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14.2: Introduction to the Skeletal System

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• Suzanne Wakim & Mandeep Grewal
• Butte College

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Skull and Cross-Bones

The skull and cross-bones symbol has been used for a very long time to represent death, perhaps because after death and decomposition, bones are all that remain. Many people think of bones as being dead, dry, and brittle. These adjectives may correctly describe the bones of a preserved skeleton, but the bones of a living human being are very much alive. Living bones are also strong and flexible. Bones are the major organs of the skeletal system.

The skeletal system is the organ system that provides an internal framework for the human body. Why do you need a skeletal system? Try to imagine what you would look like without it. You would be a soft, wobbly pile of skin containing muscles and internal organs but no bones. You might look something like a very large slug. Not that you would be able to see yourself — folds of skin would droop down over your eyes and block your vision because of your lack of skull bones. You could push the skin out of the way if you could only move your arms, but you need bones for that as well!

Components of the Skeletal System

In adults, the skeletal system includes 206 bones, many of which are shown in Figure \(\PageIndex{2}\). Bones are organs made of dense connective tissues, mainly the tough protein collagen. Bones contain blood vessels, nerves, and other tissues. Bones are hard and rigid due to deposits of calcium and other mineral salts within their living tissues. Locations, where two or more bones meet, are called joints. Many joints allow bones to move like levers. For example, your elbow is a joint that allows you to bend and straighten your arm.

Besides bones, the skeletal system includes cartilage and ligaments.

• Cartilage is a type of dense connective tissue, made of tough protein fibers. It is strong but flexible and very smooth. It covers the ends of bones at joints, providing a smooth surface for bones to move over.
• Ligaments are bands of fibrous connective tissue that hold bones together. They keep the bones of the skeleton in place.

Axial and Appendicular Skeletons

The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton, both of which are pictured in Figure \(\PageIndex{3}\).

• The axial skeleton forms the axis of the body. It includes the skull, vertebral column (spine), and rib cage. The bones of the axial skeleton, along with ligaments and muscles, allow the human body to maintain its upright posture. The axial skeleton also transmits weight from the head, trunk, and upper extremities down the back to the lower extremities. In addition, the bones protect the brain and organs in the chest.
• The appendicular skeleton forms the appendages and their attachments to the axial skeleton. It includes the bones of the arms and legs, hands and feet, and shoulder and pelvic girdles. The bones of the appendicular skeleton make possible locomotion and other movements of the appendages. They also protect the major organs of digestion, excretion, and reproduction.

Functions of the Skeletal System

The skeletal system has many different functions that are necessary for human survival. Some of the functions, such as supporting the body, are relatively obvious. Other functions are less obvious but no less important. For example, three tiny bones (hammer, anvil, and stirrup) inside the middle ear transfer sound waves into the inner ear.

Support, Shape, and Protection

The skeleton supports the body and gives it shape. Without the rigid bones of the skeletal system, the human body would be just a bag of soft tissues, as described above. The bones of the skeleton are very hard and provide protection to the delicate tissues of internal organs. For example, the skull encloses and protects the soft tissues of the brain, and the vertebral column protects the nervous tissues of the spinal cord. The vertebral column, ribs, and sternum (breast bone) protect the heart, lungs, and major blood vessels. Providing protection to these latter internal organs requires the bones to be able to expand and contract. The ribs and the cartilage that connects them to the sternum and vertebrae are capable of small shifts that allow breathing and other internal organ movements.

The bones of the skeleton provide attachment surfaces for skeletal muscles. When the muscles contract, they pull on and move the bones. The figure below, for example, shows the muscles attached to the bones at the knee. They help stabilize the joint and allow the leg to bend at the knee. The bones at joints act like levers moving at a fulcrum point, and the muscles attached to the bones apply the force needed for movement.

Hematopoiesis

Hematopoiesis is the process in which blood cells are produced. This process occurs in a tissue called red marrow, which is found inside some bones, including the pelvis, ribs, and vertebrae. Red marrow synthesizes red blood cells, white blood cells, and platelets. Billions of these blood cells are produced inside the bones every day.

Mineral Storage and Homeostasis

Another function of the skeletal system is storing minerals, especially calcium and phosphorus. This storage function is related to the role of bones in maintaining mineral homeostasis. Just the right levels of calcium and other minerals are needed in the blood for the normal functioning of the body. When mineral levels in the blood are too high, bones absorb some of the minerals and store them as mineral salts, which is why bones are so hard. When blood levels of minerals are too low, bones release some of the minerals back into the blood. Bone minerals are alkaline (basic), so their release into the blood buffers the blood against excessive acidity (low pH), whereas their absorption back into bones buffers the blood against excessive alkalinity (high pH). In this way, bones help maintain acid-base homeostasis in the blood.

Another way bones help to maintain homeostasis is by acting as an endocrine organ. One endocrine hormone secreted by bone cells is osteocalcin, which helps regulate blood glucose and fat deposition. It increases insulin secretion and also the sensitivity of cells to insulin. In addition, it boosts the number of insulin-producing cells and reduces fat stores.

• What is the skeletal system? How many bones are there in the adult skeleton?
• Describe the composition of bones.
• Besides bones, what other organs are included in the skeletal system?
• Identify the two major divisions of the skeleton.
• List several functions of the skeletal system.
• Discuss sexual dimorphism in the human skeleton.
• Bones, cartilage, and ligaments are all made of types of ____________ tissue.
• True or False. Bones contain living tissue and can affect processes in other parts of the body.
• True or False. Bone cells contract to pull on muscles in order to initiate a movement.
• If a person has a problem with blood cell production, what type of bone tissue is most likely involved? Explain your answer.
• Are the pelvic girdles part of the axial or appendicular skeleton?
• What are three forms of homeostasis that the skeletal system regulates? Briefly explain how each one is regulated by the skeletal system.
• What do you think would happen to us if we did not have ligaments? Explain your answer.

b. How is cartilage related to joints?

c. Identify one joint in the human body and describe its function.

Explore More

Attributions.

• Fighter squadron 84 by US Navy , public domain via Wikimedia Commons
• Human skeleton front by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
• Axial skeleton by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
• Appendicular skeleton by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
• Knee anatomy by Blausen.com staff (2014). " Medical gallery of Blausen Medical 2014 ". WikiJournal of Medicine 1 (2). DOI : 10.15347/wjm/2014.010 . ISSN 2002-4436 . CC BY 3.0 via Wikimedia Commons
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

6.1 The Functions of the Skeletal System

Learning objectives.

By the end of this section, you will be able to:

• Define bone, cartilage, and the skeletal system
• List and describe the functions of the skeletal system

Bone , or osseous tissue , is a hard, dense connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints), cartilage , a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the body system composed of bones and cartilage and performs the following critical functions for the human body:

• supports the body
• facilitates movement
• protects internal organs
• produces blood cells
• stores and releases minerals and fat

Support, Movement, and Protection

The most apparent functions of the skeletal system are the gross functions—those visible by observation. Simply by looking at a person, you can see how the bones support, facilitate movement, and protect the human body.

Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin.

Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of view, bones act as levers and joints serve as fulcrums ( Figure 6.2 ). Unless a muscle spans a joint and contracts, a bone is not going to move. For information on the interaction of the skeletal and muscular systems, that is, the musculoskeletal system, seek additional content.

Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain ( Figure 6.3 ).

Career Connection

Orthopedist.

An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery ( Figure 6.4 ).

While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.

Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.

Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.

Mineral Storage, Energy Storage, and Hematopoiesis

On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses.

Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most bone is referred to as bone marrow ( Figure 6.5 ). There are two types of bone marrow: yellow marrow and red marrow. Yellow marrow contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy. Red marrow is where hematopoiesis —the production of blood cells—takes place. Red blood cells, white blood cells, and platelets are all produced in the red marrow.

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• Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
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• Biology Article

Skeletal System

Human skeletal system.

The skeletal system functions as the basic framework of a body and the entire body are built around the hard framework of Skeleton. It is the combination of all the bones and tissues associated with cartilages and joints. Almost all the rigid or solid parts of the body are the main components of the skeletal system. Joints play an important role in the skeletal system as it helps in permitting the different types of movements at different locations. If the skeleton were without joints, then there would be no sign of the movements in the human body.

Skeletal System Anatomy

This skeletal system can be divided into the axial and appendicular systems. In an adult body, it is mainly composed of 206 individual bones which are organized into two main divisions:

Axial skeleton

• Appendicular skeleton.

The axial skeleton runs along the body’s central axis, therefore it is called the central core of the human body. The axial skeleton is composed of 80 bones and it consists of:

• Skull Bone – It includes 8 cranial bones, 14 facial bones, 6 auditory ossicles, and the Hyoid Bone
• The bone of the Thoracic Cage – It includes 25 bones of the thorax- a breastbone and 24 ribs.
• The bone of the Vertebral column- It includes 24 vertebrae bones, the sacrum bone, and the coccyx bone.

Also check: Function of Parietal Bones

Appendicular skeleton

The appendicular skeleton is composed of 126 bones and it comprises of the-

• Pelvic girdle
• Upper Limbs
• Lower Limbs
• Shoulder Girdle or the Pectoral

Read more: Parts and Names of Human Skeleton

Skeletal System Physiology

The primary functions of the skeletal system include movement, support, protection production of blood cells , storage of minerals and endocrine regulation.

The primary function of the skeletal system is to provide a solid framework to support and safeguard the human body and its organs. This helps in maintaining the overall shape of the human body.

Also check: Function of Short Bones

The skeletal system also helps to protect our internal organs and other delicate body organs, including the brain, heart, lungs and spinal cord by acting as a buffer. Our cranium (skull) protects our brain and eyes, the ribs protect our heart and lungs and our vertebrae (spine, backbones) protect our spinal cord.

Bones provide the basic structure for muscles to attach themselves onto so that our bodies are able to move. Tendons are tough inelastic bands that attach our muscle to that particular bone.

Also read: Femur Structure and Function

The bone matrix of the skeletal system is mainly involved in storing or preserving different types of essential minerals which are required to facilitate growth and repair of the body cells and tissues. The cell-matrix acts as our calcium bank by storing and releasing calcium ions into the blood cell when required.

Regulation of Endocrine glands

The bone cells present within the skeletal system plays an important role in releasing the synthesized hormones from the respective endocrine glands for the further requirement by the body for different metabolisms . Apart from these functions, the skeletal system also contributes to the regulation of blood sugar.

To learn more about the structure and functions of the skeletal system, visit BYJU’S.

Also check:

• How Many Bones Does A Human Have?
• How many bones does a female body have?
• How many bones are present in the human face?
• What is the meaning of appendicular skeleton?
• What is the function of the cranium?

Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

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High school biology

Course: high school biology   >   unit 8.

• Skeletal structure and function
• Ligaments, tendons, and joints
• Three types of muscle
• Anatomy of a skeletal muscle cell
• LeBron Asks: What muscles do we use when shooting a basket?

The musculoskeletal system review

The musculoskeletal system, the human skeleton, the skeleton.

• the skull, which protects the brain and supports facial structure
• the vertebral column (spine), which surrounds and protects the spinal cord and support the head
• the thoracic (rib cage), which surrounds and protects the organs within the chest (including heart and lungs)
• the pectoral girdle (shoulders):
• upper and lower limbs (arms and legs)
• the pelvic girdle (hip bones)

Joints, cartilage, ligaments, and tendons

Muscle contraction, want to join the conversation.

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Human Anatomy Worksheets and Study Guides

This is a collection of free human anatomy worksheets. The completed worksheets make great study guides for learning bones, muscles, organ systems, etc. The worksheets come in a variety of formats for downloading and printing. In most cases, the PDF worksheets print the best. But, you may prefer to work online with Google Slides or print the PNG images.

Do you need a particular worksheet, but don’t see it? Ideas for worksheet topics you want covered are welcome!

Human Anatomy Worksheets

These worksheets cover major organs and organ systems.

Label the Heart

Label the parts of the human heart.

[ Google Apps worksheet ][ worksheet PDF ][ worksheet PNG ][ answers PNG ]

Label the Eye

Label the parts of the eye.

[ Google Apps worksheet ][ worksheet PDF ][ answers PDF ][ worksheet PNG ]

Types of Blood Cells

Identify the types of blood cells.

[ worksheet Google Apps ][ worksheet PDF ][ worksheet PNG ][ answers PNG ]

Label the Muscles

Label the major anterior muscles.

[ worksheet PDF ][ worksheet PNG ][ answers PNG ]

Label the Ear

Label the human ear.

[ Google Apps worksheet ][ Worksheet PDF ][ Worksheet PNG ][ Answers PNG ]

Label the Lungs

Identify the parts of the lungs.

Label the Kidney

Label the parts of the kidney.

Label the Liver

Identify the anatomy of the liver.

Label the Large Intestine

Label the parts of the large intestine.

Label the Stomach

Label the human stomach.

[ Google Apps worksheet ] [Worksheet PDF ][ Worksheet PNG ][ Answers PNG ]

External Nose Anatomy

Identify the parts of the nose.

[ Worksheet PDF ][ Worksheet Google Apps ][ Worksheet PNG ][ Answers PNG ]

Parts of the Nose

Here’s another way of identifying nose anatomy.

Label Bones of the Skeleton

Identify major bones of the skeleton.

[ Google Apps worksheet ][ worksheet PDF ][ answers PDF ][ worksheet PNG ][ answers PNG ]

Label the Lymph Node

Label the lymph node.

Label the Human Skull

[ worksheet PDF ][ worksheet Google Apps ][ worksheet PNG ][ answers PNG ]

Label the Skull (Advanced)

Label the Parts of the Brain

Identify parts of a human brain.

Label the Lobes of the Brain

Identify the different lobes of the brain.

Brain Anatomical Sections

Explore anatomical sections using a human brain as a reference.

Arteries of the Brain

Identify major brain arteries.

Label the Pancreas

Label the parts of the human pancreas.

Label the Spleen

Label spleen anatomy.

Label the Digestive System

Identify parts of the human digestive system.

Label the Respiratory System

Label the respiratory system.

Parts of a Neuron

Identify parts of a neuron.

Label the Lips

Label human lips.

Label the Skin

Label layers and structures in skin.

Label the Circulatory System

Label the circulatory system.

The Urinary Tract

[ Worksheet PDF ][ Worksheet Google Apps ][ Worksheet PNG ][ Answer Key PNG ]

The Bladder

The Female Reproductive System

Label Human Teeth

Identify Organs #1

Identify Organ Systems #1

Identify Organs #2

Identify Organ Systems #2

• Diagram of the Human Eye [ JPG ]

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6.1 The Functions of the Skeletal System

Learning objectives.

By the end of this section, you will be able to:

List and describe the functions of the skeletal system

• Attribute specific functions of the skeletal system to specific components or structures

The skeletal system is the body system composed of bones, cartilages, ligaments and other tissues that perform essential functions for the human body. Bone tissue, or osseous tissue , is a hard, dense connective tissue that forms most of the adult skeleton, the internal support structure of the body. In the areas of the skeleton where whole bones move against each other (for example, joints like the shoulder or between the bones of the spine), cartilages, a semi-rigid form of connective tissue, provide flexibility and smooth surfaces for movement. Additionally, ligaments composed of dense connective tissue surround these joints, tying skeletal elements together (a ligament is the dense connective tissue that connect bones to other bones). Together, they perform the following functions:

Support, Movement, and Protection

Some functions of the skeletal system are more readily observable than others. When you move you can feel how your bones support you, facilitate your movement, and protect the soft organs of your body. Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilages of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin. Bones facilitate movement by serving as points of attachment for your muscles. Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (see Figure 6.1.1 ).

Mineral and Fat Storage, Blood Cell Formation

On a metabolic level, bone tissue performs several critical functions. For one, the bone tissue acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and are involved in the transmission of nerve impulses.

Bones also serve as a site for fat storage and blood cell production. The unique connective tissue that fills the interior of most bones is referred to as bone marrow . There are two types of bone marrow: yellow bone marrow and red bone marrow. Yellow bone marrow contains adipose tissue, and the triglycerides stored in the adipocytes of this tissue can be released to serve as a source of energy for other tissues of the body. Red bone marrow is where the production of blood cells (named hematopoiesis, hemato- = “blood”, -poiesis = “to make”) takes place. Red blood cells, white blood cells, and platelets are all produced in the red bone marrow. As we age, the distribution of red and yellow bone marrow changes as seen in the figure ( Figure 6.1.2 ).

Career Connection – Orthopedist

An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery ( Figure 6.1.3 ).

While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.

Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.

Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.

Section Review

The major functions of the skeletal system are body support, facilitation of movement, protection of internal organs, storage of minerals and fat, and blood cell formation.

Review Questions

Critical thinking questions.

• Suppose your red bone marrow could not be formed. What functions would your body not be able to perform?
• Suppose your osseous tissue could not store calcium. What functions would your body not be able to perform?

Answers for Critical Thinking Questions

• Without red bone marrow, you would not be able to produce blood cells. The red bone marrow is responsible for forming red and white blood cells as well as platelets. Red blood cells transport oxygen to tissues, and remove carbon dioxide. Without red blood cells, your tissues would not be able to produce ATP using oxygen. White blood cells play a role in the immune system fighting off foreign invaders in our body – without white blood cells you would not be able to recover from infection. Platelets are responsible for clotting your blood when a vessel ruptures. Without platelets you would bleed to death and die.
•  The calcium in osseous tissue provides mineral support to bones. Without this calcium, the bones are not rigid and cannot be supportive. The calcium in osseous tissue is also an important storage site, that can release calcium when needed. Other organ systems rely on this calcium for action (specifically, muscle contraction and neural signaling). Without calcium storage, blood calcium levels change dramatically and affect muscle contraction and neural signaling.

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Skeletal System Anatomy and Physiology

Dive into the intricate framework of the human body with our skeletal system study guide —perfect for nursing students eager to understand the anatomy and physiology behind every bone and joint.

Table of Contents

Functions of the skeletal system, classification of bones, gross anatomy, microscopic anatomy, axial skeleton, facial bones, the hyoid bone, fetal skull, cervical vertebrae, thoracic vertebrae, lumbar vertebrae, appendicular skeleton, bones of the shoulder girdle, bones of the pelvic girdle, fibrous joints, cartilaginous joints, types of synovial joints based on shape.

Besides contributing to body shape and form, our bones perform several important body functions.

• Support. Bones, the “steel girders” and “reinforced concrete” of the body, form the internal framework that supports the body and cradle its soft organs; the bones of the legs act as pillars to support the body trunk when we stand, and the rib cage supports the thoracic wall.
• Protection. Bones protect soft body organs; for example, the fused bones of the skull provide a snug enclosure for the brain, the vertebrae surround the spinal cord , and the rib cage helps protect the vital organs of the thorax.
• Movement. Skeletal muscles, attached to bones by tendons, use the bones as levers to move the body and its parts.
• Storage. Fat is stored in the internal cavities of bones; bone itself serves as a storehouse for minerals, the most important of which are calcium and phosphorus; because most of the body’s calcium is deposited in the bones as calcium salts , the bones are a convenient place to get more calcium ions for the blood as they are used up.
• Blood cell formation. Blood cell formation, or hematopoiesis, occurs within the marrow cavities of certain bones.

Anatomy of the Skeletal System

The skeleton is subdivided into two divisions: the axial skeleton, the bones that form the longitudinal axis of the body, and the appendicular skeleton, the bones of the limbs and girdles.

The adult skeleton is composed of 206 bones and there are two basic types of osseous, or bone, tissue: compact bone and spongy bone, and are classified into four groups according to shape: long, short, flat, and irregular.

• Compact bone. Compact bone is dense and looks smooth and homogeneous.
• Spongy bone. Spongy bone is composed of long, needle-like pieces of bone and lots of open space.
• Long bones. Long bones are typically longer than they are wide; as a rule, they have a shaft with heads at both ends, and are mostly compact bone.
• Short bones. Short bones are generally cube-shaped and mostly contains spongy bone; sesamoid bones , which form within tendons, are a special type of short bone.
• Flat bones. Flat bones are thin, flattened, and usually curved; they have two thin layers of compact bone sandwiching a layer of spongy bone between them.
• Irregular bones. Bones that do not fit one of the preceding categories are called irregular bones.

The structure of a long bone is shown both through gross anatomy and microscopic anatomy.

The gross structure of a long bone consists of the following:

• Diaphysis. The diaphysis, or shaft, makes up most of the bone’s length and is composed of compact bone; it is covered and protected by a fibrous connective tissue membrane, the periosteum .
• Sharpey’s fibers. Hundreds of connective tissue fibers called perforating or Sharpey’s , fibers secure the periosteum to the underlying bone.
• Epiphyses. The epiphyses are the ends of the long bone; each epiphysis consists of a thin layer of compact bone enclosing an area filled with spongy bone.
• Articular cartilage. Articular cartilage, instead of a periosteum, covers its external surface; because the articular cartilage is glassy hyaline cartilage, it provides a smooth, slippery surface that decreases friction at joint surfaces.
• Epiphyseal line. In adult bones, there is a thin line of bony tissue spanning the epiphysis that looks a bit different from the rest of the bone in the area; this is the epiphyseal line.
• Epiphyseal plate. The epiphyseal line is a remnant of the epiphyseal plate (a flat plate of hyaline cartilage) seen in young, growing bone; epiphyseal plates can cause the lengthwise growth of a long bone; by the end of puberty, when hormones inhibit long bone growth, epiphyseal plates have been completely replaced by bones, leaving only the epiphyseal lines to mark their previous location.
• Yellow marrow. In adults, the cavity of the shaft is primarily a storage area for adipose (fat) tissue called the yellow marrow, or medullary , cavity.
• Red marrow. However, in infants, this area forms blood cells and red marrow is found there; in adult bones, red marrow is confined to cavities in the spongy bone of flat bones and epiphyses of some long bones.
• Bone markings. Even when looking casually at bones, one can see that their surfaces are not smooth but scarred with bumps, holes, and ridges; these bone markings reveal where muscles, tendons, and ligaments were attached and where blood vessels and nerves passed.
• Categories of bone markings. There are two categories of bone markings: (a) projections , or processes , which grow out from the bone surface, and (b) depressions , or cavities which are indentations in the bone; a little trick for remembering some of the bone markings are all the terms beginning with T are projections, while those beginning with F (except facet) are depressions.

To the naked eye , spongy bone has a spiky, open appearance, whereas compact bone appears to be very dense.

• Osteocytes. The mature bone cells, osteocytes, are found within the matrix in tiny cavities called lacunae .
• Lamellae. The lacunae are arranged in concentric circles called lamellae around central ( Haversian ) canals.
• Osteon. Each complex consisting of central canals and matrix rings is called an osteon, or Haversian system .
• Canaliculi. Tiny canals, canaliculi, radiate outward from the central canals to all lacunae; the canaliculi form a transportation system that connects all the bone cells to the nutrient supply through the hard bone matrix.
• Perforating canals. The communication pathway from the outside of the bone to its interior (and the central canals) is completed by perforating ( Volkmann’s ) canals, which run into the compact bone at right angles to the shaft.

The axial skeleton, which forms the longitudinal axis of the body, is divided into three parts: the skull, the vertebral column, and the bony thorax.

The skull is formed by two sets of bones: the cranium and the facial bones.

The cranium encloses and protects the fragile brain tissue and is composed of eight large flat bones.

• Frontal bone. The frontal bone forms the forehead, the bony projections under the eyebrows, and the superior part of each eye’s orbits.
• Parietal bones. The paired parietal bones form most of the superior and lateral walls of the cranium; they meet in the midline of the skull at the sagittal suture and form the coronal suture , where they meet the frontal bone.
• Temporal bones. The temporal bones lie inferior to the parietal bones; they join them at the squamous sutures.

There are several bone markings that appear at the temporal bone :

• External acoustic meatus. The external acoustic meatus is a canal that leads to the eardrum and middle ear; it is the route by which sound enters the ear.
• Styloid process. The styloid process, a sharp, needlelike projection , is just inferior to the external auditory meatus.
• Zygomatic process. The zygomatic process is a thin bridge of bone that joins with the cheekbone ( zygomatic bone ) anteriorly.
• Mastoid process. The mastoid process, which full of air cavities ( mastoid sinuses ), is a rough projection posterior and inferior to the external acoustic meatus ; it provides an attachment site for some muscles of the neck.
• Jugular foramen. The jugular foramen, at the junction of the occipital and temporal bones, allows passage of the jugular vein , the largest vein of the head, which drains the brain; just anterior to it in the cranial cavity is the internal acoustic meatus , which transmits cranial nerves VII and VIII.
• Occipital bone . The occipital bone joins the parietal bones anteriorly at the lambdoid suture; in the base of the occipital bone is a large opening, the foramen magnum , which surrounds the lower part of the brain allows the spinal cord to connect with the brain.
• Sphenoid bone. The butterfly-shaped sphenoid bone spans the width of the skull and forms part of the floor of the cranial cavity; in the midline of the sphenoid is a small depression , the sella turcica or Turk’s saddle , which forms a snug enclosure for the pituitary gland.
• Foramen ovale. The foramen ovale, a large oval opening in line with the posterior end of the sella turcica, allows fibers of cranial nerve V to pass to the chewing muscles of the lower jaw.
• Optic canal. The optic canal allows the optic nerve to pass to the eye.
• Superior orbital fissure. The slitlike superior orbital fissure is where the cranial nerves controlling eye movements pass.
• Sphenoid sinuses. The central part of the sphenoid bone is riddled with air cavities, the sphenoid sinuses.
• Ethmoid bone. The ethmoid bone is very irregularly shaped and lies anterior to the sphenoid; it forms the roof of the nasal cavity and part of the medial walls of the orbits.
• Crista galli. Projecting from its superior surface is the crista galli; the outermost covering of the brain attaches to this projection.
• Cribriform plates . These holey areas, the cribriform plates, allow nerve fibers carrying impulses from the olfactory receptors of the nose to reach the brain.
• Superior and middle nasal conchae . Extensions of the ethmoid bone, the superior and middle nasal conchae, form part of the lateral walls of the nasal cavity and increase the turbulence of air flowing through the nasal passages.

Fourteen bones compose the face; twelve are paired, only the mandible and vomer are single.

• Maxillae. The two maxillae, or maxillary bones , fuse to form the upper jaw; all facial bones except the mandible join the maxillae; thus, they are the main or “keystone”, bones of the face; the maxillae carry the upper teeth in the alveolar margin .
• Palatine bones. The paired palatine bones lie posterior to the palatine processes of the maxillae; they form the posterior part of the hard palate.
• Zygomatic bones. The zygomatic bones are commonly referred to as the cheek bones; they also form a good-sized portion of the lateral walls of the orbits, or eye sockets.
• Lacrimal bones. The lacrimal bones are finger-sized bones forming part of the medial walls of each orbit; each lacrimal bones has a groove that serves as a passageway for tears.
• Nasal bones. The small rectangular bones forming the bridge of the nose are the nasal bones.
• Vomer bone. The single bone in the medial line of the nasal cavity is the vomer; the vomer forms most of the bony nasal septum.
• Inferior nasal conchae. The interior nasal conchae are thin, curved bones projecting medially from the lateral walls of the nasal cavity.
• Mandible. The mandible, or lower jaw, is the largest and strongest bone of the face; it joins the temporal bones on each side of the face, forming the only freely movable joints in the skull; the horizontal part of the mandible ( the body ) forms the chin; two upright bars of bone ( the rami ) extend from the body to connect the mandible to the temporal bone.

Though not really part of the skull, the hyoid bone is closely related to the mandible and temporal bones.

• Location. It is suspended in the midneck region about 2 cm (1 inch) above the larynx,  where it is anchored by ligaments to the styloid processes of the temporal bones.
• Parts. Horseshoe-shaped, with a body and two pairs of horns, or cornua, the hyoid bone serves as a movable base for the tongue and as an attachment point for neck muscles that raise and lower the larynx when we swallow and speak.

The skull of a fetus or newborn infant is different in many ways from an adult skull.

• Size. The adult skull represents only one-eighth of the total body length, whereas that of a newborn infant is one-fourth as long as its entire body.
• Fontanels. In the newborn, the skull also has a fibrous regions that have yet to be converted to bone; these fibrous membranes connecting the cranial bones are called fontanels.
• Anterior fontanel. The largest fontanel is the diamond-shaped anterior fontanel; the fontanel allows the fetal skull to be compressed slightly during birth.

Vertebral Column (Spine)

Serving as the axial support of the body, the vertebral column, or spine, extends from the skull, which it supports, to the pelvis, where it transmits the weight of the body to the lower limbs.

• Composition. The spine is formed from 26 irregular bones connected and reinforced by ligaments in such a way that a flexible, curved structure results.
• Spinal cord. Running through the central cavity of the vertebral column is the delicate spinal cord, which the vertebral column surrounds and protects.
• Vertebrae. Before birth, the spine consists of 33 separate bones called vertebrae , but 9 of these eventually fuse to form the two composite bones, the sacrum and the coccyx , that construct the inferior portion of the vertebral column.
• Cervical vertebrae. Of the 24 single bones, the 7 vertebrae of the neck are cervical vertebrae.
• Thoracic vertebrae. The next 12 are the thoracic vertebrae.
• Lumbar vertebrae. The remaining 5 supporting the lower back are lumbar vertebrae.
• Intervertebral discs. The individual vertebrae are separated by pads of flexible fibrocartilage-intervertebral discs- that cushion the vertebrae and absorb shock while allowing the spine flexibility.
• Primary curvatures. The spinal curves in the thoracic and sacral regions are referred to as primary curvatures because they are present when we are born.
• Secondary curvatures. The curvatures in the cervical and lumbar regions are referred to as secondary curvatures because they develop some time after birth.
• Body or centrum. Disc-like, weight-bearing part of the vertebra facing anteriorly in the vertebral column.
• Vertebral arch. Arch formed from the joining of all posterior extensions, the laminae and pedicles , from the vertebral body.
• Vertebral foramen. Canal through which the spinal cord passes.
• Transverse processes. Two lateral projections from the vertebral arch.
• Spinous process. Single projection arising from the posterior aspect of the vertebral arch (actually the fused laminate).
• Superior and inferior articular processes. Paired projections lateral to the vertebral foramen, allowing a vertebra to form joints with adjacent vertebrae.

The seven cervical vertebrae (C1 to C7) form the neck region of the spine.

• Atlas. The atlas ( C1 ) has no body; the superior surfaces of its transverse processes contain large depressions that receive the occipital condyles of the skull.
• Axis. The axis ( C2 )  acts as a pivot for the rotation of the atlas (and skull) above; it has a large upright process, the dens , which acts as the pivot point.
• Foramina. The transverse processes of the cervical vertebrae contain foramina ( openings ) through which the vertebral arteries pass on their way to the brain above.

The twelve thoracic vertebrae (T1 to T12) are all typical.

• Size. They are larger than the cervical vertebrae and are distinguished by the fact that they are the only vertebrae to articulate with the ribs.
• Shape . The body is somewhat heart-shaped and has two costal facets on each side, which receive the heads of the ribs.
• Transverse processes. The two transverse processes of each thoracic vertebrae articulate with the nearby knoblike tubercles of the ribs.
• Spinous process. The spinous process is long and hooks sharply downward, causing the vertebra to look like a giraffe’s head viewed from the side.

The five lumbar vertebrae (L1 to L5) have massive, blocklike bodies.

• Spinous processes. Their short, hatchet-shaped spinous processes make them look like a moose head from the lateral aspect.
• Strength. Because most of the stress on the vertebral column occurs in the lumbar region, these are the sturdiest of the vertebrae.

The sacrum is formed by the fusion of five vertebrae.

• Alae. The winglike alae articulate laterally with the hip bones, forming the sacroiliac joints.
• Median sacral crest. Its posterior midline surface is roughened by the median sacral crest, the fused spinous processes of the sacral vertebrae.
• Posterior sacral foramina. This is flanked laterally by the posterior sacral foramina.
• Sacral canal. The vertebral canal continues inside the sacrum as the sacral canal and terminates in a large inferior opening called the sacral hiatus .

The coccyx is formed from the fusion of three to five tiny, irregular shaped vertebrae.

• Tailbone. It is the human “tailbone”, a remnant of the tail that other vertebrate animals have.

Thoracic Cage

The sternum, ribs, and thoracic vertebrae make up the bony thorax; The bony thorax is routinely called the thoracic cage because it forms a protective, cone-shaped cage of slender bones around the organs of the thoracic cavity.

The sternum (breastbone) is a typical flat bone and the result of the fusion of three bones- the manubrium , body , and xiphoid process .

• Landmarks. The sternum has three important bony landmarks- the jugular notch , the sternal angle , and the xiphisternal joint .
• Jugular notch. The jugular notch (concave upper border of the manubrium) can be palpated easily, generally it is at the level of the third thoracic vertebra.
• Sternal angle. The sternal angle results where the manubrium and the body meet at a slight angle to each other, so that a transverse ridge is formed at the level of the second ribs.
• Xiphisternal joint. The xiphisternal joint, the point where the sternal body and xiphoid process fuse, lies at the level of the ninth thoracic vertebra.

Twelve pairs of ribs form the walls of the bony thorax.

• True ribs. The true ribs, the first seven pairs, attach directly to the sternum by costal cartilages.
• False ribs. False ribs, the next five pairs, either attach indirectly to the sternum or are not attached to the sternum at all.
• Floating ribs. The last two pairs of false ribs lack the sternal attachments, so they are called the floating ribs.

The appendicular skeleton is composed of 126 bones of the limbs and the pectoral and pelvic girdles, which attach the limbs to the axial skeleton.

Each shoulder girdle, or pectoral girdle , consists of two bones – a clavicle and a scapula.

• Clavicle. The clavicle, or collarbone , is a slender, doubly curved bone; it attaches to the manubrium of the sternum medially and to the scapula laterally, where it helps to form the shoulder joint; it acts as a brace to hold the arm away from the top of the thorax and helps prevent shoulder dislocation.
• Scapulae. The scapulae, or shoulder blades , are triangular and commonly called “wings” because they flare when we move our arms posteriorly.
• Parts of the scapula. Each scapula has a flattened body and two important processes- the acromion and the coracoid .
• Acromion. The acromion is the enlarged end of the spine of the scapula and connects with the clavicle laterally at the acromioclavicular joint .
• Coracoid. The beaklike coracoid process points over the top of the shoulder and anchors some of the muscles of the arm; just medial to the coracoid process is the large suprascapular notch, which serves as a nerve passageway.
• Borders of the scapula. The scapula has three borders- superior, medial (vertebral), and lateral (axillary).
• Angles of the scapula. It also has three angles- superior, inferior, and lateral; the glenoid cavity , a shallow socket that receives the head of the arm bone, is in the lateral angle.
• Factors to free movement of the shoulder girdle. Each shoulder girdle attaches to the axial skeleton at only one point- the sternoclavicular joint ; the loose attachment of the scapula allows it to slide back and forth against the thorax as muscles act; and, the glenoid cavity is shallow, and the shoulder joint is poorly reinforced by ligaments.

Bones of the Upper Limb

Thirty separate bones form the skeletal framework of each upper limb; they form the foundations of the arm, forearm, and hand.

The arm is formed by a single bone, the humerus, which is a typical long bone.

• Anatomical neck. Immediately inferior to the head is a slight constriction called anatomical neck.
• Tubercles. Anterolateral to the head are two bony projections separated by the intertubercular sulcus – the greater and lesser tubercles , which are sites of muscle attachment.
• Surgical neck. Just distal to the tubercles is the surgical neck, so named because it is the most frequently fractured part of the humerus.
• Deltoid tuberosity. In the midpoint of the shaft is a roughened area called the deltoid tuberosity, where the large, fleshy deltoid muscle of the shoulder attaches.
• Radial groove. Nearby, the radial groove runs obliquely down the posterior aspect of the shaft; this groove marks the course of the radial nerve, an important nerve of the upper limb.
• Trochlea and capitulum. At the distal end of the humerus is the medial trochlea, which looks somewhat like a spool, and the lateral ball-like capitulum; both of these processes articulate with the bones of the forearm.
• Fossa. Above the trochlea anteriorly is a depression, the coronoid fossa ; on the posterior surface is the olecranon fossa ; these two depressions, which are flanked by medial and lateral epicondyles , allow the corresponding processes of the ulna to move freely when the elbow is bent and extended.

Two bones, the radius, and the ulna, form the skeleton of the forearm.

• Radius. When the body is in the anatomical position, the radius is the lateral bone; that is, it is on the thumb side of the forearm; when the hand is rotated so that the palm faces backward, the distal end of the radius crosses over and ends up medial to the ulna.
• Radioulnar Joints. Both proximally and distally the radius and ulna articulate at small radioulnar joints and the two bones are connected along their entire length by the flexible interosseous membrane .
• Styloid process. Both the ulna and the radius have as styloid process at their distal end.
• Radial tuberosity. The disc-shaped head of the radius also forms a joint with the capitulum of the humerus; just below the head is the radial tuberosity, where the tendon of the biceps muscle attaches.
• Ulna. When the upper limb is in the anatomical position, the ulna is the medial bone (on the little-finger side) of the forearm.
• Trochlear notch. On its proximal end are the coronoid process and the posterior olecranon process, which are separated by the trochlear notch; together, these two processes grip the trochlea of the humerus in a pliers-like joint.

The skeleton of the hand consists of carpals, the metacarpals, and the phalanges.

• Carpal bones. The eight carpal bones, arranged in two irregular rows of four bones each, form the part of the hand called carpus , or, more commonly, the wrist ; the carpals are bound together by ligaments that restrict movements between them.
• Metacarpals. The metacarpals are numbered 1 to 5 from the thumb side of the hand to the little finger; when the fist is clenched, the heads of the metacarpals become obvious as the “ knuckles “.
• Phalanges. The phalanges are the bones of the fingers; each hand contains 14 phalanges; there are three in each finger (proximal, middle, and distal), except in the thumb, which has only two )proximal and distal.

The pelvic girdle is formed by two coxal bones, or ossa coxae, commonly called hip bones.

• Pelvic girdle. The bones of the pelvic girdle are large and heavy, and they are attached securely to the axial skeleton; bearing weight is the most important function of this girdle because the total weight of the upper body rests on the bony pelvis.
• Sockets. The sockets, which receives the thigh bones, are deep and heavily reinforced by ligaments that attach the limbs firmly to the girdle.
• Bony pelvis. The reproductive organs, urinary bladder , and part of the large intestine lie within and are protected by the bony pelvis.
• Ilium. The ilium, which connects posteriorly with the sacrum at the sacroiliac joint , is a large, flaring bone that forms most of the hip bone; when you put your hands on your hips, they are resting over the alae , or winglike portions, of the ilia.
• Iliac crest. The upper edge of an ala, the iliac crest, is an important anatomical landmark that is always kept in mind by those who give intramuscular injections; the iliac crest ends anteriorly in the anterior superior iliac spine and posteriorly in the posterior superior iliac spine .
• Ischium. The ischium is the “sit-down” bone, so called because it forms the most inferior part of the coxal bone.
• Ischial tuberosity. The ischial tuberosity is a roughened area that receives weight when you are sitting.
• Ischial spine. The ischial spine, superior to the tuberosity, is another important anatomical landmark, particularly in pregnant women, because it narrows the outlet of the pelvis through which the baby must pass during the birth process.
• Greater sciatic notch. Another important structural feature of the ischium is the greater sciatic notch, which allows blood vessels and the large sciatic nerve to pass from the pelvis posteriorly into the thigh.
• Pubis. The pubis, or pubic bone , is the most anterior part of the coxal bone.
• Obturator foramen. An opening that allows blood vessels and nerves to pass into the anterior part of the thigh.
• Pubic symphysis. The pubic bones of each hip bones fuse anteriorly to form a cartilaginous joint, the pubic symphysis.
• Acetabulum. The ilium, ischium, and pubis fuse at a deep socket called the acetabulum, which means “vinegar cup”; the acetabulum receives the head of the thigh bone.
• False pelvis. The false pelvis is superior to the true pelvis; it is the area medial to the flaring portions of the ilia.
• True pelvis. The true pelvis is surrounded by bone and lies inferior to the flaring parts of the ilia and the pelvic brim; the dimensions of the true pelvis of the woman are very important because they must be large enough to allow the infant’s head to pass during childbirth.
• Outlet and inlet. The dimensions of the cavity, particularly the outlet (the inferior opening of the pelvis measured between the ischial spines, and the inlet (superior opening between the right and left sides of the pelvic brim) are critical, and thus they are carefully measured by the obstetrician.

Bones of the Lower Limbs

The lower limbs carry the total body weight when we are erect; hence, it is not surprising that the bones forming the three segments of the lower limbs (thigh, leg, and foot) are much thicker and stronger than the comparable bones of the upper limb

The femur , or thigh bone, is the only bone in the thigh; it is the heaviest, strongest bone in the body.

• Parts. Its proximal end has a ball-like head, a neck, and greater and lesser trochanters (separated anteriorly by the intertrochanteric line and posteriorly by the intertrochanteric crest ).
• Gluteal tuberosity. These markings and the gluteal tuberosity, located on the shaft, all serve as sites for muscle attachment.
• Head. The head of the femur articulates with the acetabulum of the hip bone in a deep, secure socket.
• Neck. However, the neck of the femur is a common fracture site, especially in old age.
• Lateral and medial condyles. Distally on the femur are the lateral and medial condyles, which articulate with the tibia below; posteriorly these condyles are separated by the deep intercondylar fossa .
• Patellar surface. Anteriorly on the distal femur is the smooth patellar surface, which forms a joint with the patella, or kneecap.

Connected along their length by an interosseous membrane , two bones, the tibia and fibula, form the skeleton of the leg.

• Tibia. The tibia, or shinbone , is larger and more medial; at the proximal end, the medial and lateral condyles articulate with the distal end of the femur to form the knee joint.
• Tibial tuberosity. The patellar (kneecap) ligament attaches to the tibial tuberosity, a roughened area on the anterior tibial surface.
• Medial malleolus. Distally, a process called medial malleolus forms the inner bulge of the ankle.
• Anterior border. The anterior surface of the tibia is a sharp ridge, the anterior border, that is unprotected by the muscles; thus, it is easily felt beneath the skin.
• Fibula. The fibula, which lies along the tibia and forms joints with it both proximally and distally, is thin and sticklike; the fibula has no part in forming the knee joint.
• Lateral malleolus. Its distal end, the lateral malleolus, forms the outer part of the ankle.

The foot, composed of the tarsals, metatarsals, and phalanges, has two important functions.; it supports our body weight and serves as a lever that allows us to propel our bodies forward when we walk and run.

• Tarsus. the tarsus, forming the posterior half of the foot, is composed of seven tarsal bones .
• Calcaneus and Talus. Body weight is carried mostly by the two largest tarsals, the calcaneus, or heel bone , and the talus (ankle), which lies between the tibia and the calcaneus.
• Metatarsals. Five metatarsals form the sole.
• Phalanges. 14 phalanges form the toes; each toe has three phalanges, except the great toe, which has two.
• Arches. The bones in the foot are arranged to form three strong arches: two longitudinal (medial and lateral) and one transverse.

Joints, also called articulations , have two functions: they hold the bones together securely, but also give the rigid skeleton mobility .

• Classification. Joints are classified in two ways- functionally and structurally.
• Functional classification. The functional classification focuses on the amount of movement the joint allows.
• Types of functional joints. There are synarthroses or immovable joints; amphiarthroses , or slightly movable joints, and diarthrosis , or freely movable joints.
• Diarthroses. Freely movable joints predominate in the limbs, where mobility is important.
• Synarthroses and amphiarthroses. Immovable and slightly movable joints are restricted mainly to the axial skeleton, where firm attachments and protection of internal organs are priorities.
• Structural classification. Structurally, there are fibrous , cartilaginous , and synovial joints; these classifications are based on whether fibrous tissue, cartilage, or a joint cavity separates the bony regions at the joint.

In fibrous joints, the bones are united by fibrous tissue.

• Examples. The best examples of this type of joint are the sutures of the skull; in sutures, the irregular edges of the bones interlock and are bound tightly together by connective tissue fibers, allowing essentially no movement.
• Syndesmoses. In syndesmoses, the connecting fibers are longer than those of sutures; thus the joint has more “give”; the joint connecting the distal ends of the tibia and fibula is a syndesmosis.

In cartilaginous joints, the bone ends are connected by cartilage.

• Examples. Examples of this joint type that are slightly movable are the pubic symphysis of the pelvis and the intervertebral joints of the spinal column, where the articulating bone surfaces are connected by pads (discs) of fibrocartilage.
• Synarthrotic cartilaginous joints. The hyaline cartilage epiphyseal plates of growing long bones and the cartilaginous joints between the first ribs and the sternum are immovable cartilaginous joints.

Synovial Joints

Synovial joints are joints in which the articulating bone ends are separated by a joint cavity containing a synovial fluid ; they account for all joints of the limbs.

• Articular cartilage. Articular cartilage covers the ends of the bones forming the joints.
• Fibrous articular capsule . The joint surfaces are enclosed by a sleeve or a capsule of fibrous connective tissue, and their capsule is lined with a smooth synovial membrane (the reason these joints are called synovial joints).
• Joint cavity. The articular capsule encloses a cavity, called the joint cavity, which contains lubricating synovial fluid.
• Reinforcing ligaments. The fibrous capsule is usually reinforced with ligaments.
• Bursae. Bursae are flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid; they are common where ligaments, muscles, skin, tendons, or bones rub together.
• Tendon sheath. A tendon sheath is essentially an elongated bursa that wraps completely around a tendon subjected to friction, like a bun around a hotdog.

The shapes of the articulating bone surfaces determine what movements are allowed at a joint; based on such shapes, our synovial joints can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket joints.

• Plane joint. In a plane joint, the articular surfaces are essentially flat, and only short slipping or gliding movements are allowed; the movements of plane joints are nonaxial , that is, gliding does not involve rotation around any axis; the intercarpal joints of the wrist are best examples of plane joints.
• Hinge joint. In a hinge joint, the cylindrical end of one bone fits into a trough-shaped surface on another bone; angular movement is allowed in just one plane, like a mechanical hinge; hinge joints are classified as uniaxial ; they allow movement in only one axis, and examples are the elbow joint, ankle joint, and the joints between the phalanges of the fingers.
• Pivot joint. In a pivot joint, the rounded end of one bone fits into a sleeve or ring of bone; because the rotating bone can turn only around its long axis, pivot joints are also uniaxial joints ; the proximal radioulnar joint and the joint between the atlas and the dens of the axis are examples.
• Condyloid joint. In a condyloid joint, the egg-shaped articular surface fits into an oval concavity in another; condyloid joints allow the moving bone to travel (1) from side to side and (2) back and forth but the bone cannot rotate around its long axis; movement occurs around two axes, hence these are biaxial joints .
• Saddle joints. In saddle joints, each articular surface has both convex and concave areas, like a saddle; these biaxial joints allow essentially the same movements as condyloid joints; the best examples of saddle joints are the carpometacarpal joints in the thumb.
• Ball-and-socket joint. In a ball-and-socket joint, the spherical head of one bone fits into a round socket in another; these multiaxial joints allow movement in all axes, including rotation, and are the most freely moving synovial joints; the shoulder and hip are examples.

Craving more insights? Dive into these related materials to enhance your study journey!

• Anatomy and Physiology Nursing Test Banks . This nursing test bank includes questions about Anatomy and Physiology and its related concepts such as: structure and functions of the human body, nursing care management of patients with conditions related to the different body systems.

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Skeletal system.

Did you know that human babies are born with more bones than an adult? Human babies have more than 270 bones. Human adults have 206 bones. What's the reason for the difference? Many baby bones fuse together as they develop and grow to become one single bone. Here's another fun fact: your skeleton comprises 30-40% of your body weight.

Learning about the skeleton can be tons of fun. Children are often amazed to learn about their bodies and how the skeleton functions. You can amaze your young students with fun facts and guessing games. For example, ask your class how many bones they have in their feet? The answer is 26 bones.

Classroom Resources of Teaching about the Human Skeletal System

While a classroom skeleton model would be fantastic, it's not always in the school budget. Clip art can be a fantastic resource and substitute for a full scale model. Worksheets and activities also help children embrace learning and remembering. Teacher Planet has all the resources, lesson plans and even worksheets you need to bring the skeletal system to your classroom.

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Resources on the skeletal system.

Skeletal System Unit Skeleton Label Using Google Slides Label and Color the Long Bone Color the Bone Matrix Label the Bones of the Skull Color the Bones of the Foot Color the Bones of the Hand