An adult has 206 bones, which together weigh about 9 kg. A baby has 350 bones. Many bones in a baby’s skeleton are not fully grown, such as those of the skull. Cartilages are present at these points, and turn into bone at a later point. Bones are made up of different materials, and provide strength and elasticity. The bone mass is covered by a periosteum (bone skin), which contains nerves, blood vessels, and bone-building cells. The bone marrow, which makes the red blood cells, is present inside long bones. The longest bone in our body is the thigh bone.
We can judge the direction of sound only because we have two ears. The sound waves reach the right and the left ear with a small time difference because the ears are at slightly different distances from the source of the sound. The eardrums vibrate, and the ear ossicle transmits the impulse to the inner ear. The inner ear sends it further to the brain where the signals from both ears reach at marginally different times. The brain calculates the direction of the sound from the tiny difference in these times.
Without the sense of touch we would be quite helpless. We would not be able to feel one, we would not feel our feet hitting the floor while walking and injuries would go unnoticed because no pain would be felt. We would also be deprived of many pleasant experiences important for social interaction. For blind people the sense of touch is very important. They move around safely by feeling and touching objects in their surroundings. Blind people compensate for their lack of sight by highly developed sense of touch.
We can think of the eye as a kind of camera. The iris behind the cornea is light-sensitive and corresponds to the aperture of the camera. This can be set near or far, depending on the amount of light falling on it. The light rays penetrate the lens of the eye, and fall on the retina. Millions of light-sensitive cells convert the light into signals sent to the brain via the optic nerve. These are put together to make images, and are interpreted by the brain. There are two types of cells in the retina: the light-sensitive rods that help us see in the dark and the less sensitive cones that help us see colours.
Whatever we perceive is not always real. For example, in a magic show, we may think what we see is real, but actually it is an illusion. Even our sense of balance can be deceived, and then one would react with uncertainty and dizziness. Hallucinations often occur because things do not happen the way we expect them to, and the brain gets ‘confused’.
Just as each human being is unique; his grooved fingertips are also unique. Even identical twins do not have the same patterns of grooves on their fingertips. Minor cuts and some skin diseases may cause temporary changes in the patterns, but on healing, the same pattern appears. Since 1901, police have used this knowledge to compare the fingerprints found at a crime site with those of the suspects. In early days, this comparison was done manually and used to be very cumbersome and time-consuming. Today, fingerprints are matched by the computer in a fraction of the time used earlier.
The skin, as a sense organ, perceives all kinds of touch sensations as signals. These signals are sent to the brain, are interpreted, and the body reacts accordingly. The brain can differentiate between the touch stimuli that we create ourselves and the stimuli given by others. If we try to tickle ourselves, our brain anticipates this type of touch from our hands and prepares itself for it. Since the element of unexpectedness is missing, the body does not respond the same way as it would if someone else were to tickle us.
The colour of a person’s skin depends on his environment. In regions with abundant sunlight, a dark skin colour is necessary to provide protection against the harmful UV rays. Light-skinned people in north Europe, where there is very little sunlight, do not need this protection. UV rays are not always harmful: they are needed for the formation of vitamin D, which the body needs for building bones. The darker the skin, the less sunlight can enter the skin and vitamin D formation is reduced. A dark skin may, therefore, result in Vitamin D deficiency in areas where there is not much sunlight.
Goose pimples help in regulating the body temperature. When we are exposed to cold temperature or get scared, our body hairs stand up so that the air between them can form an insulating layer. Each hair is permanently anchored to the skin with its root. Small muscles are present at these roots, which contract in case of cold or fear. This is visible on the skin as small bumps. The contraction of the muscles also produces heat in the body. This was an excellent warm-up device for our thickly haired curly ancestors who did not wear clothes.
In the summers, our skin is often exposed to UV radiation from strong sun that can damage the skin. A pigment, called melanin, is produced in the skin to protect against the harmful radiations from the sun. The more this pigment is produced, the darker, i.e., more tanned, is the skin and the higher is the protection against sunburn. That is why; many light-skinned people get sunburnt very quickly. Sunburn always means damage to the skin. Even when the skin has apparently recovered from the burn, it remains damaged from inside and there is always a high risk of getting skin cancer later.
Our skin is the largest, heaviest, and the most versatile organ. The skin forms the outer covering of our bodies. The colour of the skin and hair determines our appearance. Without the skin, we would not have any protection against UV rays, injuries, and germs invading from outside. Our sense of touch would not work without it. The skin reacts to the outside stimuli and becomes red, brown, or gets goose pimples. Each person has a unique set of fingerprints, and police can often catch criminals by matching their fingerprints.
Muscle stiffness is a painful result of overstrained muscles. It frequently occurs on days when the body suffers very strong physical stress. Physical strain causes micro cracks in the muscle fibres, which slowly get filled with water. As a result of this, the tissue gets stretched, which causes pain, and gives rise to inflammation. After a few days of recuperation, the pain usually subsides.
Many of our bones are joined to one another through joints, which permit different kinds of movements. The ball joints of our hips and shoulders offer the highest freedom of movement. Here, the ball-shaped end of one bone moves inside the bowl-shaped end of the other. Knees and elbows are joined by means of a hinge joint, which permits only the back and forth movement, like the hinge in a door. Our head sits on a condyloid joint, which allows it to move sideways, front or back, but not rotate. Fluid is present in the joints as a lubricant so that the joints and the bones do not rub against each other.
Most of our organs are protected by the skull and the body skeleton. Some of our most sensitive organs lie embedded in the powerful cranial bone of the skull—the brain, the eyes, and the hearing organs. The mouth cavity protects the only movable bone of the skull, the lower jaw. The body skeleton with the ribs fanning out of the backbone protects the area of the chest. Internal organs such as the heart, lung, liver, and spleen are protected behind the rib cage, and the backbone covers the vital spinal cord.