From how much of the body is muscle to what causes colour-blindness

How much of my body is muscle?

About two-fifths of a normal adult’s body weight is muscle, but the actual figure depends on the individual. Most teenagers have slightly less muscle because more forms as our bodies mature. 

Which is the smallest muscle in the body? Which is the largest?

The smallest muscles in the body, like the smallest bones in the body, are in the middle ear. Called the stapedius and the tensor tympani, these tiny muscles allow the delicate hearing apparatus to move so we can hear sounds clearly.

Identifying the largest muscle is more difficult. The most powerful muscle is probably the gluteus maximus – the muscle that makes up the bottom; the longest is the sartorius, which runs from the hip to the knee; and the largest in surface area is the latissimus dorsi, the broad muscle that covers the back.

How many bones are there in the human body?

An adult skeleton contains 206 distinct bones: 26 in the vertebral columns, eight in the cranium, 14 in the face, seven 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 to 4.3mg.

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 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.

What causes colour-blindness, and why is it mainly a male problem?

Colour-blindness occurs when cones – the light sensors in the retina that respond to colour – are damaged. There are three types of colour-sensitive cones, responding to either red, green or blue light. Activation of combinations of these three lets us perceive all the colours of the rainbow. Loss of just one type of cone disrupts your colour vision; loss of two turns the multi-coloured world into black and white. A colour-blind person can see clearly, but cannot distinguish certain colours, most frequently red or green (sometimes both). Red/green colour-blindness is mainly a male problem: about 8 per cent of male western Europeans are colour-blind. Of these, about 75 per cent are green colour-blind, and about 25 per cent red colour-blind. The genes responsible are on the X sex chromosome, so men can inherit it from their mothers, but a woman has to get the fault from both parents, which is much rarer.