The body has five main senses that detect what is happening in the outside world. Four are shown here. The fifth is touch, sensed by the skin.
SIGHT
Each eyeball is about 25 millimetres across and well protected in a socket, called the orbit, inside the skull bone. Light rays enter the eye through its transparent domed front, the cornea. They pass into the eye through the pupil, a hole in a ring of muscle known as the iris. The iris makes the pupil smaller in bright conditions, to prevent too much light damaging the eye’s delicate interior. The rays are bent or focused by the lens, and shine a clear image on to the retina, which lines the rear of the eyeball. When light hits the retina, its 130 million microscopic cells make nerve signals, which pass along the optic nerve to the brain.
HEARING
The curly flap of skin and gristle on the side of the head, which we call the ear, is simply a funnel shape for collecting sound waves from the air around. The waves travel along a slightly curved tube, the ear canal, and strike the ear drum, a small, thin piece of skin, which vibrates. The vibrations pass along a row of three tiny bones, the hammer, anvil and stirrup. The stirrup sends the vibrations into a snail-shaped part filled with fluid, known as the cochlea. The vibrations ripple through the fluid and shake microscopic hairs sticking out of nerve cells. When the hairs shake, the cells produce nerve signals, which travel along the auditory nerve to the brain.
TASTE
The tongue’s upper surface is covered with many pimple-like lumps, known as papillae. These grip food to move it around while chewing. Scattered between the papillae are about 8000 taste buds. Each one has some 30 microscopic taste cells and looks like a tiny onion set into the tongue’s surface. When substances of a certain flavour in foods touch micro-hairs sticking up from the taste cells, the cells generate nerve signals which pass to the brain. Different tongue areas respond to different flavours.
SMELL
In each side of the nose is an air chamber about as large as a thumb. Lining the roof of the chamber is a patch of 25 million smell cells. Each has more than 20 tiny hairs sticking from it. When certain odour substances touch the hairs, their cells send nerve signals to the brain. The nose can detect 10,000 different scents and smells.
HORMONES
Two systems help the body’s parts and organs work together. One is the nervous system. The other is the hormonal or endocrine system, based on body chemicals called hormones. There are more than 50 different hormones. Each is made in a gland. Hormones flow around the bloodstream and affect certain cells, tissues and organs. They may cause them to work faster or slower, or release their products. For example, adrenaline from the adrenal gland makes the heart beat faster and more blood flow to the muscles, so the body is ready for action. The pituitary gland near the brain makes hormones that control other hormonal glands.
Picture Credit : Google
