My Science School

Over millions of years, evolution is changing the way humans look. Over a shorter period, improved nutrition and medical discoveries have meant that people in some parts of the world today are generally bigger and stronger than their ancestors. But we are also losing some abilities that no longer seem useful. The smallest toe, for example, can no longer be moved independently by most people. As recently as Roman times, some people may have been able to “prick up their ears”, moving them slightly towards sounds as some animals can.

Humans are getting taller; they’re also fatter than ever and live longer than at any time in history. And all of these changes have occurred in the past 100 years, scientists say. So is evolution via natural selection at play here? Not in the sense of actual genetic changes, as one century is not enough time for such changes to occur, according to researchers.

Most of the transformations that occur within such a short time period “are simply the developmental responses of organisms to changed conditions,” such as differences in nutrition, food distribution, health care and hygiene practices, said Stephen Stearns, a professor of ecology and evolutionary biology at Yale University.

But the origin of these changes may be much deeper and more complex than that, said Stearns, pointing to a study finding that British soldiers have shot up in height in the past century. ”Evolution has shaped the developmental program that can respond flexibly to changes in the environment,” Stearns said. “So when you look at that change the British army recruits went through over about a 100-year period, that was shaped by the evolutionary past.”

And though it may seem that natural selection does not affect humans the way it did thousands of years ago, such evolutionary mechanisms still play a role in shaping humans as a species, Stearns said.

“A big take-home point of all current studies of human is that culture, particularly in the form of medicine, but also in the form of urbanization and technological support, clean air and clean water, is changing selection pressures on humans,” Stearns told Live Science.

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Veins are blood vessels that carry blood to the heart, while arteries carry it from the heart. The heart acts as a pump, pushing blood to every part of the body. Adults have between five and six litres (between nine and ten pints) of blood. As well as containing red cells to carry oxygen to the body’s organs, blood also plays an important part in fighting infection. White blood cells attack and digest harmful bacteria, while platelets in the blood form clots so that wounds can heal and no further infection can enter the body.

Arteries and veins both carry blood around the body, and they each have three main layers of tissue (a ring of endothelial tissue at the centre of the blood vessel surrounded by a layer of muscle and elastic fibres, which is surrounded by a layer of connective tissue). However, there are several differences between them:

1. Arteries carry blood from the heart to the rest of the body, whereas veins carry blood from the rest of the body back to the heart.


2. Almost all arteries carry oxygenated blood and almost all veins carry deoxygenated blood. The only exceptions are the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, and the pulmonary vein, which carries oxygenated blood from the lungs to the heart.


3. Arteries have a thick elastic muscle layer, whereas the muscle layer for veins is much thinner. This is because the heart pumps blood into the arteries at high pressures, so the walls of the arteries must be able to cope with the changes in pressure during a heartbeat. Veins carry blood at much lower pressures so do not need such a thick wall.


4. Arteries have a much narrower lumen (the hole at the centre that the blood flows through) than veins. This helps keep higher blood pressures in the arteries, which is needed to keep blood flowing quickly to body tissues.


5. Veins have valves and arteries do not. In arteries, blood flows in the right direction because of the heart pumping it forwards at high pressures. The lower blood pressure in veins means that valves are needed to stop blood flowing backwards (for example, in veins in the legs, blood needs to flow upwards against the pull of gravity).

Food is the fuel that our bodies need for movement. But we also need some fuel simply to maintain all the parts of our bodies. Individual cells are being renewed all the time. And even if we do not move the outside of our bodies at all, there are many parts inside that are constantly in motion. How much food we need depends on our size, age, gender and level of activity.

Nutrition is how food affects the health of the body. Food is essential—it provides vital nutrients for survival, and helps the body function and stay healthy. Food is comprised of macronutrients including protein, carbohydrate and fat that not only offer calories to fuel the body and give it energy but play specific roles in maintaining health. Food also supplies micronutrients (vitamins and minerals) and phytochemicals that don't provide calories but serve a variety of critical functions to ensure the body operates optimally.

Protein: Found in beef, pork, chicken, game and wild meats, fish and seafood, eggs, soybeans and other legumes included in traditional Central America cuisine, protein provides the body with amino acids. Amino acids are the building blocks of proteins which are needed for growth, development, and repair and maintenance of body tissues. Protein provides structure to muscle and bone, repairs tissues when damaged and helps immune cells fight inflammation and infection.

Carbohydrates: The main role of a carbohydrate is to provide energy and fuel the body the same way gasoline fuels a car. Foods such as corn, chayote, beans, plantains, rice, tortilla, potatoes and other root vegetables such as yucca, bread and fruit deliver sugars or starches that provide carbohydrates for energy.

Energy allows the body to do daily activities as simple as walking and talking and as complex as running and moving heavy objects. Fuel is needed for growth, which makes sufficient fuel especially important for growing children and pregnant women. Even at rest, the body needs calories to perform vital functions such as maintaining body temperature, keeping the heart beating and digesting food.

Fat: Dietary fat, which is found in oils, coconut, nuts, milk, cheese, meat, poultry and fish, provides structure to cells and cushions membranes to help prevent damage. Oils and fats are also essential for absorbing fat-soluble vitamins including vitamin A, a nutrient important for healthy eyes and lungs.

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There are more than 600 muscles in the human body. Over 100 of these are in our faces, which is why we can have so many different expressions. Although we can perform a great variety of movements, each muscle can only do one thing: contract. That is why muscles often work in pairs, so that one muscle can move a part of the body in one direction, while its partner can move it back again. Perhaps the most important muscle in the human body is the heart, which is contracting and relaxing all the time to pump blood around the body.

There are about 700 named skeletal muscles in the human body, including roughly 400 that no one cares about except specialists. There is just one important cardiac muscle. And there are literally countless smooth muscles (which do the work of the autonomic nervous system, mostly squeezing and squishing stuff in tubes).

It’s surprisingly hard to tell. You wouldn’t think the total number would be ambiguous, but it’s difficult to know what to include and exclude, and anatomists don’t always agree. Some muscle tissue really can’t be separated into countable muscles. And, believe it or not, the science of anatomy is still advancing. No, entirely new muscles aren’t being discovered — but novel variations in individual muscle anatomy are found more or less constantly, and supernumerary muscles — extra muscles — are not unusual. Many muscles, like the four-part quadriceps, are normally split into different parts that may or may not traditionally count as separate muscles — but then some people’s muscles are more divided than others. It makes a firm count just about impossible.

There are only about 200 to 300 muscles that anyone, even a massage therapist, might actually be interested in knowing about. When most people ask how many muscles are in the human body, they mean the serious bone-movers — Pecs, delts, lats, traps, glutes, biceps & triceps, hams & quads & let’s not forget the cloits & dloits!muscles that do real work, muscles like pecs, delts, lats, traps, glutes, biceps and triceps, hams and quads, and let’s not forget the cloits and dloits! There are maybe another hundred muscles if you include the fiddly little muscles of the hands and feet, and the major face muscles.

But that’s including about 600 muscles that, mostly, no one cares about except specialists. I am aware of a few that have clinical importance to a massage therapist, but I’m mostly just barely aware of their existence — like the smaller facial muscles, like the mess of little muscles around and under the tongue and around the voice box, like the muscles around the eyeball, or the crazy trampoline of muscles on the pelvic floor.

Our bodies are very complicated. It is impossible to think about all the processes that are going on inside them at the same time, so doctors often consider the body as being made up of several different systems, each one with different organs and mechanisms working together to perform particular functions.

Organ Systems

Different organs can work together to perform a common function, like how the parts of your digestive system break down food. We refer to an integrated unit as an organ system. Groups of organ systems work together to make complete, functional organisms like us! There are 11 major organ systems in the human body, which include the circulatory, respiratory, digestive, excretory, nervous and endocrine systems. The immune, integumentary, skeletal, muscle and reproductive systems are also part of the human body.

The Circulatory & Respiratory Systems

The circulatory system is responsible for transporting blood throughout the body. It consists of the heart and blood vessels known as veins, arteries and capillaries. Think of blood vessels as the highways of the body, bringing important cargo to and from the cells. In the circulatory system, blood is pumped from the heart to the lungs, so they'll get oxygen, and then pumped to the body's cells. Here is a diagram of the human circulatory system, including the heart and major arteries, which are in red, and veins, which are in blue.

In order for blood to provide oxygen to the body, the body must have a way of obtaining that oxygen. The respiratory system allows air to enter the lungs and for oxygen to diffuse into the blood en route to the body's tissues. The entrance to the respiratory system can be found in the nose and the mouth, where air enters the body and then travels through the larynx and pharynx in the throat to the trachea or windpipe. From the trachea, right and left branches, known as bronchi, carry oxygen to the alveoli, where oxygen moves into the blood, while carbon dioxide moves into the lungs to be exhaled.

Digestive & Excretory Systems

The digestive system is responsible for bringing food into the body and breaking it down to useable components. It starts at the mouth, where we ingest our food and use our saliva, teeth and tongue to bite and mash it. The food then travels through the esophagus into the stomach, where strong acids break it down even further. During the last two stages of digestion, nutrients and water are absorbed through the small intestine and the large intestine, respectively. Any remaining waste products are stored in the rectum and eliminated through the anus.

The urinary or excretory system is where liquid waste is eliminated as urine. The excretory system starts with the kidneys, important organs for cleaning the blood and balancing water in the body. In the excretory system, the liquid part of the blood, or plasma, enters through the kidneys, where important nutrients, like sugar and some salt, are reabsorbed into the body. Compounds we don't need, like urea or excess water, are sent to the bladder in the form of urine. Urine leaves the body through the urinary tract and exits the body at the urethra.

Nervous, Endocrine & Immune Systems

Without a master control system that tells our bodies what to do, none of the organ systems we've talked about so far would work. The organs in the human nervous system are made up of cells, called neurons that use chemicals and electricity to send messages. This system has two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and the spinal cord, which serve as the main control centers for the body and process all incoming and outgoing messages. The peripheral nervous system includes all the nerves in your body that bring messages to the central nervous system and from the CNS to the muscles.

Whereas the nervous system mainly uses electrical signals to communicate between cells, the endocrine system relies upon chemicals, called hormones, to send long distance messages through the body. The main organs found in the human endocrine system are located in the brain and include the hypothalamus, thalamus and pituitary gland. They talk to other endocrine organs, like the adrenal glands, testes and ovaries to assist with other organ systems.

There is much that we do not yet know about how the brain works, but we do know that the brain communicates with the rest of the body through a thick cord of nerves running down the middle of the spine and branching off to reach the limbs and internal organs. The nerves are pathways for messages to the brain, to inform it about what is happening elsewhere in the body, and from the brain to tell the rest of the body how to act. These messages, and the processes happening within the brain, are made up of tiny electrical impulses. By far the largest part of the brain is the cerebrum, which is divided into two halves, called hemispheres. The rest of the brain is made up of the cerebellum, the pons and the medulla, which join together at the top of the spinal cord.

With 80-100 billion nerve cells, known as neurons, the human brain is capable of some astonishing feats. Each neuron is connected to more than 1,000 other neurons, making the total number of connections in the brain around 60 trillion! Neurons are organized into patterns and networks within the brain and communicate with each other at incredible speeds.

The largest part of the human brain is the cerebrum, which is divided into two hemispheres, according to the Mayfield Clinic. Underneath lies the brainstem, and behind that sits the cerebellum. The outermost layer of the cerebrum is the cerebral cortex, which consists of four lobes: the frontal, parietal, temporal and occipital.

Like all vertebrate brains, the human brain develops from three sections known as the forebrain, midbrain and hindbrain. Each of these contains fluid-filled cavities called ventricles. The forebrain develops into the cerebrum and underlying structures; the midbrain becomes part of the brainstem; and the hindbrain gives rise to regions of the brainstem and the cerebellum.

The cerebral cortex is greatly enlarged in human brains and is considered the seat of complex thought. Visual processing takes place in the occipital lobe, near the back of the skull. The temporal lobe processes sound and language, and includes the hippocampus and amygdala, which play roles in memory and emotion, respectively. The parietal lobe integrates input from different senses and is important for spatial orientation and navigation.

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It is not gravity that causes food to move through the long tube that is our digestive tract. In fact, even if you stood on your head, food would still move through your oesophagus and intestines. Muscles in the walls of these organs squeeze and release rhythmically to move the partly digested food along.

The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle.

The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.

The first major muscle movement occurs when food or liquid is swallowed. Although we are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves.

The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass.

The food then enters the stomach, which has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material.

The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. (The mixture is referred to as chyme.)

The third task of the stomach is to empty its contents slowly into the small intestine. Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and the degree of muscle action of the emptying stomach and the next organ to receive the contents (the small intestine).

As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion.

Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa. These materials are propelled into the colon, where they remain, usually for a day or two, until the feces are expelled by a bowel movement.

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The food that we eat travels slowly through our bodies, a journey of up to ten metres (nearly eleven yards), taking about two days. As it passes through the various stages of our digestive system, chemicals called enzymes act on the food to make different parts of it useful to the body. Anything that cannot be used is passed out when we go to the toilet.

Food moves through your GI tract by a process called peristalsis. The large, hollow organs of your GI tract contain a layer of muscle that enables their walls to move. The movement pushes food and liquid through your GI tract and mixes the contents within each organ. The muscle behind the food contracts and squeezes the food forward, while the muscle in front of the food relaxes to allow the food to move.

Mouth: Food starts to move through your GI tract when you eat. When you swallow, your tongue pushes the food into your throat. A small flap of tissue, called the epiglottis, folds over your windpipe to prevent choking and the food passes into your esophagus.

Esophagus: Once you begin swallowing, the process becomes automatic. Your brain signals the muscles of the esophagus and peristalsis begins.

Lower esophageal sphincter: When food reaches the end of your esophagus, a ring-like muscle—called the lower esophageal sphincter —relaxes and lets food pass into your stomach. This sphincter usually stays closed to keep what’s in your stomach from flowing back into your esophagus.

Stomach: After food enters your stomach, the stomach muscles mix the food and liquid with digestive juices. The stomach slowly empties its contents, called chyme, into your small intestine.

Small intestine: The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine, and push the mixture forward for further digestion. The walls of the small intestine absorb water and the digested nutrients into your bloodstream. As peristalsis continues, the waste products of the digestive process move into the large intestine.

Large intestine: Waste products from the digestive process include undigested parts of food, fluid, and older cells from the lining of your GI tract. The large intestine absorbs water and changes the waste from liquid into stool. Peristalsis helps move the stool into your rectum.

Rectum: The lower end of your large intestine, the rectum, stores stool until it pushes stool out of your anus during a bowel movement.

In the days of sailing ships, sailors could be at sea for months on end. Fresh fruits and vegetables, containing vitamin C, could not be kept fresh for long voyages. As a result of a lack of vitamin C, also known as ascorbic acid, sailors developed a condition called scurvy. This distressing condition caused bleeding gums, weak-ness and dizziness. In the eighteenth century it was discovered that limes could cure these symptoms.

Imagine you are on a sailing ship in 1747. You left England only a couple of months ago and you felt fine. Now you are so tired you can barely walk. Your gums are swollen and so sore it hurts when you are eating. Your teeth are falling out. When you look at your legs you notice they are swollen and purple from bruising.

Lucky for you, a passenger on your ship is very interested in your condition. His name is Dr. James Lind. Dr. Lind wants to discover what is causing you so much pain that you can't work. After examining all the sailors on the ship he finds 11 more sailors that feel the same as you do. Dr. Lind divides you into six groups with two sailors in each group. On May 20, 1747 he is ready to begin the first clinical nutrition experiment.

Dr. Lind hypothesized that something was missing from the diet of all the sailors who were sick. By giving each group different treatments he hoped to locate what was missing from the diet of you and your fellow sailor's. Here is a list of what Dr. James Lind gave each group:

Group 1 drank one quart of cider a day, group 2 gargled with sulfuric acid, group 3 had two spoonfuls of vinegar, 3 times a day, group 4 drank 1/2-pint seawater a day, group 5 drank barley water and group 6 (you & another sailor) ate two oranges and 1 lemon a day.

You and the other lucky sailor who ate the oranges and lemon felt better. In fact, in only six days you felt great and were able to start working again.

Can you guess what made you feel better? If you guessed that it was from the vitamin C in the oranges and the lemon you ate, you are correct! The disease you and your fellow sailors were suffering from is scurvy. Scurvy is a disease caused by a vitamin C deficiency.

When the sailors began their voyage they had fresh fruits and vegetables on their ship. Fruits and vegetables are hard to keep fresh, so the sailors had to eat them right away. If the sailors were at sea for many months they would not have fruits and vegetables for most of this time.

Scurvy was a huge problem for English sailors in the 1600s and 1700s. Because of what Dr. James Lind discovered, the Royal Navy made sure that all sailors had lemon juice to drink when they were at sea for longer than one month. The sailors thought that it was the acid content of the lemon juice that cured scurvy (vitamin C is also called ascorbic acid). Doctors thought that lime juice would work better because it has more acid than lemon juice, so they substituted lime juice for lemon juice on the English Royal Navy ships.

And that is how the English sailors became known as Limeys! Since that time the word has been used as a negative slang term to describe British Nationals, and is therefore not considered a polite word to use.

Vitamins are chemicals that we need to stay healthy. They are referred to by the letters A, B, C and so on. Some of them are stored in the body but others, such as vitamin C, need to be eaten every day.

If you are like most people, you've probably heard at least one of these sayings: 'Don't forget to take your vitamins!' or 'Eat your veggies -- they are packed with vitamins!' or maybe 'Need more energy? Take your vitamins!' what exactly are vitamins?

Vitamins are nutrients your body needs to function and fight off disease. Your body cannot produce vitamins itself, so you must get them through food you eat or in some cases supplements. There are 13 vitamins that are essential to your body working well. Knowledge of the different types and understanding the purpose of these vitamins are important for good health.

There are two types of vitamins: fat-soluble and water-soluble. Fat-soluble vitamins are stored in your fat cells, consequently requiring fat in order to be absorbed. Water-soluble vitamins are not stored in your body; therefore, they need to be replenished daily. Your body takes what it needs from the food you eat and then excretes what is not needed as waste. Here is a list of some vitamin types and common food sources:

Fat-Soluble Vitamins

• Vitamin A - comes from orange colored fruits and vegetables; dark leafy greens, like kale


• Vitamin D - can be found in fortified milk and dairy products; cereals; (and of course, sunshine!)


• Vitamin E - is found in fortified cereals; leafy green vegetables; seeds; nuts


• Vitamin K - can be found in dark green leafy vegetables; turnip/beet greens

Water-Soluble Vitamins

• Vitamin B1, or Thiamin - come from whole grains; enriched grains; liver; nuts; seeds


• Vitamin B2, or Riboflavin - comes from whole grains; enriched grains; dairy products


• Vitamin B3, or Niacin - comes from meat; fish; poultry; whole grains


• Vitamin B5, or Pantothenic Acid - comes from meat; poultry; whole grains


• Vitamin B6, or Pyridoxine - comes from fortified cereals; soy products


• Vitamin B7, or Biotin - is found in fruits; meats


• Vitamin B9 or Folic Acid (Folate) - comes from leafy vegetables


• Vitamin B12 - comes from fish; poultry; meat; dairy products


• Vitamin C - comes from citrus fruits and juices, such as oranges and grapefruits; red, yellow, and green peppers

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Human beings need a certain amount of food each day to supply them with energy. Almost all foods can supply some energy, but our bodies have other requirements as well. In order to make sure that we are taking in everything we need, we should eat a wide variety of foods, with the correct amounts of carbohydrates, fat and protein. A diet that fulfills these requirements is called a balanced diet.

Every diet needs a balance of three main categories of nutrients: carbohydrates, fats, and proteins. Learn how to create a balanced diet, which contains the correct amounts of nutrients, vitamins, and minerals.

Before we look at creating a balanced diet, let's talk a little bit about calories. Calories are units of energy in our food. If you have too many calories, your body will store the extra energy as fat. If you have too few calories, your body will burn fat to make up the difference.

From this perspective, it might seem like fewer calories is better. However, our body always wants to be in balance, including having a specific number of calories and balanced nutrients. Our body spends a considerable amount of energy maintaining these balanced conditions, or homeostasis.

Everyone will need a different number of total calories depending on your sex, age, and activity levels. The United States Department of Agriculture (USDA) recommends that moderately active males between 26 and 30 years old get about 2,600 calories per day, while females of the same age and activity level get 2,000 calories per day. Although everyone's body is slightly different, scientists and doctors recommend a general division of calories by nutrient category for all humans.

Nutritionists recommend that we should eat something from each of these food groups every day. Carbohydrates give us energy. Protein is needed to build and repair cells and to keep our bones, muscles, blood and skin healthy. Fruits and vegetables contain energy and a wide range of essential vitamins and minerals. Dairy foods contain protein and calcium for healthy bones and teeth.

Food is our fuel, and its nutrients give our bodies' cells the energy and substances they need to work. But before food can do that, it must be digested into small pieces the body can absorb and use.

The first step in the digestive process happens before we even taste food. Just by smelling that homemade apple pie or thinking about how delicious that ripe tomato is going to be, you start salivating — and the digestive process begins in preparation for that first bite.

Digestion begins as soon as we put food in our mouths. Saliva starts to digest the carbohydrates in the food as we chew it. Chewing breaks the food up into small pieces that can pass easily down the esophagus and into the stomach, where powerful acids begin to digest proteins and kill harmful bacteria. The stomach is not still. Its muscular walls churn the food into a thick, soupy consistency.

Along the way, food is broken down into tiny molecules so that the body can absorb nutrients it needs:

Protein must be broken down into amino acids.

Starches break down into simple sugars.

Fats break down into fatty acids and glycerol.

The waste parts of food that the body can't use are what leave the body as feces.

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Living things have evolved and adapted over millions of years to live successfully in very different temperatures. Penguins at the South Pole may live quite happily in temperatures of —50°C (-58°F), while some bacteria live near deep-sea vents that are gushing out water at close to boiling point.

The current temperatures are excellent for the northern hemisphere. Today the weather and climate are mild with an excellent growing season. If there was a little more CO2 for the plants and the temperature goes up some it would be ideal. The fear that the planet will crumble as the thermostat goes up is sheer nonsense. Man has adapted to not only a warmer climate, but extreme climates. Just remember, cold kills faster than heat.

There is so much fear mongering regarding climate change, while humans have not had it this good for some time. Regardless if there were an ideal global temperature, mankind does not have the ability or technology to regulate the climate. Good thing too! Every time man has tried to control nature he has made a mess of things.

I value humans, so the mean temperature should be something suitable for humans. On the upside, what’s suitable for humans is also suitable for everything that’s been living on the planet for the past 10,000 years.

The current global average temperature is about 59 degrees F. This is almost two full degrees warmer than the global average from 1951–1980. What is the average global temperature now? However, for the past 10,000 years or so the global climate has only fluctuated a couple of degrees F around the current global mean. Climate change during the Holocene (Past 1200).

In that case, I would want the global mean to be the average for the past 10,000 years. This is the climate in which humans developed civilization, and is the climate which all living things on the planet are currently adapted to. Keeping to this temperature isn’t going to happen, however. The climate has already warmed nearly 2.5 degrees F since the Industrial Revolution, and by 2100 is expected to warm an additional 3.6 degrees F (2 degrees C) to 6 degrees C, which is an absurd amount of warming.

Milankovitch Cycles includes variations in the Earth's eccentricity, axial tilt, and precession comprising the three dominant cycles with a periodicity of 100,000 years. Taken in unison, variations in these three cycles create alterations in the seasonality of solar radiation reaching the Earth's surface. These times of increased or decreased solar radiation directly influence the Earth's climate system, thus impacting the advance and retreat of Earth's glaciers.

Man may try to change his carbon footprint but celestial mechanics are beyond human grasp. The hottest temperature ever recorded was 159.3 F (70.7 C) and the coldest temperature was minus 128.6 F. The average temperature on Earth is 61 degrees F (16 C). A few degrees up or down represents a smaller change than daylight and nighttime swings. In the final analysis there is no ideal temperature, at least as far as the planet is concerned.

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The back of the eye is very sensitive, but at one point the optic nerve leads to the brain. If the image falls on this point, called the blind spot, it cannot be seen. The blind spot is particularly important for drivers, as there is a point behind them on either side where their vision can be misleading as they check for vehicles behind before overtaking.

Blind spot, small portion of the visual field of each eye that corresponds to the position of the optic disk (also known as the optic nerve head) within the retina. There are no photoreceptors (i.e., rods or cones) in the optic disk, and, therefore, there is no image detection in this area. The blind spot of the right eye is located to the right of the centre of vision and vice versa in the left eye. With both eyes open, the blind spots are not perceived because the visual fields of the two eyes overlap. Indeed, even with one eye closed, the blind spot can be difficult to detect subjectively because of the ability of the brain to “fill in” or ignore the missing portion of the image.

The optic disk can be seen in the back of the eye with an ophthalmoscope. It is located on the nasal side of the macula lutea, is oval in shape, and is approximately 1.5 mm (0.06 inch) in diameter. It is also the entry point into the eye for major blood vessels that serve the retina. The optic disk represents the beginning of the optic nerve (second cranial nerve) and the point where axons from over one million retinal ganglion cells coalesce. Clinical evaluation of the optic nerve head is critical in the diagnosis and monitoring of glaucoma and other optic neuropathies that may lead to vision loss.

The human eye is a ball containing a kind of jelly that keeps it round, just as the air in a balloon keeps that round. At the front is a lens, through which light can enter the eye. The lens can change shape to focus light from different distances. Like a slide projector, the lens throws an image onto the back of the eye, which is called the retina. In fact, the image is upside down, but your brain, which makes sense of everything you see, sorts out the image so that you “see” it the right way up.

Our vision allows us to be aware of our surroundings. Eighty per cent of everything we learn is through our sight. Your eye works in a similar way to a camera. When you look at an object, light reflected from the object enters the eyes through the pupil and is focused through the optical components within the eye.

The front of the eye is made of the cornea, iris, pupil and lens, and focuses the image onto the retina. The retina is the light sensitive membrane that covers the back of the eye. This membrane consists of millions of nerve cells which gather together behind the eye to form a large nerve called the optic nerve.

When the light enters the eye, it is focused to a pinpoint on the macula, a small area in the centre of the retina at the back of the eye. The macula is responsible for central detailed vision, allowing you to see fine detail and colour, read and recognise faces.

When light stimulates the nerve cells in the retina, messages are sent along the optic nerve to the brain. The optic nerves from the two eyes join inside the brain. The brain uses information from each optic nerve to combine the vision from the two eyes allowing you to see one image.