for the body’s most vital structures; for example – the skull protects the brain, while the sternum and ribs protect the heart and lungs.
Bones aid in the mechanics of movement. While muscles provide the force via contraction, the bones provide the vehicle for the movement. Muscles are attached to bones, and during contraction, they shorten, pulling on the bones and moving them. Bones also perform a significant role in the storage and maintenance of certain metabolic functions, such as calcium and phosphate regulation.
Figure 4-2 Red blood cells.
Lastly, bones are responsible for one of the most vital physiological processes in the body. That process is called hematopoiesis, or blood cell formation. Blood cell production is carried out in the red bone marrow, a soft connective tissue found deep inside the medulla, or center, of specific bones.
■Classification of Bones
Six different types of bones are defined and named according to their shape2:
•long bones
•short bones
•flat bones
•sesamoid bones
•sutural bones
•irregular bones
Examples of long bones are the humerus (bone of the upper arm), and the femur (bone of the upper leg).
Short bone examples include the carpals (wrist bones) and the tarsals (ankle bones).
Examples of flat bones can be seen in the skull, such as the frontal bone or in the pelvis, such as the ilium.
Sesamoid bones are small, rounded bones implanted with a tendon (a type of connective tissue that attaches skeletal muscles to bones); the most easily recognizable one being the patella, or knee cap.
Sutural bones are small and flat bones that sit between the flat bones of the skull; they vary in size and number.
Finally, irregular bones are bones that cannot be classified as any other type of bone and have varying shapes that can be found in the vertebrae, or spinal bones and in the skull, such as the sphenoid and ethmoid bones.
We will take a thorough look at the structural components of a long bone, which will allow us to recognize all of the fundamental characteristics of the entire collection of bones.
Bone structure (gross)
Gross anatomy defines a branch of anatomy focusing on macroscopic (large enough to be observed by the naked eye) structures of organs and tissues.
Seven main components to the gross structure of a long bone are discussed below:
•Diaphysis – shaft; the cylindrical mid-section of a long bone,3 constructed out of solid compact bone (discussed later). It is dense, hard, and extremely strong.
•Medullary cavity – the hollow area inside and deep to the diaphysis of a bone. It contains yellow bone marrow, the fatty, inactive form of marrow found in the adult skeleton.
•Epiphysis – the outer ends of bone, constructed of spongy bone, which houses red bone marrow, the active form of marrow that is responsible for hematopoiesis. The epiphysis is also the location of secondary ossification during development (discussed later).
•Periosteum – A thick, strong, outer fibrous membrane that covers most of a long bone, except for the ends of joint surfaces.
•Endosteum – A thin, interior membrane of connective tissue that lines the medullary cavity.
•Articular cartilage – a thin layer of cartilage that covers the ends of bones, at the outermost regions of the epiphysis; they function like shock absorbers that cushion the ends of bones, where a joint is formed.
•Physis – the epiphyseal plate, also called the growth plate. This is the site of active primary endochondral ossification in growing bone (discussed later).
Figure 4-3 Bone structure.
■Bone Histology
Histology defines structure and organization, so in regard to the skeletal system, bone histology means a branch of anatomy that focuses on the minute structures of human (or animal) tissues that is discernable through a microscope.
Several types of bone cells4 are found in the human body:
•Osteoblasts – bone-forming cells
•Osteocytes – responsible for the maintenance of mineral and organic elements in bone
•Osteoclasts – involved in breakdown of bone tissue and involved with resorption processes
Figure 4-4 Bone cells.
The skeletal system contains two main types of connective tissue: bone and cartilage. Further, each bone is comprised of two types of osseous tissue: compact bone, and spongy bone.
Compact bone is a dense and solid type of bone. Spongy bone, also called cancellous bone,5 is a relatively weaker and porous type of bone (hence, resembling a sponge). It is composed of “needle-like threads” of bone, called trabeculae encompassed by gaps of space filled with red marrow.
Compact bone, because it does not contain a network of gaps or open spaces, is a dense and rigid structure, organized into a matrix of several functional units of bone called osteons, or Haversian systems.6
All osteons are packed tightly together and oriented the same way, creating immense strength and providing a great basis for support. The Haversian systems are circular structures composed of a hardened medium arranged in multiple layers, resembling the rings of an onion.
Each concentric ring or layer in the osteon is called a lamella. The lamellae (plural) surround and encircle the central canal, also known as the Haversian canal, named after Dr. Clopton Havers (1657–1702),7 which contains a blood vessel.
In contrast to compact bone, spongy bone often lacks complete osteons or Haversian systems due to extremely thin trabeculae. However, spongy bone is more metabolically active than its denser counterpart, due to its much larger surface area.
Bones, because of their hard, rigid matrix, are often thought to be lifeless structures. However, wedged in between the hard layers of the lamellae in tiny pockets called lacunae are living bone cells; the aforementioned osteocytes.
Lacunae are connected to one another and to the central canal via microscopic channels or canals, called canaliculi. Because of this interconnection, nutrients
