between the diaphysis and epiphysis. It contains the epiphyseal plate (growth plate) in children, which is responsible for bone growth, and it becomes the epiphyseal line in adults after growth has ceased.

The periosteum is the outer covering of bone (except at joints). It has two layers:

1. Outer fibrous layer - provides protection and serves as an attachment for muscles and tendons.
2. Inner osteogenic layer - contains bone-forming and bone-destroying cells, and is rich in blood vessels and nerves.

The medullary cavity is a hollow central cavity inside the diaphysis of long bones. It is filled with:

• Yellow bone marrow (fat) in adults.
• Red marrow (responsible for blood cell formation) in children.

The cortex, or compact bone, is the hard, dense outer layer of bone. Its primary function is to provide strength and support to the bone structure.

The endosteum is a thin inner membrane that lines the medullary cavity. It contains osteoblasts (bone-forming cells) and osteoclasts (bone-destroying cells).

The main structural differences include:

The main structural components of a long bone include:

1. Spongy Bone - Located at the ends, characterized by a porous texture.
2. Medullary Cavity - The hollow center of the bone, which contains marrow.
3. Thick Compact Bone - The dense outer layer that provides strength and support.
4. Marrow - Fills the medullary cavity, involved in blood cell production.

The main components of a long bone's structure include:

1. **Proximal Articular End (Head)**n2. Neckn3. **Shaft (Diaphysis)**n4. **Distal Articular End (Head)**n5. **Spongy Bone (Epiphysis)**n6. **Compact Bone (Diaphysis)**n7. Metaphysisn8. Periosteum

The periosteum serves several functions:

• Protection of the bone
• Pain sensation
• Muscle attachment site
• Some cells can differentiate into osteoblasts for bone formation

Spongy bone is also known as cancellous bone and is made of trabeculae, which are small, rod-like structures that create a porous network within the bone.

The main components of a long bone's internal structure include:

1. Epiphyses - Contains spongy bone and is located at the ends of the bone.
2. Articular cartilage - Covers the exterior of the epiphyses.
3. Compact bone - Forms the shaft of the bone.
4. Periosteum - A membrane that covers the outer surface of the bone.
5. Yellow marrow - Found in the middle of the shaft.
6. Medullary cavity - Located at the bottom of the shaft near the head of the bone.
7. Artery - A small branch from the shaft that supplies blood.
8. Osteons - The structural units of compact bone, visible in cross-section.
9. Epiphyseal fusion - Indicates the growth plate area.

Compact Bone (Cortical Bone) is the dense, solid outer layer of bone that forms the shaft of long bones and the outer layer of all bones. It is composed of osteons (Haversian systems) and provides strength and rigidity. Key characteristics include:

• Rich in blood vessels and nerves within Haversian canals
• Supports weight and resists bending and torsion
• Comprises 80% of the human skeleton
• Forms a shell around cancellous bone, primarily in long bones of the arm and leg where greater strength is needed.

The main components of an osteon include:

1. Lamellae - concentric rings of bone matrix.
2. Central (Haversian) canal - contains blood vessels and nerves.
3. Perforating (Volkmann's) canals - channels that connect Haversian canals.
4. Compact bone - dense outer layer of bone.
5. Spongy bone - porous inner structure of bone.
6. Periosteum - outer membrane covering the bone.
7. Blood vessels - supply nutrients and oxygen to the bone.
8. Marrow - found in the central cavity, involved in blood cell production.

Mature compact bone is lamellar, or layered, in structure. It is organized into concentric layers around interconnecting vascular canals known as haversian systems, which supply blood to the osteocytes. These concentric layers form structural units called osteons.

Immature compact bone does not contain osteons and has a woven structure. It forms around a framework of collagen fibers and is eventually replaced by mature bone through a remodeling process.

The remodeling process involves bone resorption and new bone formation, which leads to the creation of osteons. This process is significant as it transforms immature compact bone into mature compact bone, ensuring proper structure and function.

Spongy bone has a light, porous, honeycomb-like inner structure that encloses large spaces. It is mainly found at the epiphyses of long bones and in flat/irregular bones. The bone matrix is organized into a three-dimensional latticework of trabeculae, which are struts of bone arranged along lines of stress. The spaces within spongy bone are filled with red bone marrow, which is essential for hematopoiesis. The functions of spongy bone include:

1. Lightening bone weight
2. Supporting marrow
3. Absorbing shock

Additionally, osteoporosis primarily affects cancellous bone due to its large surface area and high metabolic activity, making it more susceptible to bone loss.

The main regions of a long bone include:

1. Epiphysis - The end part of the bone, which is usually wider than the shaft.
2. Metaphysis - The region between the epiphysis and diaphysis, where growth occurs in children.
3. Diaphysis - The long shaft of the bone.
4. Cancellous (spongy) bone - The inner layer of bone that is less dense and contains marrow.
5. Compact bone - The dense outer layer of bone that provides strength.
6. Medullary canal - The central cavity of the bone that contains bone marrow.
7. Periosteum - The outer membrane that covers the bone, providing nourishment and attachment for tendons and ligaments.

The key features of compact bone include:

1. Osteons - The basic structural unit of compact bone, consisting of concentric layers of bone matrix.
2. Haversian canals - Channels that run through the osteons, containing blood vessels and nerves.
3. Volkmann canals - Perpendicular channels that connect Haversian canals, allowing for communication between them.
4. Cancellous (spongy) bone - Found within the compact bone, providing structural support and housing marrow.
5. Marrow - The soft tissue found in the medullary canal and within cancellous bone, responsible for blood cell production.
6. Periosteum - The outer layer that covers the compact bone, aiding in protection and growth.

• Immature bone
• Collagen fibers arranged irregularly (haphazard)
• Found in embryonic development and during fracture healing
• Less organized, mechanically weaker
• Normal in early development and fracture repair; persistence in adults indicates a pathological condition
• Associated with diseases like Paget's disease, fibrous dysplasia, osteogenesis imperfecta, and bone tumors (e.g., osteosarcoma).

• Mature bone
• Collagen fibers arranged in regular parallel or concentric layers (lamellae)
• Strong and organized
• Replaces woven bone during growth and remodeling

Subtypes:

1. Compact (cortical) bone - dense, forms outer layer.
2. Spongy (cancellous) bone - porous, found in epiphyses and flat bones.

• Rare, primitive type of bone
• Dominated by collagen fibers
• Found in some lower vertebrates and in pathological cases.

• Dentine: forms bulk of tooth beneath enamel.
• Cementum: covers tooth root, anchors tooth to periodontal ligament.

The two types of bone tissue are Woven and Lamellar bone.

The main components of a human tooth include:

1. Enamel - Covers the crown
2. Dentin - Underneath the enamel, extends into the root
3. Gingiva - Covers the neck of the tooth
4. Pulp chamber - Contains blood vessels and nerves
5. Cementum - Covers the root
6. Jaw bone - The tooth is embedded in this structure

Fibrous bone tissue appears pink with elongated and irregularly arranged cells and fibers, along with small, irregularly shaped white spaces known as lacunae.