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WonderBody fact

An adult fingernail grows about 3.5 millimetres a month, while a toenail grows about 1.6. That is why a fingernail takes three to six months to replace itself completely, and a toenail twelve to eighteen months.

Human biology

Health

The story behind the card

A nail is made by the nail matrix, a layer of living tissue hidden under the skin at the base of the nail. The pale half-moon visible at the base of some nails, the lunula, is the edge of that matrix showing through. New cells made there fill with keratin, the tough protein also found in hair, and push older cells forward; the nail plate they form is dead tissue, with no nerves or blood vessels. Growth speed depends on age, season, diet and health, and it is linked to finger length: the nail on the index finger grows faster than the one on the little finger. Contrary to a popular belief, nails do not keep growing after death; the skin dries and shrinks back, which only makes them look longer.

Türkçe oku
Yetişkinlerde el tırnağı ayda yaklaşık 3,5 milimetre, ayak tırnağı ise yaklaşık 1,6 milimetre uzar. Bu yüzden bir el tırnağı üç ila altı ayda, bir ayak tırnağı ise on iki ila on sekiz ayda baştan sona yenilenir.

Tırnağı üreten, tırnak dibinde derinin altında kalan canlı bir dokudur: tırnak matriksi. Bazı tırnakların dibindeki soluk yarım ay (lunula), bu dokunun dışarıdan görünen ucudur. Burada oluşan yeni hücreler, saçta da bulunan sert bir protein olan keratinle dolar ve eskileri öne doğru iter. Tırnağın görünen kısmı bu ölü hücrelerden oluşur; içinde sinir ya da kan damarı yoktur. Uzama hızı yaşa, mevsime, beslenmeye ve sağlığa göre değişir. Parmak boyu da etkilidir: İşaret parmağının tırnağı, serçe parmağınkinden daha hızlı uzar. Sanılanın aksine tırnaklar ölümden sonra uzamaz; deri kuruyup çekildiği için yalnızca daha uzun görünür.

Sources and further reading

Background reading: Nail (anatomy)
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WonderHistory

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The plan grew out of the 1942 Beveridge Report, in which the economist William Beveridge proposed a comprehensive health service as part of a welfare system for post-war Britain. Aneurin Bevan, a former coal miner from Wales and health minister in Clement Attlee's Labour government, pushed it through despite opposition from many doctors. On its first day the NHS took over some 2,700 hospitals in England and Wales. Before 1948 people relied on charity hospitals, council hospitals and insurance schemes, but coverage was patchy and many women and children were left out. The care did not stay entirely free: charges for glasses and false teeth came in 1951, leading Bevan to resign, and prescription charges followed in 1952.

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Deliberately smiling can make people feel slightly happier. In 2022 a study of nearly 4,000 people in 19 countries found that posing a smile, or copying a smiling face, raised reported happiness a little; a subtler method, holding a pen between the teeth, gave less clear results.

The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

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A 2016 estimate found that a typical adult man weighing 70 kg has about 30 trillion human cells and 38 trillion bacteria. That overturned the old claim that bacteria outnumber our own cells ten to one: the two numbers are roughly equal.

The estimate comes from Ron Sender, Shai Fuchs and Ron Milo of the Weizmann Institute of Science in Israel. Their reference man is 20 to 30 years old, 170 cm tall and weighs 70 kg. The ten-to-one figure, they found, went back to a single rough calculation from 1972 that had been repeated for decades. Almost all the bacteria live in the colon, the last part of the gut, and together they weigh only about 0.2 kg. Among human cells, red blood cells dominate the count, making up about 84 per cent, while fat and muscle cells, far fewer but much larger, make up most of the body's cell mass. The balance also shifts through the day: a single trip to the toilet empties a quarter to a third of the colon's contents.

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An adult body replaces about 330 billion cells every day, around 80 grams of cells, according to a 2021 estimate by scientists at the Weizmann Institute of Science. Close to nine in ten of those cells are blood cells.

Ron Sender and Ron Milo combined the number, mass and lifespan of every major cell type in the body. Short-lived cells dominate the count. Red blood cells live about four months, and neutrophils, the most common white blood cells, only days, so the bone marrow must make vast numbers of them every day. By mass, the largest shares come from blood cells and from the cells lining the gut, which are shed and replaced within days. At this pace, a number of cells equal to all the roughly 30 trillion human cells in the body is replaced in about three months. That does not mean the whole body is renewed: most nerve cells in the brain last a lifetime, and heart muscle cells are replaced only very slowly.

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Scientists call this link the gut–brain axis. The gut has its own network of about 500 million nerve cells, the enteric nervous system, sometimes called the “second brain”, which can run digestion largely by itself. It is connected to the brain mainly through the vagus nerve, a long nerve running from the brainstem down to the abdomen, and most of its fibres carry messages upwards, from the gut to the brain. More than 90 per cent of the body's serotonin, a chemical messenger, is found in the gut. Gut bacteria play a part too: in a 2004 Japanese study, mice raised entirely free of germs overreacted to stress. Stress can speed up or slow down the gut, one reason it often worsens irritable bowel syndrome, a common gut disorder. Most research on the microbes' role has been in animals, and human evidence is still limited.

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