It is common knowledge that the human skeletal system is composed of a total of 206 bones. These bones are categorized into five different types based on their shape, with one of the primary classifications being the extended bone.
What Constitutes an Extended Bone
An extended bone is characterized by its elongated, robust, and dense structure. These bones are predominantly found in the limbs, specifically the arms and legs. The bones of the fingers and toes also fall under the category of extended bones, as the classification is based on shape rather than size.
Illustrations of Extended Bones in the Organism
In the Upper Limb
- Humerus
- Radius
- Ulna
- Scapula
In the Lower Limb
- Femur
- Tibia
- Fibula
In the Phalanges and Metacarpals
- Metacarpals
- Metatarsals
- Phalanges
Roles
- Bearing the body’s weight
- Operating as a lever to aid in movement
- Supplying strength and framework to the upper and lower extremities
Construction of an Extended Bone
An extended bone consists of two main components: (1) the central shaft or diaphysis, and (2) the epiphyses (singular: epiphysis), the bulky ends on both sides. This is interspersed with a narrow segment known as (3) metaphysis, positioned between the diaphysis and epiphysis. Additionally, a membrane known as (4) periosteum covers the bone.
1. Diaphysis
The central tubular shaft links the two ends of the bone. Its walls consist of dense and robust compact bone, establishing an internal hollow area referred to as the medullary cavity (as depicted in the above cross-section image). This cavity houses yellow bone marrow, aiding in fat storage, and is covered internally by a delicate membrane called the endosteum, which assists in bone growth, remodeling, and recovery from fractures.
2. The Epiphyses
As mentioned earlier, these are the two extremities of the bone. The end situated closest to the body’s center is termed the proximal epiphysis or proximal end, while the end farthest from the body’s center is referred to as the distal epiphysis or distal end.
The epiphyses are composed of spongy bone filled with red bone marrow, which contains hematopoietic stem cells responsible for producing red and white blood cells and platelets.
The extreme ends of the epiphyses, where they articulate with other bones, are coated with articular cartilage. This cartilage safeguards the surfaces from friction and absorbs mechanical shocks at the joints.
3. Metaphysis
This is the juncture where the diaphysis and epiphysis converge. In a developing bone, this region encompasses a growth plate known as the epiphyseal plate, comprised of a layer of transparent hyaline cartilage. Around the age of 18-21, the cartilage is replaced by osseous tissues, signaling the end of bone growth. At this stage, the ossification of the area is complete, and the epiphyseal plate transforms into the epiphyseal line.
4. Periosteum
‘Peri’ denotes ‘around’ or ‘surrounding’, while ‘osteo-’ signifies ‘bone’. Therefore, it is evident from the name that the periosteum is a fibrous membrane enveloping the external surface of an extended bone. The only parts of the bone not covered by the periosteum are the ends coated in cartilage.
This membrane contains blood vessels, nerves, and lymphatic vessels that transport essential nutrients and respiratory gases to the compact bone. Furthermore, the ligaments and tendons attached to an extended bone anchor onto the periosteum.
Frequently Asked Questions
Q.1. Where does elongation occur in an extended bone?
A: Elongation in an extended bone takes place at the epiphyseal plate, promoting lengthwise bone growth.
Q.2. How many extended bones are present in the organism?
A: There are approximately 90 extended bones in the human body.
References
- Extended bones — Mountsinai.org
- Extended bones — Medlineplus.gov
- Bone classification — Training.seer.cancer.gov
- Structure of an Extended Bone — Bio.libretexts.org
- Bone Configuration — Pressbooks-dev.oer.hawaii.edu
