- INTRODUCTION
Bones are connected in a system of moveable and immoveable joints to form the skeleton. In addition to its other functions, the skeleton serves as frameworks to which voluntary striated muscles are attached. The skeleton and the attached muscles constitute what is known, after an animal’s death, as the carcass. A butcher may cut up the carcass to form the meat cuts.
- 2. ANATOMY OF BONE
| Calcified: Harden by deposition of or conversion into calcium carbonate or some other insoluble calcium compounds. Rigidity: unable to bend or be forced out of shape, not flexible. |
Bones are living structures. They have blood vessels, lymphatic vessels and nerves. They undergo growth, but often repair themselves.
About one third of the weight of a bone consists of an organic framework of fibrous tissue and cells. Two-thirds consists of inorganic salts which are deposited within the organic framework. These salts are chiefly calcium and phosphorus and give hardness and rigidity to the bones. Calcium phosphate accounts for some 80% of these deposited salts.
Mature bone consists of osteocytes (bone cells) surrounded by an intercellular matrix composed of calcified osteoid material. The osteocytes are located in small cavities in the bone called lacunae. A system of tiny canals called canaliculi connects the lacunae and these convey tissue fluids essential for the maintenance of the osteocytes.
Both the lacunae and canaliculi are formed because the osteoblasts (bone forming cells) are interconnected through cytoplasmic processes at the time the osteoid material is laid down. Thus the cells and their processes act as a mould until the osteoid tissue is set and properly calcified. The cell processes are then partially withdrawn leaving the cells, now known as osteocytes, in the lacunae, which are connected by canaliculi.
The osteoblasts appear to be responsible for the formation of osteoid tissue and almost immediately secrete the enzyme phosphatase which is necessary for the deposition of calcium salts in the osteoid tissue, thus forming true bone.
Osteoblasts come from mesenchymal cells which are the parent cells for all connective tissue. Osteoblasts divide readily but only a portion of the new cells actually secrete said substance and form bone. The rest are held in reserve as the osteogenic layer. These cells function (divide and form more osteoblasts) whenever more bone is needed. This occurs in the repair of fractures or simply to increase the bone size. Bone intercellular matrix is unyielding and the osteocytes (mature osteoblasts) have lost the ability to divide.
Re-absorption of the bone occurs both under normal and abnormal conditions. Whenever bone is being re-absorbed, large multi-nucleated cells called osteoclasts are found.
The sequence of bone formation consists of osteoblasts laying down osteoid tissue that is subsequently calcified under the influence of phosphatase. A localised area of bone formation is called a centre of ossification.
There are 3 types of ossification set out as follows:
- Heteroplastic Ossification
The bone formation in tissue other than the skeleton. Generally pathologic except for Os penis of certain animals and Os cordis of the bovine heart.
| Extremities: The furthest point or limit of something |
Endochondral Ossification
The bone is formed as cartilage in the foetus. Most long bones develop by this method. Long bones grow as a result of the epiphysal cartilage.
- Intra-membranous Ossification
The ossification of many of the flat bones. These bones are preformed in a fibrous membrane or matrix which
is infiltrated with osteoid tissue. This bone, like osteoid tissue, becomes calcified to form true bone.
Of the mineral salts deposited in the organic framework, calcium phosphate makes up about 80%. The remainder is largely composed of calcium carbonate and magnesium phosphate.
Ossification is the formation of true bone by the deposition of calcium salts in matrix of osteoid tissue.
Bones are developed from cartilage under the influence of bone-forming cells (osteoblasts). These cells lay down osteoid tissue and immediately secrete an enzyme called phosphatase which results in the deposition of calcium salts in the osteoid material. The bone-forming cells then cease their function and become mature bone cells called osteocytes. The depositing of these calcium salts is known as the process of ossification.
- 3. CLASSIFICATION OF BONE
Bones may be classified into the following categories:
- Long Bones
These are greater in one dimension than another. They consist of a cylindrical shaft and two extremities. They serve as levers and aid in the support and movement of the body.
- Short Bones
These are approximately equal in size in all three dimensions. They have no marrow cavity. They are found in complex joints, e.g. the wrist. They are used to absorb shock.
- Sesamoid Bones
These bones are found along the course of tendons where they help reduce friction or change the course of tendons. A good example is the knee cap (or patella.)
- Pneumatic Bones
These bones contain air spaces and are found in the skull, e.g. sinuses in the nasal bones.
- Irregular Bones
These are unpaired bones situated on the median plane – and include the vertebrae. They are there for protection, support and muscle attachment.
- 4. STRUCTURE AND GROWTH OF BONE
For the generalised structure of bones we shall look at the long bones. Structure of a typical long bone.
Figure 1: Diagram of a Typical Long Bone
The outer shell of the bone is known as compact bone. This is a hard layer covering the surface of most of the long bone.
Within the bone we have at the two extremities, spongy bone. This is composed of plates arranged to form a porous network. These spaces are usually filled with marrow.
THE MEDULLARY CAVITY
This cavity is hollow within the long bone and is filled with marrow. This marrow is red in the young, but gradually changes to a fatty yellow in older mammals.
THE ARTICULAR CARTILAGE
This is a thin layer of smooth cartilage covering the surfaces of the bones involved in joint movement.
THE PERIOSTEUM
This is a thin fibrous membrane covering the whole surface of the bone, except where there is articular cartilage. Bone-forming cells are located here and are responsible for laying down bone to increase the width of long bones. It also lays down bone in response to healing at places where fractures have occurred.
The long bone extremities are known as epiphyses and the shaft as the diaphysis. Between the epiphyses and diaphysis is a disc of cartilage known as the epiphyseal cartilage. Osteoblasts are located in this disc and these lay down bone which is responsible for the growth of the long bones. Obviously, in the growing human or animal, this disc is active until mature size is reached. After this, the discs also ossify. In humans, this stage is reached in the late teens or early twenties.
GROWTH OF BONES
As the foetus develops, so the skeletal system becomes organised into a cartilage framework. Before birth, there is a certain amount of ossification or hardening of the limb bones to allow the animal to stand shortly after delivery. However, the greater part of the skeleton remains cartilage until after birth. After birth, ossification of all parts of the skeleton begins. This process continues throughout life until, in the old animal very little cartilage is left, and the bones are hard and brittle. Ossification is simply a laying down of salts of calcium and magnesium as hard plates within the bone tissue.
FRACTURES AND FRACTURE HEALING
A fracture of bone is simply a break in the continuity of a bone. If the broken ends of a fractured bone are brought together and immobilised, the normal process of healing will take place. At the time the fracture occurs, some blood vessels are ruptured, pouring blood around the broken ends of the bone. This forms a clot which is invaded by connective tissue cells forming granulation tissue and new blood capillaries. The osteoblasts from the surface of the bone, from the periosteum, from the endosteum and from the linings of the haversian canals divide rapidly and produce a massive amount of osteoid tissue called a callus. This fills the gap between the broken ends of the bone. It fills the marrow cavity for a distance, and completely encircles the broken ends of the bone forming an effective splint which usually prevents movements between the segments. As soon as the callus becomes calcified, it becomes true bone. The healing process is then completed by the re-organisation of the callus to form a typical bone shaft with a marrow cavity. Misalignment of the fractured bone will be corrected to some extent by the action of osteoblasts which will also remove excessive external and internal calluses.
| Definition Synovial: relating to or denoting a type of joint which is surrounded by a thick flexible membrane forming a sack into which is secreted a viscous fluid that lubricates the joint. |
Figure 2: Fractures and Fracture Healing
- 5. JOINTS OF BONES
Bones are joined to one another to form the skeleton. These unions are commonly called joints. Joints can be:
- Immoveable:
An example of this is the sutures between the bones of the skull. Another example is the hole in the head of a baby’s skull, where the bones still have to knit together into an immoveable joint or suture. See the diagram below:
Figure 3: Immoveable Joints in the Skull of a Cow
- Slightly Moveable:
An example of this type of joint is the joints between vertebrae in the spinal column. These vertebrae are separated by discs of cartilage.
- Freely Moveable:
An example of this type is the knee joint shown in the diagram below:
Figure 4: Freely Moveable Joints such as the Knee
Freely moveable joints are able to move in nearly every direction, such as, the ball and socket joint (enartardial), or the movement may be in one place only, such as the hinged knee-joint (Ginglymus). The joint may only allow a slight gliding movement such as the wrist bones (carpal). Such a joint is known as an arthrodial joint. Finally, the movement of the joint may be that of rotation (trochoid or pivot joint), e.g. the atlanto-axial joint.
The basic structure of a freely moveable joint is shown in the diagram. The joint is surrounded by a capsule which consists of a thick strong outer fibrous ligament, and a thin delicate inner synovial membrane. This membrane secretes synovial fluid or joint oil to lubricate the joint. The surfaces of the bones involved in movement are covered by very smooth articular cartilage.
- 6. THE SKELETON
Bones are connected together by a system of joints in a framework called the skeleton: the diagram below shows a simple skeleton of a mammal.
Figure 4: The Skeleton of a Mammal
The skeleton carries out several functions in the body, the most important of these are:
- To provide protection for the vital organs of the body. The brain is protected by the skull, and the remainder of the central nervous system, the spinal cord, is protected by the vertebral column, or the backbone. The rib cage protects the heart and lungs, and the pelvic girdle covers the uro-genital system;
- The skeleton provides the base for the general structure and outline for the mammal. It is the frame onto which are attached the muscles and skin;
- It gives the body rigidity;
- Some of the bones of the skeleton are arranged to act as levers so that movements like walking, running or lifting can be performed;
- The skeleton is a storage site for minerals, particularly calcium, phosphorus and magnesium. These are stored within the bones and can be used if they are deficient in the animal’s diet and;
- The bones of the skeleton contain marrow, which play a part in the formation of blood cells, mainly the red blood cells, but also some of the white blood cells. This is a very important function of bones.
The various parts of the skeleton are shown in the following series of diagrams, but remember that these rough diagrams have been simplified, and are only intended to give you a general idea.
Figure 5: The Skull of a Cow
Figure 6: The Chest of the Cow
Figure 7: The Fore and Hind Limbs of a Bovine Cow
