My Science School

           The place where two or more bones meet in the body is called a joint. Depending on their structure, joints permit different degrees of movement of bones. Do you know the different types of joints in our body?

           Basically, there are three kinds of joints in our body. They have been grouped on the basis of the degree of movement they allow. They are: (I) Immovable or fixed joints, (II) Partially movable or cartilaginous joints, (Ill) Freely movable or synovial joints. 

 

Immovable joints: In an immovable joint, the bones are held tightly together. Because dense bundles of tough and strong fibres hold the bones, they cannot change the position relative to each other. The elasticity essential for joint movements is almost absent. Joints in the tooth sockets and between the skulls bones are of this type. Immovable joints are also known as fibrous joints. 

 

 

 

Partially movable joints: In a partially movable joint, the bones are linked by a cartilage. A cartilage is a very tough, spiny material. These joints have very little movement. The different vertebras of the spinal column are joined together by a cartilage. The joints of the spine are partially movable so that the spine can bend. Between the vertebrae there are disks of cartilage. They are called intervertebral disks. In addition to allowing the vertebrae to move against each other, they also act as shock absorbers. At the front of the chest where the ribs meet the breastbone are also partially movable joints. 

 

 

 

Freely movable joints: These are the main joints of the body. These are found in the bones of the ankle, hip, wrist, elbow and knee. The ends of the bones that are the part of the movable joints are covered with caps of tough cartilage. A cartilage does not wear out easily. Its surfaces are very smooth and reduce friction between the moving bones.

In a freely movable joint, the space between the bones is filled with a special oily fluid called synovial fluid. It helps the parts to move easily. It is produced by a thin membrane that lies around the joint and is kept within the joint by this synovial membrane. Outside the membrane are tough, flexible ligaments (connecting tissues). The ligaments in addition to joining one bone to another also allow movement. The membrane and the ligaments together form what is called the joint capsule. Synovial (freely movable) joints may be of different types depending upon the nature of articulation and degree of freedom. They are explained below:

Ball and Socket joints: In this the ball of one bone fits into the socket of another. So it makes movement possible in all directions – bending and stretching from side to side and rotation. The shoulder and hip joints are of this type.

Hinge joints: These are found in the elbow, knee and fingers. The ends of the bones fit into each other in such a way that the movement is possible in one direction only. Angular joints: In joints like the wrist, movement is possible around two axes. Such joints are called angular joints.

Pivot joints: In these joints, turning or rotatory movement is possible around one axis only. Skull joint to the spine is a pivot joint.

Gliding joints: In joints between tarsal bones in the ankle, carpal bones in the wrist and between sternum and clavicle, the end of one bone glides across a certain portion of the surface of the other bone.

Injuries and diseases may affect joints. Arthritis is a painful disease that attacks cartilage in joints.  

          Albinism has been derived from the Latin word ‘albus’ which means ‘white’. Albinism is an inherited disease caused by genetic changes. It can be passed on from one generation to another.

          Albinism is caused by the absence of yellow, red, brown or black pigments in the eyes, skin scales, feathers or hair. The natural pigments usually provide protective colouration and act as a screen against the light rays. Since albino animals lack them, they rarely survive in the wild.

          Albinos are found in plants, animals and human beings. In human beings, it is caused by the absence of melanin, the dark brown pigment normally present in the skin, hair and eyes. It varies from complete albinism to localized albinism or spotting.

          In the case of complete albinism, the person doesn’t have any pigment in any of their cells. They have milk-white skin and hair. Their eyes appear pink because of the colour of the blood vessels. Since the light-absorbing pigments are absent, an albino is extremely sensitive to bright light like that of the sun.

          In partial albinism, only some tissues and organs lack in pigment. Some animals are also partial albinos. One case of complete albinism is found in every 20,000 people.

          However some plants too, with white flowers, are partial albinos. A complete albino plant lacks even the green pigment - chlorophyll. As a result, it is unable to make its food by photosynthesis, and dies shortly after its food supply in the seed is exhausted.

          Normally mammals have bigger brains in relation to their size when compared with other living creatures and the human brain is the biggest and most developed among all the mammals. It controls all the activities of the body throughout our life. It remains active every moment directing and guiding all other organs of the body. That is why it is called the control centre of the body? But what is our brain made of and how does it carry out its different functions?

          The human brain is largely made up of grey and white matter. The grey matter contains nerve cells and the white matter contains the nerve fibres. The nerve fibres carry messages from the nerve cells to different parts of the body. Thousands of electrical impulses are constantly passing through these nerve cells. All messages are first sent to the brain through different nerves from the sense organs in all parts of our body. Consequently it sends signals to different muscles and glands in the body to carry out necessary actions. The most important human activity ‘thinking’ takes place in our brain. All the energy produced in the body is used by the brain. 

          The central nervous system consists of the brain and the spinal cord. Different parts of the brain are interconnected and they control different activities of the body. The medulla and hypothalamus control the involuntary activities such as breathing and heart-beat. The cerebellum controls muscles and organs of balance in activities like walking or riding a bicycle that is, carrying out work automatically once we have learnt them. The cortex controls conscious feeling and voluntary movements such as writing and running. The front part is called cerebrum which has two cerebral hemispheres – the left hemisphere and the right hemisphere. This part (cerebrum) does most of the thinking and is responsible for our memory and emotions. The left hemisphere is associated with the right side of our body and the right hemisphere, with the left side. People with a more developed right hemisphere are left-handed and vice-versa.

          Our brain works even when we are asleep. An adult’s brain weighs about 1.4 kg and has 14 billion nerve cells in it. The fastest messages pass along the nerves at a speed of 400 km/hr. 

          Immunity is described as the ability of the body to resist or to recover from the invasion of disease-causing microbes (bacteria, viruses, and protozoas) and larger parasites (helminthes). Thus a person said to be immune to a particular disease would not contact it although others might do so.

          Microbes and parasites cause several diseases in man. The disease-causing germs often release toxins (poisonous substances). Normally our body is able to defend itself against most disease-causing microbes. First the skin acts as a barrier to the entry of many microbes. And secondly the white blood cells destroy many microbes. But if the number of microbes exceeds the capacity of white blood cells they fail to protect the body against them.

          Many persons are able to resist diseases to certain extent due to immunity mechanisms in their systems. The blood of a person produces substances called antibodies which fight the invading organisms. Each kind of antibody acts against only one type of microbe. Different antibodies have different characteristics. Some antibodies neutralize the toxins released by microbes. Certain others clump them together which can then be easily attacked by white blood cells. Some other antibodies dissolve the bacteria. Sometimes a certain amount of antibodies is permanently left in the blood plasma and this serves to protect the individual from future attacks. Such persons are said to be immune to that particular disease. 

          Immunological mechanisms are either specific or non-specific. Specific immunity, also called acquired immunity, which refers to mechanisms that are activated individually after a microbe or some other foreign material, invades the body. Non-specific immunity refers to general protective mechanisms that either kill or prevent the multiplication of microbes and other parasites.

          Some persons are immune to certain diseases right from the birth. These people are said to have natural immunity or inborn immunity. In an epidemic of a particular disease, say cholera, people with natural immunity do not suffer from cholera. If someone gets smallpox or chickenpox once, he will not get it again during his whole lifetime. The body in such a case retains adequate levels of antibodies as a protection against future infection.

          A person can also develop immunity by treatment with appropriate antigens. This is called artificial immunity. For instance, in the case of epidemics of smallpox, the health authorities vaccinate the people by which immunity is produced artificially. This vaccine was invented by Edward Jenner. It is produced by infecting a calf or horse with the smallpox virus. The virus in the calf or horse gets weakened. This weakened virus is collected as a vaccine and introduced into the human body. Since the virus is weak, it is not able to cause a severe attack of smallpox in man. But its presence induces the body, to produce antibodies and provides immunity against smallpox for several years. The introduction of weakened microbes in the body is called active immunity, the effect of which may last for even a lifetime. Immunity may also be passive. In this readymade antibodies are injected into the human body. An animal like a horse is infected with disease-producing microbes. The antibody is produced in the blood of the horse. The serum of the horse containing the antibody is extracted and introduced into the human body. The human body makes use of antibodies against the disease thus producing passive immunity. Passive immunity is produced almost instantly when the serum has been injected, but the effect lasts only for a short period.

          Vaccines are now produced to provide immunity against diseases like whooping cough, diphtheria, measles, tetanus, typhoid, polio, rabies tuberculosis, mumps, scarlet fever, German measles, and chickenpox. All infants should be provided immunity by vaccination for different diseases.

          In our country Haffkine Institute at Bombay and the Virus Institute at Poona produce several kinds of vaccines.