Although the word skeleton comes from the Greek word meaning “dried-up body”, our internal framework is so beautifully designed and engineered that it puts any modern skyscraper to shame.
Functions of the Skeletal System
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.
Classification of Bones
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 human skeleton is made up of 206 bones. The functions of the skeleton are to provide support, give our bodies shape, provide protection to other systems and organs of the body, to provide attachments for muscles, to produce movement and to produce red blood cells.
Click here to download and print a diagram of the human skeleton without labels so you can test yourself.
The main bones of the human skeleton are:
- The Skull - Cranium, Mandible and Maxilla
- Shoulder girdle - clavicle and scapula
- Arm - humerus, radius and ulna
- Hand - Carpals, Metacarpals and Phalanges
- Chest - Sternum and Ribs
- Spine - Cervical area (top 7 vertibrae), Thoracic (next 12), Lumbar (bottom 5 vertebrae), Sacrum (5 fused or stuck together bones) and Coccyx (the tiny bit at the bottom of the spine).
- Pelvic girdle - Ilium, Pubis and Ischium.
- Leg - Femur, Tibia and Fibula
- Ankle - Talus and calcaneus (not shown above)
- Foot - Tarsals, Metatarsals and Phalanges.
The skeleton can be divided into two parts known as axial and the appendicular. The axial skeleton consists of the central core of the skull, spine and ribs whilst the appendicular is composed of the arms and legs.
How are bones formed?
- Bones are formed by the ossification of cartilage. What this really means is all bones start off as cartilage (normally in the womb) and they gradually turn to hard bone (ossification) over a period of years.
- Calcuim is needed for strong bone growth. Read more on the structure of bone.
What is the function of the skeleton?
- It provides protection to the major organs in particular the chest and rib cage.
- Muscles attach to bones to enable movement.
- Production of red blood cells within the bone marrow (a spongy substance in found in the cavities of long bones). Red blood cells carry oxygen around the body which is important in the production of energy.
How many bones in the skeleton?