快猫短视频

Dead end

I鈥檝e just had my appendix removed. My surgeon told me I wouldn鈥檛 miss it because it no longer serves a purpose in humans. But does it serve a purpose in some animals? What exactly?

鈥� The equivalent of the true appendix in most animals is known as the caecum, which is at the junction of the small and large intestines. In general, carnivorous mammals have a small caecum that serves the same purpose as it does in humans. However, in many herbivorous mammals the caecum is greatly enlarged to create all sorts of wonderful anatomical arrangements. The function of the caecum in these mammals is to ferment the complex carbohydrates from the herbivorous diet into volatile fatty acids and then to absorb these as a source of energy. A functioning caecum is also vital for providing the energy needs of hindgut fermenters like horses, rabbits, rats, guinea pigs and swine. The ruminant stomach of cattle and sheep performs a similar function in these animals and so they are less dependent on their caeca.

An additional function of the caecum is to reabsorb water from the gastrointestinal tract 鈥� this job is performed by the colon in humans and carnivorous mammals.

Richard Luong

Lidcombe, New South Wales, Australia

鈥� Your surgeon was a little out of date. Although it used to be believed that the appendix had no function and was an evolutionary relic, this is no longer thought to be true. Its greatest importance is the immunological function it provides in the developing embryo, but it continues to function even in the adult, although it鈥檚 not so important and we can live without it.

The function of the appendix appears to be to expose circulating immune cells to antigens from the bacteria and other organisms living in your gut. That helps your immune system to tell friend from foe and stops it from launching damaging attacks on bacteria that happily co-exist with you.

There are other parts of the body that appear to do the same thing. Peyer鈥檚 patches in the intestine help to expose your immune system to the usual contents of the intestine. By the time you are an adult, it seems your immune system has already learned to cope with the foreign substances in the gastrointestinal tract, so your appendix is no longer important. But defects in these immune sampling areas may be involved in autoimmune diseases and intestine inflammation.

Interestingly, the appendix has been used as a personal 鈥渟pare part鈥� in surgery. It can be removed and its tissue used in reconstructive surgery of the bladder without risking the immune reaction that would be triggered by using tissue from another individual.

Kathleen James

Chicago, Illinois

鈥� The true appendix is a worm-like, narrow extension beginning abruptly at the apex of the caecum. It is only present in anthropoid apes (gibbons, orang-utans, chimpanzees and gorillas), a few rodents (rabbits and rats) and a few marsupials such as the wombat and the South American opossum. However, in many herbivorous mammals, the large appendix-like pouch of the caecum is an alternative site for fermentation of food. It contains microorganisms that break down cellulose in plant cell walls.

In humans, it was thought to have no physiological function. However, it is now known to play a role in fetal immunity and in young adults. During the early years of development, the appendix functions as a 鈥渓ymphoid organ鈥�, assisting with the maturation of B lymphocytes (a type of white blood cell) and in the production of immunoglobulin A antibodies. In addition, at around the 11th week of fetal development, endocrine (hormone-producing) cells appear in the appendix. These cells produce peptide hormones that control various biological mechanisms.

Johan Uys

Bellville, South Africa

Less cargo?

What happens to a snail鈥檚 anatomy when it appears to shrink and retreat back into its shell? It looks like it turns itself inside out like a sock.

鈥� To make room for the body, hard-shelled snails have a large air space called the mantle cavity. To extend out of its shell, the snail squeezes fluids that inflate its body cavities to pull the body into shape. Meanwhile, the mantle cavity expands inwards to take up the space vacated in the shell. You can see the opening to the cavity just above the back of an extended garden snail, below the edge of the shell. Altogether it鈥檚 a neat application of hydraulics.

A bit of sock-inversion also takes place during the process of retraction. The snail鈥檚 eyes and tentacles are closed tubes, as you can see by watching them being extended. When retracted, they get drawn into the body from the tip inwards, like socks being inverted.

To squeeze into the shell, other muscles contract until only part of the foot is visible. In snails such as winkles with a lid (called an operculum) over their head, this fits protectively into the shell mouth. As the body is squeezed into the shell it is inverted somewhat, but not as much as eyes and tentacles.

Jon Richfield

Somerset West, South Africa

This week鈥檚 questions

The black stuff?

When I buy a pint of Guinness there is no doubt the liquid is black. Yet the bubbles that settle on top, which are made of the same stuff, are white. The same is true of many types of beer. Why?

Stewart Brown

Bristol, UK

Number game

Reading car number plates in a long, dreary queue one day, I noticed if you take any three-digit number, reverse the digits then subtract the smaller from the larger, the answer is always divisible by 9. For example, 761 鈭� 167 = 594, and 594 梅 9 = 66. This obviously doesn鈥檛 work for symmetrical numbers such as 555 or 919 but, in other cases, how and why does it happen?

Robin Anderson

Stockport, Greater Manchester, UK

Smashing song

About 50 years ago, Lake Erie froze and became covered with a glassy sheet of ice about 50 millimetres thick. As an onshore wind rose, I heard the lake sing with a pure, mid-range, organ-like tone, which rapidly intensified to an almost intolerable level with no visible motion in the lake. After a few minutes the ice fractured and the pure tone was replaced by cracking, echoing at about the same pitch and, abruptly, by the atonal roar of shattering ice. Has anybody else witnessed such an event? How was the tone created?

Charles Sawyer

Camptonville, California, US