Body parts - Curious Folks Ask: 162 Real Answers on Amazing Inventions, Fascinating Products, and Medical Mysteries - Sherry Seethaler

Curious Folks Ask: 162 Real Answers on Amazing Inventions, Fascinating Products, and Medical Mysteries - Sherry Seethaler (2009)

Chapter 3. Body parts

Toe the line

Why do some people, like me, have second toes that are longer than their big toes? Is it a genetic specificity? Is it a characteristic that occurs more in women than in men, or more in particular ethnic groups?

You are in good company. The Statue of Liberty has short big toes, or what is referred to as the Greek foot. Lady Liberty’s sculptor, Frédéric Auguste Bartholdi, was trained in the classical tradition, and Greek and Roman statues often have short big toes. Leonardo da Vinci drew skeletons with Greek feet, rather than so-called Egyptian feet, in which the big toe is longest.

Some cultures have considered short big toes to be a sign of intelligence. (Disclaimer: I have Greek feet.) Unfortunately, Greek feet got a bad rap when, in 1927, a doctor named Dudley Morton published a paper describing a foot disorder associated with short big toes.

According to Morton’s findings, the head of a short big toe cannot readily reach the ground and therefore does not carry its full share of the body’s weight. As a result, the second toe carries extra weight. Calluses develop on the ball of the feet beneath the second and third toes, and tenderness may develop in this area.

However, a study of more than 3,500 men enlisted in the Canadian Army during World War II revealed absolutely no relationship between toe length and distribution of weight on the foot or foot pain. The researchers concluded that feet develop ways to compensate for variants in structure.

The reason for the discrepancy between these findings and Morton’s may be that the vast majority of patients who came to see Morton complaining of foot pain caused by “Morton’s Toe” were women. Women are more likely to wear high-heeled shoes, which compound the problem by forcing the weight toward the front of the foot.

Researchers have reported varying incidences of Greek feet in different populations, ranging from 3 percent to 40 percent. Extreme cases, where the big toes are less than two-thirds the length of the second toes, are rare. The trait is thought to be genetic, with Greek feet being recessive and Egyptian feet being dominant.

Gender differences in the relative lengths of fingers and toes are small. Nearly all of the research on gender differences in digit length has focused on the ratio of the lengths of the index and ring fingers. Studies indicate that minor differences in this ratio, possibly caused by exposure to hormones in the womb, are associated with certain personality traits, susceptibility to disease, and even sexual orientation. These claims are controversial, because research in many areas of human health and behavior has shown that most traits are a result of complex interactions between nature and nurture.

Surgeons’ favorite organ

If the appendix is a relatively useless organ in our bodies, why do we have it? Did the appendix used to have a purpose in the bodies of early man?

Someone once said that the only function of the appendix is financial support of the surgical profession. About 7 percent of the population in developed countries will suffer from appendicitis in their lifetime, but appendicitis seems to be rare in undeveloped countries. It is unclear if diet or some other factor contributes to this difference.

In humans, the appendix is a wormlike pouch, three-and-a-half inches long on average, attached to the first part of the large intestine. In herbivorous mammals, such as rabbits, a much larger analogous structure houses bacteria that help break down cellulose, a large plant molecule. The appendix is present in many vertebrates, including other primates.

The human appendix does not contain cellulose-digesting bacteria, so humans cannot digest cellulose (which is why lettuce is roughage). Therefore, the appendix is often called a vestigial organ—a structure that has become diminished in size and lost its original physiological function.

That does not mean the human appendix has no function. Of the many functions hypothesized, a role in immunity is considered the most likely, although this remains controversial. The appendix, along with other parts of the digestive system, produces immune system cells, which can respond to ingested, disease-causing microbes. Whether the appendix contributes significantly to the immune response is unknown, since the lack of an appendix does not cause any obvious health problems.

Longer and longer

How can your nails continue growing throughout your life? How are they formed?

The first sign of nail formation occurs at week 10 of prenatal development, when a thickened area of skin called the primary nail field appears at the tip of each finger. The nail fields burrow into the skin, and the side and lower borders become thickened to form nail folds. The cells in the bottom nail fold continue to divide to produce the nail.

The fingernails reach the tips of the fingers by the end of the eighth month of pregnancy. The toenails, which begin development later than the fingernails, reach the tips of the toes just before birth. The extent of nail growth can be used as an indicator of how prematurely a baby has been born.

Most of what we can see of our nails is tightly packed layers of dead cells that are full of a tough protein called keratin. Keratin is also an important component of hair, feathers, beaks, horns, hooves, and the outermost layer of skin.

As new cells are produced in the nail’s germinal matrix (base or root), which is located under the skin behind the fingernail, they are pressed forward and upward into the nail. They die, but remain firmly attached to their neighbors to create the solid nail. As the nail streams along the nail bed, new cells produced in the bed are added to it, helping compensate for surface wear.

Tough tips

What purpose do toenails and fingernails serve?

They serve as mini body armor to protect the tips of our fingers and toes. Of course, fingernails also come in handy for scratching itchy spots and picking up small objects. A less obvious, but important, function of fingernails is to enhance the sensation in the fingertip.

When we use our fingertips to feel something, the nail acts as a counterforce. It increases the compression of the sensory organs between the pad of the finger and the nail, which makes us better able to distinguish fine detail on the surface we are touching.

Ashes to ashes

When a person is cremated, how much do the ashes weigh? Is there anything that doesn’t burn?

The weight of the cremains (cremated remains) depends on several factors: the temperature of the cremator furnace and the duration of the cremation, and the individual’s weight, height, age, and gender. On average, the cremains of a fully developed adult weigh 5 pounds (2.3 kilograms), or approximately 3.5 percent of body weight. The range is from around 2 pounds to 8 pounds.

Cremains are not really ashes. Most of what remains after cremation is bone, often sizeable fragments. The larger and heavier a person’s skeleton is, the greater the weight of his or her cremains. As a result, the cremains of men are 2 pounds heavier on average than the cremains of women. Also, the cremains of older adults are lighter on average than those of younger adults because bone density decreases with age.

The chemical composition of cremains is mainly calcium and phosphate—the major constituents of bone. Smaller amounts of carbon, potassium, sodium, chloride, magnesium, iron, and other minerals also remain. Melted metal from dental fillings and surgical implants, such as artificial hip joints, is usually removed, and the cremains are pulverized to give them the consistency of coarse sand.

Holey lids

When I was inserting my contact lenses, I noticed a tiny hole on the inside corner of my bottom eyelids. What are these holes?

They are called puncta and are the openings to the tiny canals through which tears drain. Tears flow from these canals into a tear sac and then down the tear duct into the nose. In fact, you can taste eyedrops after they flow from the puncta into your nose and drip onto the back of your tongue.

Producing peepers

When a human is developing, how are the eyes formed?

We start out as a ball of indistinguishable, genetically identical cells. A cell in what becomes the eye is different from a muscle cell or skin cell because it makes different proteins—the cell’s workhorses. For example, proteins called crystallins pack the lens of the eye and help focus light onto the retina.

During development, chemical signals released by other cells and physical contacts with other cells can tell a cell to switch on certain genes, thereby making it produce certain proteins. A gene called PAX6 is a master gene that initiates eye development. The same gene initiates eye development in fruit flies, and if researchers activate PAX6 randomly, flies end up with eyes in unusual places.

Eye formation begins in a 22-day-old human embryo. At this stage, the brain and head are tube-shaped and consist of sheets of cells. Outward bulges form in an inner sheet of cells. When these bulges—called optic vesicles—contact the outer sheet of cells, the formation of the eye’s lens is initiated. As the optic vesicles grow outward, their base narrows to form a stalk. This stalk eventually forms the optic nerve.

The side of the optic vesicle opposite the stalk pushes inward to become bowl-shaped, with the developing lens at its center. After many rounds of cell division, cell migration, and cell death, the layers of tissue making up the bowl behind the lens become the retina, with its ordered arrangement of light-sensitive rod and cone cells, supporting cells, and nerve cells that send electrical impulses to the brain.

A small opening, the pupil of the eye, remains in the bowl of the optic vesicle. The iris—the colored part of the eye, which is a muscle that expands and contracts the pupil—develops from the tissue in the optic vesicle that surrounds the future pupil.

The cornea, eyelids, and other parts of the eye develop in similar ways, with signals from other cells being paramount in switching on the appropriate genes for cells to take on the correct identity.

Both optic vesicles begin from a single patch of cells. Activation of a gene called sonic hedgehog (scientists have a lot of fun naming genes) is necessary for the splitting of this patch of cells so that two optic vesicles form. A mutation in the sonic hedgehog gene can result in cyclopia, a single eye in the center of the face. Infants born with cyclopia do not survive past birth because the condition is accompanied by brain defects.

Guardian lashes

What is the purpose of the lashes on your lower eyelid?

Their function is partly cosmetic—to frame those baby blues (or greens or browns)—but they also help protect the eye. They can deflect dust, foil insects, and shield the eye from reflected sunlight. If you gently touch the tips of your upper or lower lashes, you will see how exquisitely sensitive the nerves at the base of the lashes are to the deflection of the lashes. Because the lashes project outward, they trigger a protective blink reflex when an object comes too close to your eye.

Stopping short

Why don’t eyelashes grow beyond a certain length, unlike the hair on one’s head?

Some people who want thicker eyelashes have hair follicles from their scalp transplanted onto their eyelids. The transplanted hairs act like head hairs. They keep growing and require trimming.

Within each hair follicle (a pit containing the hair) is a biological “clock” that determines speed of hair growth and how long the hair grows before it falls out. Unfortunately for folks hoping for more hair on their heads, or less on their backs, the exact genes and molecules responsible for the hair cycle clock are still an enigma.

Grating habit

What happens when you crack your knuckles/joints? Is it bad for you?

Different parts of a joint—ligaments, tendons, cartilage, and synovial fluid—can snap, crackle, and pop for different reasons.

Ligaments connect bone to bone to strengthen the joint. Tendons connect muscle to bone and move the bone by transmitting the force created by the muscle. When a joint moves, cracking noises are created by the loosening and tightening of ligaments, as well as the change in position of tendons and their snapping back into place. Such noises are normal and are especially common in the knee and ankle joints.

On the other hand, grinding cartilage is a sign of an arthritic or injured joint. Smooth cartilage coats the ends of the bones that come together to form the joint. A joint capsule containing a lubricant surrounds the cartilage surfaces.

In a normal joint, the lubricated cartilage surfaces glide past each other with less friction than a skate on ice. Unfortunately, cartilage has very little ability to repair itself. Worn cartilage can make noise as it grates together. Loose pieces of cartilage can even break off and get caught in the joint, causing it to lock.

Some people can pop their knuckles by pulling on their fingers, which increases the space in the joint capsules. This reduces the pressure on the synovial fluid—the lubricant in the joints. Synovial fluid contains dissolved gases (carbon dioxide, oxygen, and nitrogen). Like the bubbles of gas that form when a bottle of sparkling water is opened, the reduced pressure on the synovial fluid can cause a bubble of gas to pop into existence. The bubble can be seen on an X-ray and takes approximately 20 minutes to redissolve in the synovial fluid.

Microphones on a knuckle detect two separate sounds when the joint is cracked. One is the sound of the gas bubble forming. The other is probably the sound of the joint capsule (which would be pulled inward slightly as the pressure in the joint decreased) snapping back into place because the formation of the gas bubble increases the pressure within the capsule.

Habitual knuckle crackers are not more likely to develop arthritis, but they are more likely to experience minor swelling and have poorer grip strength. However, the researchers who reported these findings pointed out that they do not prove knuckle cracking causes these problems. Only some people can crack their knuckles. It is possible that these people have looser ligaments to begin with, and the looser ligaments may predispose them to hand weakness and swelling.

Local harvest

What type of tissue would be used if stem cells were harvested from an adult?

Many adult tissues have stem cells, including the skin, gut, respiratory tract, liver, muscle, and brain, where they play a role in tissue repair and renewal. Not all of these stem cells are amenable to being harvested and transformed into other cell types.

Many studies have employed hematopoietic stem cells, which are found in bone marrow and generate all the types of cells in the blood. They have been used to treat blood disorders for three decades, and under the right conditions they can be coaxed to give rise to many other cell types.

Recently, researchers discovered stem cells in fat and have transformed them into other tissue types. It would be ideal if stem cells from fat prove to be as versatile as those from bone marrow. Liposuction is simpler than removing bone marrow, and even slender people carry a large-enough fat supply for their own treatment.

Cell selection

Embryonic stem cell research is controversial in many countries. Does adult stem cell research hold the same therapeutic promise(s)?

The jury is still out. Depending on who you ask, you will be told either that adult stem cells have demonstrated a surprising ability to transform into other cell types and repair damaged tissue, or that such transformations are relatively rare and can sometimes be accounted for by alternative explanations.

Embryonic stem cells are taken from three- to five-day-old embryos. These cells are exciting to researchers because at this stage, they have the potential to give rise to any cell type (muscle, bone, nerve, skin). On the other hand, it was initially thought that adult stem cells—which are found in many tissues (in children and adults), as well as umbilical cord blood and the placenta—could produce only progeny cells corresponding to their tissue of origin. For example, skin stem cells give rise to the various types of cells in the skin.

However, many recent studies suggest that adult stem cells can generate cell types other than those in their tissue of origin. Researchers coax stem cells into taking on a specific identity by selectively exposing them to chemicals that cells normally use to communicate with each other. Coaxing cells to take on a specific identity and verifying that they have indeed taken on that identity is technically challenging, and many studies have proven difficult to replicate.

Preliminary results of efforts to develop adult stem cell treatments do provide reason for optimism. For example, a few small human trials have shown that injecting adult stem cells into the blood stream can lead to some improvement in heart function after bypass surgery. But scientists still have much to learn about both embryonic and adult stem cells before clinical therapies live up to their promise.

If adult stem cell research advances to the point where scientists can consistently generate large numbers of cells of the tissue of interest, adult stem cells have three potential advantages over embryonic stem cells. First, ethical controversy has arisen over the destruction of embryos to obtain stem cells. Second, more research is needed to overcome the risk that rapidly dividing embryonic stem cells could lead to tumors. Third, using a patient’s own adult stem cells in a treatment would overcome the issue of immune rejection.

Whiter shade of pale

Why don’t some scars tan?

The most obvious possible explanation is that the scar tissue has fewer melanocytes—cells that produce the dark pigment melanin—than the surrounding skin. However, this does not appear to be the case.

In one study, researchers took biopsies from old, pale scars and from the adjacent normal skin of Caucasian volunteers. The researchers were surprised to discover that the number of melanocytes was about the same in scar tissue and nonscar tissue. In addition, the amount of melanin appeared to be similar in the scarred and normal skin.

The researchers proposed two hypotheses to explain why scars may appear pale even though melanocytes are present and appear to be functioning normally. First, scar tissue may have fewer blood vessels, resulting in decreased blood flow and whiter skin. Second, the structural properties of scar tissue can cause it to reflect light differently than normal skin.

In normal skin, fibers of the structural protein collagen are randomly oriented. As a result, skin scatters light in random directions. When skin is injured, the interwoven arrangement of collagen is destroyed. In an effort to repair the damage as quickly as possible, the body lays down new collagen fibers in linear strips parallel to each other. The scar reflects light mainly along a direction perpendicular to the skin.

Also, the upper layer of the skin over the scar may be thinner and may absorb less light. Thus, the scar may reflect more light toward the observer and appear whiter.

Healing potion

How does vitamin E lotion help scars?

Since vitamin E was found to be a major antioxidant in skin, physicians have recommended that patients apply it to injured skin to reduce scarring. Antioxidants mop up free radicals—highly reactive molecules that are produced at the site of a wound. Free radicals can damage cells and can also interfere with the production of collagen. Therefore, vitamin E should protect skin and promote healing.

Yet, despite its popularity, there is little scientific evidence that vitamin E reduces scarring. In fact, some studies have found the opposite. In one carefully designed study, published in Dermatologic Surgery (April 1999), patients applied a regular ointment (Aquaphor) to one side of a surgical wound and the same ointment mixed with vitamin E to the other side of the wound. In the majority of cases (90 percent), vitamin E had no effect or actually worsened the scar’s appearance. Also, about one-third of patients developed a rash on the vitamin-E-treated skin.

Replacement parts

How can I donate an organ to give someone else a chance at a longer life?

Lack of donor organs is a big problem in the United States, where more than 100,000 people are on waiting lists, and 19 people die each day while waiting for an organ, according to the official U.S. government website for organ and tissue donation ( Some countries, including France, Spain, and Belgium, have solved the problem of lack of organ donors by adopting “opt out” policies. This means that everyone is considered a potential donor when they die unless they have said otherwise. Typically only about 2 percent of people choose to opt out.

In the United States, you must “opt in” to be considered as an organ donor. You can request an organ donor card at the Department of Motor Vehicles, or download one from the Donate Life website at The argument against an opt-out system has to do with informed consent. Silence is not consent, because if people do not know about the policy, they cannot opt out. Therefore, an opt-out policy raises ethical questions but, of course, so does the current opt-in policy and the resulting chronic donor shortage.

You can also become a “living donor” and donate a kidney, partial liver, lung, or partial pancreas. Medical costs are paid through the organ recipient’s insurance, but the donor is not compensated for taking time off from work. More information is available on the Donate Life website. To learn about becoming a bone marrow donor, see or contact your local blood bank.

Not so wise

What is Nature’s purpose for wisdom teeth?

Wisdom teeth were the greatest thing before sliced bread. The extra surface area they provide is handy for chewing nuts, coarse grains, and raw meat. In other words, they helped our long-lost ancestors extract more calories from tough stuff. As humans have found ways to make food more toothsome, wisdom teeth have become disadvantageous, except to surgeons who make a living extracting them.

Our jaws are considerably smaller than those of our ancient ancestors. It is often impossible to squeeze in an extra set of molars, and the resulting prevalence of malocclusions—poor alignment—of teeth have made braces a rite of passage for many adolescents. No other mammals, even other primates, suffer from malocclusions to the extent that we do.

Part of the explanation for our shrinking jaws lies with genetic changes dating back to early human history. Those changes led to the remodeling of the skull and made room for a larger brain. Our jaws have also gotten smaller as a result of changes in our diet that have reduced the amount of muscular force we need to chew our food and the amount of time we must spend chewing it. Teeth have also gotten smaller over time, but not as rapidly as jaws, because tooth size is more strongly controlled by genetic factors and less influenced by diet.

For at least 300,000 years, humans have fragmented food with tools and used cooking to reduce its toughness. The advent of agriculture over 10,000 years ago increased humans’ intake of cereals and other soft foods. More recently, improved techniques for milling grain and a host of other food-processing techniques have made it even easier to take in calories without exercising our jaws, a fact to which anyone who has gulped down a burger, fries, and shake can attest.

The result is a case of “use it or lose it.” The fact that exerting muscular forces during chewing has an important effect on the jaw’s development has been demonstrated experimentally. In one study, young pigs were fed a diet of soft food. After just a few months, their snouts were shorter and narrower and had thinner bones than pigs fed a diet of hard food.

“Magdalenian Girl,” a 13,000- to 15,000-year-old skeleton found in southwestern France, had the earliest recorded case of impacted wisdom teeth. Anthropologists consider it evidence that dietary changes have long been a source of tooth troubles.