When you reflect on the remarkable abilities of various animals, do you sometimes feel a twinge of envy? Perhaps you wish that you could soar like an albatross, swim like a dolphin, see like an eagle, or run like a cheetah.
Yes, animals have some amazing abilities. But so do we! Indeed, the human body has been described as the perfect machine. Of course, we are much more than a machine. We have creativity, curiosity, imagination, and ingenuity –qualities that move us to devise machines that enable us to do virtually anything we set our minds to.
We can fly, even beyond the speed of sound; navigate above or below the surface of vast oceans; gaze some 14 billion light-years into space; peer into the living cell; and design medicines, therapies, and technologies that help us diagnose and treat diseases.
Even with little or no external assistance, healthy, well-trained humans are capable of doing astonishing things. At the Olympic Games, for example, gymnasts, high divers, ice-skaters, skiers, and others perform amazing feats with a level of agility, artistry, creativity, and grace that leave audiences enthralled.
Do you appreciate the special gifts that you have as a human? Granted, you may not be an Olympic athlete, but you have many gifts for which to be thankful.
BUILT TO CONSERVE ENERGY
Our upright stance is very energy efficient, for it demands little muscle action to sustain the vertical alignment of our body. In fact, we use only 7% more energy standing than when lying down, says neuroscience researcher John R. Skoyles. He adds that a dog uses about 70% more energy when standing [on all fours] than when lying down.
The human body is outstandingly versatile. No animal has the sheer range of abilities that we humans do. One reason for our versatility is our upright stance, which not only expands our area of vision but also frees our arms and hands for any number of tasks. Imagine how our activities would be curtailed if we had to walk on all fours!
Another asset is our highly sophisticated sensory system, which will be the focus of this article. The system includes the hands, the ears, the eyes and, of course, our exceptional brain. Let’s look at these individually.
THE HUMAN HAND
Our hands are beautiful instruments of amazing precision. With them we can thread a needle or swing an ax, paint a portrait or play the piano. Our hands are also highly sensitive. Even a brief touch may reveal whether a substance is fur, paper, skin, metal, or wood. Yes, our hands are much more than implements for grasping and manipulation. They are also a source of knowledge about our world. And they are a means of conveying warmth and affection.
Why is the human hand so adept, so expressive, so sensitive, and so versatile? The reasons are many. Consider four.
1. Our two hands have a total of more than 50 bones, about a quarter of all bones in the body. The intricate assembly of the parts of the hand –the bones, the joints, the ligaments –gives the human hand extraordinary flexibility.
2. The hand has an opposable thumb mounted on a saddle joint, an ingenious configuration of two saddle-shaped surfaces at right angles to each other. This joint, along with the associated muscles and other tissues, give the thumb amazing flexibility and strength.
3. Three sets of muscles control the hand. The two most powerful sets –the extensors and the flexors –are in the forearm and operate the fingers by means of tendons. How bulky and unwieldy the hand would be if these muscles were located in it! The third set, much smaller, which does lie within the hand, gives the fingers precision of movement.
4. Your fingers are, in effect, living sensors –the fingertips having about 2,500 receptors in just one square centimeter. Moreover, the receptors are varied, each kind having its own function, enabling you to feel texture, temperature, wetness, vibration, pressure, and pain. As a result, the human finger is the most sensitive touch sensor known.
THE HUMAN EAR
Although some animals can hear sound frequencies beyond the range of human hearing, the combination of a human’s ears and brain is a formidable one, say audio experts. Our hearing enables us to determine loudness, pitch, and tone and to approximate the direction and distance of a sound source.
The frequency range of a healthy human ear is roughly 20 to 20,000 hertz, or cycles of sound oscillation per second. The most sensitive region is in the 1,000 to 5,000 hertz range. Moreover, we may be able to detect a change of just one hertz from, say, 440 hertz to 441 hertz.
Indeed, a healthy ear is so sensitive that it can detect sounds when the vibration, or to-and-fro movement, of the air at the eardrum is less than the diameter of an atom! According to a university course on hearing, “the human hearing system is close to the theoretical physical limits of sensitivity… There would be little point in being much more sensitive to sound, as all would hear would be a ‘hiss,’” the result of the random movement of the atoms and molecules that make up the air.
Eardrum vibrations are amplified mechanically by lever action and are transferred to the inner ear by means of the OSSICLES –tiny bones known as the hammer, the anvil, and the stapes. But what if your ears are suddenly struck by a deafening sound?
In that event, they have built-in protective mechanism in the form of muscle action that adjusts the OSSICLES to reduce the force of the sound. However, the ears are not equipped to deal with prolonged loud noise. Such exposure can permanently damage the hearing.
Your auditory system also helps you to detect a sound source. The secret lies in a number of factors, including the shell-like shape of the outer ears, its grooves, the separation of the two ears, and some computational brilliance on the part of your brain. Thus, if the intensity of a sound fades just slightly from ear to ear or if the sound reaches one ear just 30 millionths of a second before it reaches the other, your brain will promptly point your eyes toward the sound source.
THE HUMAN EYE
Some researchers estimate that people who can see well gain approximately 80 percent of their information about the world through their eyes. In combination with our brain, our eyes enables us to see in full color, to track moving objects and images smoothly, to recognize patterns and shapes, and to see in three dimensions. Furthermore, we can see in varying degrees of light.
The latter involves a number of complementary mechanisms. For example, the pupil can expand from 1.5 millimeters to 1.8 millimeters in diameter, resulting in a possible 30-fold increase in the amount light entering the eye. The light then passes through the lens, which focuses it onto the retina, concentrating the light energy by a factor of 100,000 times. So never look directly at the sun with the naked eyes!
The retina, in turn, houses two types of photoreceptors –cones [approximately 6 million], which give us color vision and high resolution, and rods [120-140 million], which are more than a thousand times as sensitive as the cones and help us to see in dim light. Indeed, under optimal conditions, a rod can detect a single photon, or elementary particle of light!
Another adaptive mechanism involves retinal neurons linked to the cones and rods. These neurons adapt “in seconds and can improve night vision by a factor of 10 or more. Neural adaptation is rather like having low-speed and high-speed film simultaneously available in your camera.
THE HUMAN BRAIN
With astonishing efficiency, the brain decodes the streams of signals pouring in through the nerves from the sense organs. Moreover, it links these signals with details stored in its memory. Thus, a certain smell may immediately trigger the brain to retrieve a long-forgotten experience or event.
And if you see just a small part of something familiar –the tip of your cat’s tail, for example –your brain will fill in the missing details so that you know your cat is nearby.
Of course, your brain was not preprogrammed with images of cats, just as it was not preprogrammed with the smell of a rose or the sound of running water or the feel of fur.
Your brain learned these associations. The experiences of people who were born blind but have been given the ability to see, perhaps through surgery, make this evident. Their brain had to learn to interpret the flood of visual signals now flowing to it. How do such people fare?
They soon report the ability to detect color, motion, and simple forms. But after that, progress varies. Children, especially the very young, continue to learn quite well. But that is not the case with adults. Even their ability to recognize faces remains highly impaired. And tragically, a common feature with “cured” adults “is initial euphoria followed by disappointment and disorientation with the restored vision, often leading to severe depression”.