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Divisions of Skeletal System and Types of Bone

The skeletal system consists of two groups of bones: the axial and appendicular skeletons.
Divisions of the skeletal system
The skeletal system consists of two groups of bones: the axial and appendicular skeletons.

Axial
The skull, vertebral column, ribs, and sternum make up the axial skeleton. In the human body, the bones that make up these structures collectively make up the axis, the central bony core.

Skull
The skull consists of a cranium and a face, each of which is separate from the spinal column.

A. Cranium
Bones arranged in irregular shapes and patterns make up the skull, which protects the brain from injury. Brains rest upon a base and are surrounded and covered by a vault. Dura mater, also known as the outer layer of the periosteum, lines the inner surface of the skull bones. A large number of the bone joints (sutures) in the mature skull are fixed (fibrous). Nerves, blood vessels, and lymph nodes pass through the numerous perforations of the bones (e.g., foramina, fissures).

The skull consists of the following bones:
1 Frontal bone – Also known as the forehead bone, this bone is found at the front of your head. Surrounding the eyes are ridges called supraorbital margins that constitute the orbital cavities (eye sockets).
2 Parietal bones – The skull's sides and roof are made up of these bones.
2 Temporal bones – you can see that the zygomatic and parietal bones of the head are usually joined by fibrous immovable joints with the occipital, parietal, sphenoid, and sphenoid bones. There are many important characteristics of the temporal bone.
1 Occipital bone – It makes up part of the base of the skull and is located at the back of the head. The frontal, temporal, and sphenoidal bones are immovable by fibrous joints.
1 Sphenoid bone – It is situated in the middle part of the skull and articulates with the temporal, parietal, frontal, and occipital bones.
1 Ethmoid bone – A part of the nasal cavity is composed of the ethmoid bone. It normally occupies the base of the skull's anterior part and the lateral wall by the pharynx of the orbital cavity.

B. Face
There are 13 bones plus the frontal bone in the face's skeleton. There are:
2 zygomatic (cheek) bones - Before birth, two bones fuse to form the zygomatic bone.
1 maxilla - It originates from two bones, but fuses before it is born.
2 nasal bones - In the bridge of the nose, both of these bones are lined up on the superior and lateral surfaces.
2 lacrimal bones - A medial wall of the orbit is formed by these two bones, which are posterior to the nasal bones and laterally to the nasal bones.
1 vomer - This is a slim, flat bone that forms most of the nasal septum's inferior part, extending up from the middle of the palate.
2 palatine bones - Both of these bones have an L-shaped shape. The hard palate's posterior wall is formed by the horizontal parts, while its lateral walls are made up of perpendicular parts. They form an orbital cavity in their upper extremities.
2 inferior conchae - Each shell is a long, cylindrical bone that projects into the nasal cavity from the lateral wall below the middle shell.
1 mandible - Only the lower jaw moves in the skull, making it the only bone with mobility

C. Vertebral column
A vertebra is an individual bone within the spinal column (or backbone). The spinal column contains 26 vertebrae in total. Vertebrae are named after their positions along the spinal column.

Four to five small coccygeal vertebrae fuse to form one coccyx, and seven cervical, twelve thoracic, five lumbar, and five sacral vertebrae from one sacrum. Within the neck, there are seven cervical vertebrae in total. With the odontoid process of the axis vertebra, the Atlas collaborates with the skull to support it; together, Atlas and axis form a pivot joint. Our heads can be turned using this pivot joint. There are no names for the remaining five cervical vertebrae. On the posterior side of the trunk, a thoracic vertebra articulates with the ribs (forming a joint). Lumbar vertebrae are found in the lower back and are among the strongest and largest of all spine bones. Two hip bones articulate at the sacrum, the sacroiliac joints. Coccyx, a remnant of the tail vertebrae, is attached to muscles in the perineum (pelvic floor).

There are discs of fibrous cartilage between adjacent vertebrae and the bodies of adjacent vertebrae support each other. In addition to cushioning and absorbing shock (symphysis joints), they allow some movement between vertebrae.

D. Ribs
It also contains the sternum, or breastbone, along with the 12 pairs of ribs. There is the posterior attachment of all ribs to the thoracic vertebrae. A total of 7 first pairs of ribs articulate directly with the manubrium and the body of the sternum through the cartilages of the costa, while 3 next pairs of rib cartilages join the seventh cartilage. Because of their lack of articulation with the sternum, the last two pairs are known as floating ribs. Rib cages serve a very obvious purpose by enclosing and protecting the heart and lungs.

E. Sternum
As you feel this flat, muscular bone under your chest skin, you're able to navigate your way to the Sternum, located in the middle of the chest. The sternum consists of three parts in total and they include the upper manubrium, central body, and lastly, the xiphoid process.

Attached to the ribs in the middle portion, or body.
Connecting the sternum to the clavicles and the first two pairs of ribs is the suprasternal joint, which is found at the top of the sternum.

The tip of the bone is called the xiphoid process. There are also muscles of the anterior abdominal wall attached to it, as well as the Linea alba and diaphragm.

Appendicular skeleton
With the upper limbs attached to the shoulder girdle, and with the lower limbs attached to the pelvic girdle, we have the appendicular skeleton.

Shoulder girdle and upper limb
An upper limb is connected to the trunk by the shoulder girdle (pectoral girdle).

a) Shoulder girdle
2 Scapulae and 2 clavicles make up the shoulder girdle.

1. Clavicle (collar bone)
The clavicle is a long, S-shaped bone. During the sternoclavicular joint, the manubrium forms the joint, and during the acromioclavicular joint, the acromion process forms the joint. To define the upper limb as well as the axial skeleton neatly, there’s the presence of the clavicle.

2. Scapula (shoulder bone)
This flat triangular-shaped bone lies just below the ribs, separated from them by muscles, on the posterior wall of the chest. The caveola is a shallow articular surface connecting the head of the humerus to the glenoid cavity of the joint. There is also a rough ridge extending from the scapula's posterior edge along the spine and in the glenoid cavity. The acromion process is defined as an overhang of skin that extends from the top of the shoulder blade is called the acromion process. This makes up a movable synovial joint with the clavicle, which is necessary for the shoulder girdle to function. In the shoulder joint, the coracoid process gives attachment to the muscles that move it.

b) Upper limb
1. Humerus

The bone that makes up the upper arm, In the shoulder, the joint is formed when the scapula fits into the glenoid cavity as the head enters the shallow articular surface of the scapula. An elbow joint is formed by articulating the radius and ulna together at the bone's distal end.

2. Ulna and radius
The forearm consists of these two bones. If the hand is placed in its anatomical position, the palm facing forward, the ulna and radius are parallel.

3. Carpal bones (wrist bones)
The eight carpal bones appear in two rows of four each. From the outside looking in you can observe that they are:
  • Proximal row: they include scaphoid, lunate, triquetrum, and pisiform.
  • Distal row: they include trapezium, trapezoid, capitate, and hamate.
4. Metacarpal bones
The palm is made up of these five bones. Counting begins at the thumb side. Their proximal ends are connected to the carpal bones and their distal ends to the phalanges.

5. Phalanges
There are 14 phalanges. Each finger has three phalanges, as well as two on each thumb.

Pelvic girdle and lower limb
A pelvic girdle joint connects the lower limb to the trunk.

a) Pelvic girdle
Two innominate (hip) bones from the pelvic girdle, as a form of a basin, the pelvic girdle consists of the pelvic girdle and sacrum that surround it.

1. Innominate (hip) bones
The hip joint is normally formed by two almost-spherical heads of the femur that are lying side by side in the acetabulum, which extends from the lateral surface of the acetabulum.

A flattened ilium is found near the top of the bone, and the iliac crest forms an anterior curvature. This curve has been called the anterior superior ilium. A cartilaginous joint called the symphysis pubis connects the pubis of each hip bone. Ischium refers to the inferior and posterior parts. While seated, the ischial tuberosities support the body's weight. The acetabulum is the site of the union of all three parts.

2. Pelvis
There are three main bones in the pelvis: hips, sacrum, and coccyx. The lower and upper sections of the pelvis are separated by the pelvis brim, which is composed of the sacrum promontory and innominate lines. A greater pelvis is above the brim, while a lesser pelvis is below it.

b. Lower limb
1. The femur (thigh bone)
The femur is known to be the longest and the heaviest bone in the body and it has an almost spherical head is inserted into the hip bone's acetabulum to form the hip joint.

2. Tibia
Several bones make up the lower leg. The main one is called the tibia, which is located below the femur.

3. Fibula
Essentially a long, slender lateral bone, the fibula is the long bone.

4. Patella (knee cap)
Sesamoids, such as this one, are triangular-shaped bones found in joints, such as the knee.

5. Tarsal (ankle) bone
Several bones make up the posterior part of the ankle (ankle), including a talus, a calcaneus, a navicular, a cuboid, and three cuneiforms.

6. Metatarsal (bones of the foot)
On the dorsum of the foot, five bones are numbered from inside out.

7. Phalanges (toe bones)
Phalanges make up the toes, and they are arranged the same way they do in the fingers, with two of them in the great toe (the hallux) and three in each of the remaining toes.

8. Arches of the foot
Curves or arches in the sole are a result of ligaments and muscles that support bone arrangement in the foot.

Classification of bones
1. Long bones

Those bones that are long, such as the arm, leg, hand, and foot plus it does not include the bones of the wrists and ankles. Normally, long bones are made up of a shaft that is called a diaphysis and an end is called an epiphysis. The diaphysis consists of compact bone, hollow within, and forms a canal in the shaft. Yellow bone marrow, mostly adipose tissue, is found in the marrow canal (or medullary cavity). There are two layers of bone on the epiphyses: spongy bone and compact bone. Children's bones contain red bone marrow, but in adults yellow bone marrow typically replaces it.

2. Short bones
Bones those makeup wrists and ankles belong to this group of bone types.

3. Flat bones
The flat bones of the body, such as the ribs, shoulder blades, hip bones, and cranium.

4. Irregular bones
The vertebral column and the facial bones are irregular.

Bones that are short, flat, or irregular are made up of spongy, compact bone covered by a thin layer of compact bone. The spongy bone contains red bone marrow. Bones have smooth surfaces because articular cartilage covers their joint surfaces. A fibrous connective tissue membrane, the periosteum, is a fibrous tissue coating the rest of your bones and attaching them to your tendons and ligaments. These structures are held in place by the periosteum, which contains not only blood vessels that enter the bone but also osteoblasts that enter the bone in response to bone damage.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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