Chapter 5 - Axial Skeleton


Functions of Bone
1. Support
     The skeletal system provides structural support and a framework for attachment of soft tissues and organs.
2. Storage of Minerals 
     Bones maintain a large reserve of calcium and phosphate ions. 
3. Blood Cell Production   
     The hollow spaces in some bones contains red marrow where blood cells are produced.
4. Protection
     In many places bone protects soft tissues and organs.
5. Leverage
     The articulation of bones creates levers that change the magnitude and direction of the forces exerted by muscles.
Classification of Bones  
     There are two basic types of Bone tissue
Compact bone is dense and homogeneous and forms the walls of bone.
Spongy bone is composed of slender intertwined pieces of bone enclosing a space filled with non-bone tissue. It is found in the interior of normal bone.
     Bones in their entirety can also be classified according to shape:  
Long bones are longer than they are wide. All bones of the limbs except for the wrist and ankle bones are long bones.
Short bones are cube-shaped and are found in the wrist and ankle of the limbs.
Flat bones are thin and flat and often curved. These bones include some bones of the skull, the ribs and the sternum.
Irregular bones as their name suggests do not fit conveniently into any category according to their shape.

Sesamoid bones is not a category based on shape although they tend to be short bones. A sesamoid bone is a bone that develops within a tendon. The knee cap or patella is an example.

Structure of a Long Bone  
     The middle long shaft of a long bone is the diaphysis. Either end of the long bone where it articulates with another bone it is expanded and the ends are called epiphyses (sing. epiphysis).  
     Except where the bone has a moveable articulation with another bone the bone is covered by a sheath of dense connective tissue called the periosteum. The surface of a moveable articulation is covered by articular cartilage that provides a smooth, slippery surface that decreases friction.
     The hollow space within bone is called bone marrow. Bone marrow is filled either with adipose tissue and is then called yellow marrow or blood-forming tissue in which case it is called red marrow. Within the middle of the diaphysis of the long bone there is a large cavity called the marrow or medullary cavity.  
File:Bone marrow biopsy.jpg
Bone Remodeling  
     Although bone appears solid, it is actually a very dynamic tissue. Bone remodels itself in response to two factors:
1. Calcium levels in the blood.
2. Pull of gravity and muscles on the bone.
     When the body needs calcium, bone-destroying cells called osteoclasts break down bone tissue to release its reserves of calcium. Osteoporosis is a disease that results from the removal of too much bone tissue by osteoclasts. It results as the result of inactivity (e.g. space flight) and the hormonal changes that are associated with aging particularly in the female. YouTube - The Osteoclasts
     In response to growth and the stresses that may be placed upon bone, bone tissue is formed by bone-forming cells called osteoblasts. Both osteoblasts and osteoclasts work together to model bone during growth and the changing stress that is placed on bone.
Axial Skeleton   Skull Bones Menu
     The axial skeleton forms the longitudinal axis of the body and can be divided into the skull, vertebral column and the thorax.  
  Skull
     The skull is formed by two sets of bones, the cranium and the facial bones.
Cranium  
     The cranium encloses and protects the brain. It is composed of 8 bones of which includes two pairs, the temporals and parietals. All the cranial bones are joined to one another by tight, interlocking joints called sutures.  
     The bones of the cranium are:
Frontal bone  
  The frontal bone forms the forehead and forms the superior part of the eye’s orbit supporting the eyebrows.
Parietal bones  
  The parietal bones are paired and form the superior and lateral walls of the cranium. Where they meet on the midline they form the sagittal suture and where they meet with the frontal bones they form the coronal suture.  
Temporal bones  
  The temporal bones lie inferior to the parietal bones and where they meet with the parietal bones form the squamosal sutures. The temporal bones are irregular in shape.
Occipital bone  
  The occipital bone forms the floor and the back wall of the skull. Where it joints the parietal bones anteriorly it forms the lambdoid suture. At the base of the occipital bone there is a large opening called the foramen magnum where the brain joins the spinal cord. The occipital bone articulates with the first vertebra by means of the occipital condyles.  
Sphenoid Bone  
  This bone forms part of the floor of the skull and spans the width of the skull. It is very irregular but somewhat butterfly-like in shape. In the midline it contains a depression called the sella turcica (Turkish saddle) which contains the pituitary gland.
Ethmoid Bone  
  The ethmoid bone is another irregularly shaped bone that lies anterior to the sphenoid bone in the floor of the skull. It forms the anterior roof of the nasal cavity. 
Facial Bones  
     The facial bones consist of 14 bones of which only two the mandible and the vomer are unpaired. These bones include:
Maxillae  
  These bones form the upper jaw. This bone contributes to the hard palate and holds the upper teeth. This bone also contains a paranasal sinus that can be infected during a cold.
Palatine bones  
  These bones form the posterior part of the hard palate.
Zygomatic bones
  The zygomatic bones are commonly referred to as the cheek bones.
Lacrimal bones  
  These small bones contribute to the medial wall of the orbit and have a groove that accommodates a passage that allows tears to drain into the nasal cavity.
Nasal bones  
  These bones form the bridge of the nose.
Vomer  
  This single bone, which is shaped like a plow, contributes to the nasal septum.
Inferior nasal conchae  
  As the name of this bone suggests, it forms the inferior nasal conchae of the nasal cavity.
Mandible
  The mandible is another single bone that forms the lower jaw. It is the only freely moveable joint and has an articulation with the temporal bone of the cranium. It holds the lower teeth.
  Vertebral Column (Spine) vertebral tutorial
     The vertebral column provides the axial support for the trunk and transfers the weight of the upper body onto the pelvis and lower limbs. In addition to being sturdy, the spine is also flexible and able to absorb shock. The flexibility and shock absorption come from intervertebral discs that connect the bodies of the vertebrae. The curvatures of the spine also contribute to its flexibility and ability to absorb shock.
     The vertebral column is composed of bones which are, or result from the fusion of, vertebrae. The basic structure of the vertebra includes the following features:
Body – The body is the weight-bearing part of the vertebrae.
Vertebral arch – The arch is on the posterior aspect of the body and protects the spinal cord.
Vertebral foramen – The opening through the arch is called the vertebral foramen.
Processes – There are three different types of processes which serve as attachment points for ligaments and muscles, and sites for moveable joints.
     

A total of 26 separate bones form the spine. These include 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 1 sacrum (5 fused vertebrae) and 1 coccyx (3-5 fused vertebrae).  

  Bony Thorax  
     The sternum, ribs and thoracic vertebrae make up the bony thorax. The bony thorax protects the heart and lungs. The ribs, and the muscles attaching to the ribs, also facilitate the ventilation of the lungs.