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Q How many bones are there in the human body?

An adult skeleton contains 206 distinct bones: 26 in the vertebral columns, 8 in the cranium, 14 in the face, 7 other skull bones, 25 in the sternum and ribs, 64 in the upper limbs and 62 in the lower limbs. The stirrup bone, one of the three auditory ossicles in the middle ear, is the smallest: it is between 2.6mm and 3.4mm long, and weighs from 2.0 to 4.3mg.

Q Do bacteria have sex?

Bacteria normally reproduce asexually, by simply splitting into two identical cells. However, some bacteria do actually mate. As well as long "hairs" called flagellae, bacteria have smaller hair-like structures on their surface, called pili. One bacteria can join pili with another bacteria, forming a continuous tube between the two cells. Genetic information called plasmids - little loops of DNA - can then pass from one bacterium to another. Bacterial mating and passing of plasmids is important for two main reasons. First, that's how bacteria pass on the genes for certain types of antibiotic resistance: passing these genes between generations, as well as down through the generation, means that more bacteria become antibiotic-resistant more quickly (which suits the bacteria, though not humans). Second, this process is useful to molecular biologists for their research.

Q What's the difference between white matter and grey matter?

Different areas of nerve tissue in the central nervous system are either grey(ish) or white. Grey matter contains nerve cell bodies: their nuclei are responsible for the grey colour. Grey matter also contains other cells and some nerve fibres, and it forms the surface layer of the brain and an area deep inside the brain, as well as the central column of the spinal cord. White matter consists largely of nerve fibres which are covered by a white, fatty insulating material called myelin. It is the insulation that gives the tissue its white colour. White matter forms a layer between the two areas of grey matter in the brain, and encloses the column of grey matter in the spinal cord.

Q Why is it dangerous to burn many fuels in a limited air supply?

Insufficient air means alkanes and other carbon-based fuels burn to give carbon monoxide and carbon (soot) instead of carbon dioxide. This is called incomplete combustion. Carbon monoxide is poisonous, and responsible for many accidental deaths. Gas fires that haven't been serviced regularly can burn gas incompletely. In a poorly ventilated room, the build-up of carbon monoxide can put the occupants into a coma, and kill them if they are not found. Carbon monoxide detectors are available, which, like smoke detectors, make a loud bleeping noise.

Q Where did the peculiar name of the fundamental particle called a "quark" come from?

In 1964 Murray Gell-Mann and George Zeweig thought up the theory that introduced these particles to explain how protons and neutrons and other similar particles behaved. Murray had just been reading the book Finnegan's Wake,by James Joyce, which contains the phrase "three quarks for Muster Mark". He decided it would be amusing to name his particles after this phrase.

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