True Ribs

The true ribs are attached to the sternum (breastbone) directly by their costal cartilages. There are seven true ribs. (The other ribs are termed floating or false ribs.)

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Clavicle

The clavicles, or collarbones, are a pair of long bones that connect the scapula to the sternum. The name clavicle comes from the Latin word for “little key” and describes the shape of the clavicle as an old-fashioned skeleton key. The clavicle is one of the most commonly broken bones in the human body. It also serves as an important and easily located bony landmark due to its superficial location and projection from the trunk.

The clavicles are cylindrical bones around 6 inches (15 cm) long and curved in the transverse plane like a letter S.

 They are located in the thoracic region superior and anterior to the first rib. Each clavicle runs transversely and forms a joint with the sternum on its medial end and the scapula on its lateral end. The medial end of each clavicle is a smooth, rounded cylinder known as the sternal extremity, which forms the sternoclavicular joint with the manubrium of the sternum. Viewed from the anterior position, the clavicle forms a convex curve at its medial end before forming a smaller concave curve near its lateral end. The lateral end terminates in a flattened facet known as the acromial extremity, which forms the acromioclavicular (AC) joint with the acromion process of the scapula.
The clavicles, along with the scapulae, form the pectoral girdle that attaches the bones of the arm to the trunk. In fact, the sternoclavicular joints are the only bony attachments between the pectoral girdles and the bones of the axial skeleton. The clavicles function as struts to anchor the arms to the trunk while permitting the movement of the scapulae and shoulder joints relative to the trunk. The movement of the clavicles increases the mobility of the shoulder joints beyond what would be possible with only ball-and-socket joints, allowing the arm to move in a large circle. Several muscles of the neck and shoulder also attach to the clavicle, including the pectoralis major, sternocleidomastoid, trapezius, and deltoid.

The unique position of the clavicle in the body frequently makes it the site of fractures from several types of accidents. When the arm is extended to break a fall, much of the force from the fall is transmitted through the arm to the shoulder, which shifts suddenly and can fracture the clavicle. When a strong force is applied directly to the shoulder, such as during a car accident, tackle, or sudden fall, the shoulder bones can be pushed medially and result in a fractured clavicle.

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Special thanks- DR. NITIN BHOKARE

Sacrum

The sacrum is a large wedge shaped vertebra at the inferior end of the spine. It forms the solid base of the spinal column where it intersects with the hip bones to form the pelvis. The sacrum is a very strong bone that supports the weight of the upper body as it is spread across the pelvis and into the legs. Developmentally, the sacrum forms from five individual vertebrae that start to join during late adolescence and early adulthood to form a single bone by around the age of thirty.

A ridge of tubercles along the posterior surface of the sacrum represents the spinous processes of these fused bones.
At its wide superior end, the sacrum forms the fibrocartilaginous lumbosacral joint with the fifth lumbar vertebra above it. The sacrum tapers to a point at its inferior end, where it forms the fibrocartilaginous sacrococcygeal joint with the tiny coccyx (tail bone). On the left and right lateral sides the sacrum forms the sacroiliac joints with the ilium of the hip bones to form the rigid pelvis. Many ligaments bind the sacroiliac joints together tightly to reduce motion and solidify the pelvis. Along its anterior surface the sacrum is concave to provide a larger space within the pelvic cavity. The female sacrum is shorter, wider, and curved more posteriorly than the male sacrum to provide more room for the passage of the fetus through the birth canal during childbirth.
Many nerves of the cauda equina at the inferior end of the spinal cord pass through the sacrum. These nerves enter the sacrum from the vertebral foramen of the lumbar vertebrae through the tunnel-like sacral canal. From the sacral canal these nerves branch out and exit the sacrum through four pairs of holes on the sides of the canal called the sacral foramina or through the sacral hiatus at the inferior end of the canal.

The sacrum serves several important functions in the skeletal, muscular, nervous, and female reproductive systems. Acting as the keystone of the pelvis, the sacrum locks the hip bones together on the posterior side and supports the base of the spinal column as it intersects with the pelvis. Several key muscles of the hip joint, including the gluteus maximus, iliacus and piriformis, have their origins on the surface of the sacrum and pull on the sacrum to move the leg. The sacrum also surrounds and protects the spinal nerves of the lower back as they wind their way inferiorly toward the end of the trunk and into the legs. Finally, the sacrum helps to form the pelvic cavity that supports and protects the delicate organs of the abdominopelvic cavity and provides space for a fetus to pass through during childbirth.

Prepared by- ASIM KAPSHIKAR
Specail thanks- DR. NITIN BHOKARE

Lumbar Vertebrae

The lumbar vertebrae consist of five individual cylindrical bones that form the spine in the lower back. These vertebrae carry all of the upper body’s weight while providing flexibility and movement to the trunk region. They also protect the delicate spinal cord and nerves within their vertebral canal.
Found along the body’s midline in the lumbar (lower back) region, the lumbar vertebrae make up the region of the spine inferior to the thoracic vertebrae in the thorax and superior to the sacrum and coccyx in the pelvis.The lumbar vertebrae are stacked to form a continuous column in order from superior (L1 or first lumbar vertebra) to inferior (L5 or fifth lumbar vertebra). Together they create the concave lumbar curvature in the lower back.

Connecting each vertebra to its neighboring vertebra is an intervertebral disk made of tough fibrocartilage with a jelly-like center. The outer layer of the intervertebral disk, the annulus fibrosus, holds the vertebrae together and provides strength and flexibility to the back during movement. The jelly-like nucleus pulposus acts as a shock absorber to resist the strain and pressure exerted on the lower back.
The lumbar vertebrae are the some of the largest and heaviest vertebrae in the spine, second in size only to the sacrum. A cylinder of bone known as the vertebral body makes up the majority of the lumbar vertebrae’s mass and bears most of the body’s weight. Posteriorly the body is connected to a thin ring of bone known as the arch. The arch surrounds the hollow vertebral foramen and connects the body to the bony processes on the posterior of the vertebra. The vertebral foramen is a large, triangular opening in the center of the vertebra that provides space for the spinal cord, cauda equina, and meninges as they pass through the lower back.
Extending from the vertebral arch are several bony processes that are involved in muscle attachment and movement of the lower back. The spinous process extends from the posterior end of the arch as a thin rectangle of bone. It serves as a connection point for the muscles of the back and pelvis, such as the psoas major and interspinales. On the left and right lateral sides of each vertebra are the short, triangular transverse processes. The transverse processes form important connection points for many muscles, including the rotatores and multifidus muscles that extend and rotate the trunk.
Unlike the cervical vertebrae in the neck, the lumbar vertebrae lack the transverse foramina in the transverse processes, and also lack facets to either side of the body. The fifth lumbar vertebra is distinct from the L1-4 vertebrae in being much larger on its front side than in the back. Its spinous process, on the other hand, is smaller than in the other lumbar vertebrae with a wide, four-sided shape that comes to a rough edge and a thick notch.

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Thoracic Vertebrae

The thoracic vertebrae are a group of twelve small bones that form the vertebral spine in the upper trunk. Thoracic vertebrae are unique among the bones of the spine in that they are the only vertebrae that support ribs and have overlapping spinous processes. Like all other vertebrae, the thoracic vertebrae help to support the weight of the upper body and protect the delicate spinal cord as it runs through the vertebral canal.

The thoracic vertebrae are located in the thorax posterior and medial to the ribs. They form the region of the spinal column inferior to the cervical vertebrae of the neck and superior to the lumbar vertebrae of the lower back.Each thoracic vertebra is named for its position within the spine, from the first thoracic vertebra (T1) on the superior end to the twelfth thoracic vertebra (T12) on the inferior end. The spinal column formed by the thoracic vertebrae protrudes posteriorly to form the convex thoracic curve of the spine.
The thoracic vertebrae are medium in size – larger and thicker than the cervical vertebrae above them, but smaller and thinner than the lumbar vertebrae below. The T1 vertebra is the smallest and closely resembles the cervical vertebrae, while the T12 vertebra is the largest and most similar to the lumbar vertebrae. The intermediate vertebrae all follow the trend of increasing size from superior to inferior as a result of the greater body weight supported by the inferior vertebrae.
The bulk of the bony mass of the thoracic vertebrae is located within a cylindrical region known as the vertebral body or centrum. Each thoracic vertebra supports a pair of ribs and contains a pair of smooth, concave joint-forming processes known as facets on its sides. The ribs are anchored to the spine by the planar joints formed between the vertebrae and the ribs. The first nine thoracic vertebrae (T1 through T9) contain a pair of demi-facets, where a facet is split between two adjacent vertebral bodies. Meanwhile, the first, tenth, eleventh, and twelfth (T1, T10, T11 and T12) vertebrae all contain a pair of full facets on their vertebral bodies to support ribs. T1 is unique among all thoracic vertebrae in supporting two pairs of ribs through a pair of facets and a pair of demi-facets.
Between the vertebral bodies of the thoracic vertebrae are the tough, rubbery intervertebral disks. Each disk is made of an outer shell of fibrocartilage known as the annulus fibrosus, which holds the vertebrae in place while providing a small range of motion between them. Inside the annulus fibrosus is the gel-like nucleus pulposus that acts as a soft shock absorber to prevent collisions between the vertebrae.
Posterior to the vertebral bodies are thin bony rings known as the vertebral arches. Each vertebral arch surrounds and protects a hollow vertebral foramen that provides space for the spinal cord and spinal nerves. A pair of transverse processes extends from the lateral sides of each vertebral arch to support the ribs and provide attachment sites for the rotatores and multifidus muscles of the back. At the posterior end of the vertebral arch, each thoracic vertebra extends posteriorly and inferiorly to form the spinous processes. Each spinous process supports several muscles of the back to provide movement to the trunk and spine region. The spinous processes also overlap each other slightly to provide extra support and rigidity to the thoracic region and prevent extraneous movements.
Two pairs of articular processes extend superiorly and inferiorly toward the neighboring vertebrae, in order to help stabilize the spine and connect the thoracic vertebrae to one another and to the C7 and L1 vertebrae. Flat planar joints form between the articular processes of these neighboring vertebrae, allowing the bones to move independently while maintaining the strength and stability of the spinal column. The superior articular processes end in smooth surfaces facing posteriorly to meet the articular process of the vertebra above. On the inferior end, the inferior articular processes end in smooth, flat surfaces facing anteriorly to meet the next vertebra. The T1 and T12 vertebrae are the exceptions to this rule; T1 features a superior articular process resembling those of the cervical vertebrae, while the inferior articular process of T12 resembles those of the lumbar vertebrae.
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Cervical Vertebrae

The cervical vertebrae of the spine consist of seven bony rings that reside in the neck between the base of the skull and the thoracic vertebrae in the trunk. Among the vertebrae of the spinal column, the cervical vertebrae are the thinnest and most delicate bones. Yet, in spite of their size, the cervical vertebrae have the huge jobs of supporting the head, protecting the spinal cord, and providing mobility to the head and neck.

The cervical vertebrae are stacked along the length of the neck to form a continuous column between the skull and the chest.Each cervical vertebra is named by its position in order from superior (C1 or first cervical vertebra) to inferior (C7 or seventh cervical vertebra). The C1 vertebra, which holds up the skull, is named the atlas after the mythological titan Atlas who similarly held the Earth on his shoulders. Similar to the C1 vertebra, the C2 vertebra is named the axis as it provides the axis upon which the skull and atlas rotate when the head is moved side to side.
Each cervical vertebra consists of a thin ring of bone, or vertebral arch, surrounding the vertebral and transverse foramina. The vertebral foramen is a large opening in the center of the vertebra that provides space for the spinal cord and its meninges as they pass through the neck. Flanking the vertebral foramen on each side are the much smaller transverse foramina. The transverse foramina surround the vertebral arteries and veins, which, along with the carotid arteries and jugular veins, have the vital job of carrying blood to and from the brain.
Extending from the vertebral arch are several bony processes that are involved in muscle attachment and movement of the neck. The spinous process extends from the posterior end of the arch and serves as a connection point for the muscles that extend the neck, such as the trapezius and spinalis muscles. On the left and right lateral sides of each vertebra is a transverse process that forms the insertion point for the muscles of the erector spinae group that extend and flex the neck.
A thickened region of bone known as the body lies anterior to the vertebral foramen and forms the main bone mass in all vertebrae except for the atlas. The bodies strengthen the vertebrae and support most of the weight of the tissues of the head and neck. Intervertebral disks made of rubbery fibrocartilage lie between the vertebral bodies to provide slight flexibility to the neck. Lateral to the vertebral bodies are flattened facets that form joints with the neighboring vertebrae and skull, allowing movement among the vertebrae. The axis has a very distinct shape due to the presence of the odontoid process, a tooth-like prominence that extends from its body superiorly toward the axis. The odontoid process serves as the axis upon which the atlas rotates at the atlantoaxial joint.
Despite being some of the smallest and lightest bones in the axial skeleton, the cervical vertebrae perform many important functions that are critical to the survival of the body. Vital nerves and blood vessels passing through the neck are protected from mechanical damage by the bony arches of the cervical vertebrae. The cervical vertebrae also provide support to the head and neck, including supporting the muscles that move this region of the body. The muscles that attach to the vertebral processes provide posture to the head and neck throughout the day and have the greatest endurance of all of the body’s muscles. Finally, the many joints formed between the skull and cervical vertebrae provide incredible flexibility that allows the head and neck to rotate, flex, and extend.
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Special thanks- DR NITIN BHOKARE

SPINE

Stretching down the midline of the trunk from the base of the skull to the coccyx, the spine plays an extremely important role in our bodies as it supports the upper body’s weight; provides posture while allowing for movement and flexibility; and protects the spinal cord.

The spine, also known as the vertebral column or spinal column, is a column of 26 bones in an adult body – 24 separate vertebrae interspaced with cartilage, and then additionally the sacrum and coccyx.
       Prior to adolescence, the spine consists of 33 bones because the sacrum’s five bones and the coccyx’s four do not fuse together until adolescence.
The vertebrae are named by the first letter of their region (cervical, thoracic, or lumbar) and with a number to indicate their position along the superior-inferior axis. For example, the fifth lumbar vertebra (which is most inferior one, located beneath the fourth lumbar vertebra) is called the L5 vertebra.
Each vertebra has several important parts: the body, vertebral foramen, spinous process, and transverse process.

• The body is the main weight-bearing region of a vertebra, making up the bulk of the bone’s mass.
• Extending from the body, the transverse processes are thin columns of bone that point out to the left and right sides of the body.
• The spinous process extends from the ends of the transverse processes in the posterior direction.
• Between the body, transverse processes and spinous process is the vertebral foramen, a hollow space that contains the spinal cord and meninges.

Between the vertebrae of the spine are thin regions of cartilage known as the intervertebral discs. Intervertebral discs are made of an outer shell known as the annulus fibrosus and a soft, pulpy region known as the nucleus pulposus in the middle.

• The annulus fibrosus is made of tough fibrocartilage that binds the vertebrae together but is flexible enough to allow for our movements.
• The inner nucleus pulposus acts as a shock absorber to support the body’s weight and prevent the vertebrae from painfully crashing into each other while under strain.

The vertebrae of the spine align so that their vertebral canals form a hollow, bony tube to protect the spinal cord from external damage and infection. Between the vertebrae are small spaces known as intervertebral canals that allow spinal nerves to exit the spinal cord and connect to the various regions of the body.
There are 5 major regions of the spine:

1. Cervical: The 7 vertebrae in the neck form the cervical region of the spine. Cervical vertebrae are the thinnest and most delicate vertebrae in the spine but offer great flexibility to the neck. The first cervical vertebra, C1, supports the skull and is named “atlas” after the Greek titan who held the Earth on his shoulders. The skull pivots on the atlas when moving up and down. The second cervical vertebra, C2, is also known as the “axis” because it allows the skull and atlas to rotate to the left and right.
2. Thoracic: The 12 vertebrae in the chest region form the spine’s thoracic region. Thoracic vertebrae are larger and stronger than cervical vertebrae but are much less flexible. The spinous processes of the thoracic vertebrae point inferiorly to help lock the vertebrae together. A unique feature of the thoracic vertebrae is that each one forms joints with a pair of ribs to form the sturdy rib cage that protects the organs of the chest.
3. Lumbar: The 5 vertebrae in the lower back form the lumbar region of the spine. Lumbar vertebrae are even larger and stronger than thoracic vertebrae, but are more flexible due to the lack of ribs in the lumbar region. All of the upper body’s weight bears down on the lumbar vertebrae, leading to many back problems in this region despite the size and strength of the vertebrae.
4. Sacral: The sacral region of the spine contains only the sacrum, a single bone in the adult skeleton that is formed by the fusion of 5 smaller vertebrae during adolescence. The sacrum is a flat, triangular bone found in the lower back and wedged between the 2 hip bones.
5. Coccygeal: The spine’s coccygeal region contains only the coccyx, a single bone in the adult skeleton that is formed by the fusion of 4 tiny vertebrae during adolescence. The coccyx is often referred to as the human tailbone, as this region is homologous to the tail bones of animals that have tails. In humans, the coccyx bears our body weight when sitting down and provides attachment points for muscles of the pelvic and gluteal regions. While most people have a coccyx made of 4 fused vertebrae, the coccyx may consist of as few as 3 or as many as 5 vertebrae. The length of the coccyx has no effect on the body’s function.

Prepared by-ASIM KAPSHIKAR

Special thanks- DR. NITIN BHOKARE

HUMAN ANATOMY

Skeletal System Anatomy

The skeletal system in an adult body is made up of 206 individual bones. These bones are arranged into two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton runs along the body’s midline axis and is made up of 80 bones in the following regions:

• Skull
• Hyoid
• Auditory ossicles
• Ribs
• Sternum
• Vertebral column

The appendicular skeleton is made up of 126 bones in the folowing regions:

• Upper limbs
• Lower limbs
• Pelvic girdle
• Pectoral (shoulder) girdle

Skull
The skull is composed of 22 bones that are fused together except for the mandible. These 21 fused bones are separate in children to allow the skull and brain to grow, but fuse to give added strength and protection as an adult. The mandible remains as a movable jaw bone and forms the only movable joint in the skull with the temporal bone.
The bones of the superior portion of the skull are known as the cranium and protect the brain from damage. The bones of the inferior and anterior portion of the skull are known as facial bones and support the eyes, nose, and mouth.

Hyoid and Auditory Ossicles
The hyoid is a small, U-shaped bone found just inferior to the mandible. The hyoid is the only bone in the body that does not form a joint with any other bone—it is a floating bone. The hyoid’s function is to help hold the trachea open and to form a bony connection for the tongue muscles.
Vertebrae
Twenty-six vertebrae form the vertebral column of the human body. They are named by region:

• Cervical (neck) - 7 vertebrae
• Thoracic (chest) - 12 vertebrae
• Lumbar (lower back) - 5 vertebrae
• Sacrum - 1 vertebra
• Coccyx (tailbone) - 1 vertebra

With the exception of the singular sacrum and coccyx, each vertebra is named for the first letter of its region and its position along the superior-inferior axis. For example, the most superior thoracic vertebra is called T1 and the most inferior is called T12.
Ribs and Sternum
The sternum, or breastbone, is a thin, knife-shaped bone located along the midline of the anterior side of the thoracic region of the skeleton. The sternum connects to the ribs by thin bands of cartilage called the costal cartilage.
There are 12 pairs of ribs that together with the sternum form the ribcage of the thoracic region. The first seven ribs are known as “true ribs” because they connect the thoracic vertebrae directly to the sternum through their own band of costal cartilage. Ribs 8, 9, and 10 all connect to the sternum through cartilage that is connected to the cartilage of the seventh rib, so we consider these to be “false ribs.” Ribs 11 and 12 are also false ribs, but are also considered to be “floating ribs” because they do not have any cartilage attachment to the sternum at all.
Pectoral Girdle and Upper Limb
The pectoral girdle connects the upper limb (arm) bones to the axial skeleton and consists of the left and right clavicles and left and right scapulae.
The humerus is the bone of the upper arm. It forms the ball and socket joint of the shoulder with the scapula and forms the elbow joint with the lower arm bones. The radius and ulna are the two bones of the forearm. The ulna is on the medial side of the forearm and forms a hinge joint with the humerus at the elbow. The radius allows the forearm and hand to turn over at the wrist joint.
The lower arm bones form the wrist joint with the carpals, a group of eight small bones that give added flexibility to the wrist. The carpals are connected to the five metacarpals that form the bones of the hand and connect to each of the fingers. Each finger has three bones known as phalanges, except for the thumb, which only has two phalanges.
Pelvic Girdle and Lower Limb
Formed by the left and right hip bones, the pelvic girdle connects the lower limb (leg) bones to the axial skeleton.
The femur is the largest bone in the body and the only bone of the thigh (femoral) region. The femur forms the ball and socket hip joint with the hip bone and forms the knee joint with the tibia and patella. Commonly called the kneecap, the patella is special because it is one of the few bones that are not present at birth. The patella forms in early childhood to support the knee for walking and crawling.
The tibia and fibula are the bones of the lower leg. The tibia is much larger than the fibula and bears almost all of the body’s weight. The fibula is mainly a muscle attachment point and is used to help maintain balance. The tibia and fibula form the ankle joint with the talus, one of the seven tarsal bones in the foot.
The tarsals are a group of seven small bones that form the posterior end of the foot and heel. The tarsals form joints with the five long metatarsals of the foot. Then each of the metatarsals forms a joint with one of the set of phalanges in the toes. Each toe has three phalanges, except for the big toe, which only has two phalanges.
Microscopic Structure of Bones
The skeleton makes up about 30-40% of an adult’s body mass. The skeleton’s mass is made up of nonliving bone matrix and many tiny bone cells. Roughly half of the bone matrix’s mass is water, while the other half is collagen protein and solid crystals of calcium carbonate and calcium phosphate.

Living bone cells are found on the edges of bones and in small cavities inside of the bone matrix. Although these cells make up very little of the total bone mass, they have several very important roles in the functions of the skeletal system. The bone cells allow bones to:

• Grow and develop
• Be repaired following an injury or daily wear
• Be broken down to release their stored minerals

Types of Bones
All of the bones of the body can be broken down into five types: long, short, flat, irregular, and sesamoid.

• Long. Long bones are longer than they are wide and are the major bones of the limbs. Long bones grow more than the other classes of bone throughout childhood and so are responsible for the bulk of our height as adults. A hollow medullary cavity is found in the center of long bones and serves as a storage area for bone marrow. Examples of long bones include the femur, tibia, fibula, metatarsals, and phalanges.
   
• Short. Short bones are about as long as they are wide and are often cubed or round in shape. The carpal bones of the wrist and the tarsal bones of the foot are examples of short bones.
   
• Flat. Flat bones vary greatly in size and shape, but have the common feature of being very thin in one direction. Because they are thin, flat bones do not have a medullary cavity like the long bones. The frontal, parietal, and occipital bones of the cranium—along with the ribs and hip bones—are all examples of flat bones.
   
• Irregular. Irregular bones have a shape that does not fit the pattern of the long, short, or flat bones. The vertebrae, sacrum, and coccyx of the spine—as well as the sphenoid, ethmoid, and zygomatic bones of the skull—are all irregular bones.
   
• Sesamoid. The sesamoid bones are formed after birth inside of tendons that run across joints. Sesamoid bones grow to protect the tendon from stresses and strains at the joint and can help to give a mechanical advantage to muscles pulling on the tendon. The patella and the pisiform bone of the carpals are the only sesamoid bones that are counted as part of the 206 bones of the body. Other sesamoid bones can form in the joints of the hands and feet, but are not present in all people.

Parts of Bones
The long bones of the body contain many distinct regions due to the way in which they develop. At birth, each long bone is made of three individual bones separated by hyaline cartilage. Each end bone is called an epiphysis (epi = on; physis = to grow) while the middle bone is called a diaphysis (dia = passing through). The epiphyses and diaphysis grow towards one another and eventually fuse into one bone. The region of growth and eventual fusion in between the epiphysis and diaphysis is called the metaphysis (meta = after). Once the long bone parts have fused together, the only hyaline cartilage left in the bone is found as articular cartilage on the ends of the bone that form joints with other bones. The articular cartilage acts as a shock absorber and gliding surface between the bones to facilitate movement at the joint.
Looking at a bone in cross section, there are several distinct layered regions that make up a bone. The outside of a bone is covered in a thin layer of dense irregular connective tissue called the periosteum. The periosteum contains many strong collagen fibers that are used to firmly anchor tendons and muscles to the bone for movement. Stem cells and osteoblast cells in the periosteum are involved in the growth and repair of the outside of the bone due to stress and injury. Blood vessels present in the periosteum provide energy to the cells on the surface of the bone and penetrate into the bone itself to nourish the cells inside of the bone. The periosteum also contains nervous tissue and many nerve endings to give bone its sensitivity to pain when injured.
Deep to the periosteum is the compact bone that makes up the hard, mineralized portion of the bone. Compact bone is made of a matrix of hard mineral salts reinforced with tough collagen fibers. Many tiny cells called osteocytes live in small spaces in the matrix and help to maintain the strength and integrity of the compact bone.
Deep to the compact bone layer is a region of spongy bone where the bone tissue grows in thin columns called trabeculae with spaces for red bone marrow in between. The trabeculae grow in a specific pattern to resist outside stresses with the least amount of mass possible, keeping bones light but strong. Long bones have a spongy bone on their ends but have a hollow medullary cavity in the middle of the diaphysis. The medullary cavity contains red bone marrow during childhood, eventually turning into yellow bone marrow after puberty.
Articulations
An articulation, or joint, is a point of contact between bones, between a bone and cartilage, or between a bone and a tooth. Synovial joints are the most common type of articulation and feature a small gap between the bones. This gap allows a free range of motion and space for synovial fluid to lubricate the joint. Fibrous joints exist where bones are very tightly joined and offer little to no movement between the bones. Fibrous joints also hold teeth in their bony sockets. Finally, cartilaginous joints are formed where bone meets cartilage or where there is a layer of cartilage between two bones. These joints provide a small amount of flexibility in the joint due to the gel-like consistency of cartilage.

Skeletal System Physiology

Support and Protection
The skeletal system’s primary function is to form a solid framework that supports and protects the body's organs and anchors the skeletal muscles. The bones of the axial skeleton act as a hard shell to protect the internal organs—such as the brain and the heart—from damage caused by external forces. The bones of the appendicular skeleton provide support and flexibility at the joints and anchor the muscles that move the limbs.
Movement
The bones of the skeletal system act as attachment points for the skeletal muscles of the body. Almost every skeletal muscle works by pulling two or more bones either closer together or further apart. Joints act as pivot points for the movement of the bones. The regions of each bone where muscles attach to the bone grow larger and stronger to support the additional force of the muscle. In addition, the overall mass and thickness of a bone increase when it is under a lot of stress from lifting weights or supporting body weight.
Hematopoiesis
Red bone marrow produces red and white blood cells in a process known as hematopoiesis. Red bone marrow is found in the hollow space inside of bones known as the medullary cavity. Children tend to have more red bone marrow compared to their body size than adults do, due to their body’s constant growth and development. The amount of red bone marrow drops off at the end of puberty, replaced by yellow bone marrow.
Storage
The skeletal system stores many different types of essential substances to facilitate growth and repair of the body. The skeletal system’s cell matrix acts as our calcium bank by storing and releasing calcium ions into the blood as needed. Proper levels of calcium ions in the blood are essential to the proper function of the nervous and muscular systems. Bone cells also release osteocalcin, a hormone that helps regulate blood sugar and fat deposition. The yellow bone marrow inside of our hollow long bones is used to store energy in the form of lipids. Finally, red bone marrow stores some iron in the form of the molecule ferritin and uses this iron to form hemoglobin in red blood cells.
Growth and Development
The skeleton begins to form early in fetal development as a flexible skeleton made of hyaline cartilage and dense irregular fibrous connective tissue. These tissues act as a soft, growing framework and placeholder for the bony skeleton that will replace them. As development progresses, blood vessels begin to grow into the soft fetal skeleton, bringing stem cells and nutrients for bone growth. Osseous tissue slowly replaces the cartilage and fibrous tissue in a process called calcification. The calcified areas spread out from their blood vessels replacing the old tissues until they reach the border of another bony area. At birth, the skeleton of a newborn has more than 300 bones; as a person ages, these bones grow together and fuse into larger bones, leaving adults with only 206 bones.
Flat bones follow the process of intramembranous ossification where the young bones grow from a primary ossification center in fibrous membranes and leave a small region of fibrous tissue in between each other. In the skull these soft spots are known as fontanels, and give the skull flexibility and room for the bones to grow. Bone slowly replaces the fontanels until the individual bones of the skull fuse together to form a rigid adult skull.

Long bones follow the process of endochondral ossification where the diaphysis grows inside of cartilage from a primary ossification center until it forms most of the bone. The epiphyses then grow from secondary ossification centers on the ends of the bone. A small band of hyaline cartilage remains in between the bones as a growth plate. As we grow through childhood, the growth plates grow under the influence of growth and sex hormones, slowly separating the bones. At the same time the bones grow larger by growing back into the growth plates. This process continues until the end of puberty, when the growth plate stops growing and the bones fuse permanently into a single bone. The vast difference in height and limb length between birth and adulthood are mainly the result of endochondral ossification in the long bones
Prepared by- ASIM KAPSHIKAR,
Special thanks- DR. NITIN BHOKARE