86 Skin

CHAPTER 1 Skin THE ORGAN THAT'S ALWAYS ON DISPLAY. When we think of our body's organs, words like heart, brain, or stomach come to mind. However, there is one we often forget about— our skin. Skin is so important that it even forms part of our personal identity. Our skin's color, created by a chemical in the body called melanin, is one of the first things other people notice about us. The amount of melanin in our skin depends on our genes, though it can be increased through contact with sunlight. Skin is made up of three layers: the epidermis, the dermis, and the subcutis. The epidermis, the outer layer, includes dead skin cells that are constantly falling away and being replaced. It is a defensive layer that protects us from the sun, wind, and water.

Below the epidermis lies the dermis. This layer has small organs called sweat glands. These help control our body's temperature by sending a water-rich liquid— sweat— up to the epidermis. When this sweat reaches the skin, it has a cooling effect on the body. In the dermis are also millions of tiny nerve endings. These let us feel and judge everything we come into contact with, from a breath of wind to the temperature of water. The nerve endings in the dermis are found at different depths and react to the movement of body hair, pressure, vibration, and temperature. These nerve endings let us react quickly to our environment and avoid objects that might hurt us. Our sense of touch is possibly the most noticeable of the skin's characteristics because the feelings of pleasure or pain it provides are so powerful. Below the dermis lies the subcutis. It is made up of many fat cells that help the body hold in heat. These cells also help soften the stresses placed on the body. This layer also contains the roots of our hair.

CHAPTER 2 The First Line of Defense THE SKIN PLAYS A KEY ROLE IN THE FIGHT TO STAY ALIVE. Human skin can vary from as thick as 1.5 millimeters on the hands and feet to as thin as 0.05 millimeters around the eyes, which is why this is often the first place to show signs of aging. However, some animals, the rhinoceros for example, have skin that can be four centimeters thick! Skin, however, is more than just physical armor. It can also camouflage an animal from hunters, predators, or prey. This effect makes it possible for an animal to almost disappear into the background. Perhaps the best example of the power of camouflage is the chameleon. This lizard is so good at changing its appearance that we use the word chameleon to describe people who change depending on the situation.

However, only a few species of chameleon change color to camouflage themselves. One of them is the Smith's dwarf chameleon, which can seem almost invisible when not moving. Although many species of chameleons can change color, most do so not to hide, but because of changes in their emotional state or body temperature. The Namaqua Chameleon lives in the desert. It changes color to protect itself from the extreme temperatures. In the cool morning it turns dark to absorb, or take in, heat more efficiently. In the hot sun it turns a light gray to reflect, or keep out, the sun's heat. Some animals take advantage of color changes to protect themselves from predators over a longer period of time. Arctic foxes have two different coats, one for summer and one for winter. The winter coat is white to hide the fox in the snow. The fox loses this fur in spring and replaces it with a gray coat more suited to the gray rocks of the arctic summer. Interestingly, the arctic hare, a common prey of the fox, takes advantage of exactly the same color changes!

Other animals have permanent camouflage. A polar bear's white fur makes it almost disappear against the snow and ice, despite its huge size. Strangely enough, the skin below the polar bear's fur is black! Although the patterns on the fur of tigers and leopards are very different, both serve the same purpose to make it harder for their prey to see them. The patterns are different because they live in different places.

Leopards hunt in bush country. Their spots allow them to disappear into the surrounding bushes. Tigers live in the jungle. Their stripes help them hide in the light and shade from the trees and long grass. These stripes are so effective that "tiger stripe" camouflage is used by many armies as a jungle uniform. Sometimes, however, an animal's skin color seems like a strange choice, almost a mistake. Take the zebra, which seems to draw attention to itself with its striking black and white pattern. Although scientists don't yet know the exact reason for the pattern, one theory is that when many zebras run together in a group, they make a pattern which is visually confusing. This makes it difficult for predators, such as lions, to identify one zebra to attack. This idea was copied in the First World War when British boats were painted in many different patterns to confuse the enemy. However, the technique, known as dazzle camouflage, was never proven to work. Another theory suggests that zebra stripes are actually very effective in hiding the animal in long grass because lions are in fact colorblind!