The popular Sunday evening ITV series The Knock seems to have introduced the phrase “OK, I have eyeball!” into the language. I know what it really means, of course, but could not resist using it when I picked up an elaborate model of the human eye, one of a number of anatomical models to be found listed in the NES Arnold catalogue.

Such models are needed for much the same reason as globes are: a flat map or diagram is in-capable of telling the whole story. An organ - such as the eye - has space within it, filled by a globular mass and enclosed by three-dimensional curved structures. The body itself contains organs which are tucked in behind each other. A diagram helps, but a model adds another layer of understanding.

The models are also fascinating in themselves - which undoubtedly explains why, according to suppliers, more and more primary schools are buying them, despite the fact that the science national curriculum does not call for detailed study of internal structures at that level. At key stage 1, the curriculum says, “children should be taught to name the main external parts, for example, hand, elbow, knee, of the human body”. To use a model for this, rather than a real child, would be perverse and confusing. At key stage 2, where there is a requirement to deal with the function of the heart, and with the idea that human beings have a skeleton and muscles, there is a more obvious role for models, but even here they are not essential.

In secondary school, where pupils look at how joints work, and at the structure of the eye, for example, there is a greater role for anatomical models. Good ones are expensive, so they should be long-lasting. Luckily, the internal structure of the body, unlike the political structure of the world as depicted on a globe, does not go out of date.

The models of the lower jaw, the ear, and the eye, as supplied by NES Arnold, are three times life size. They are expensive, highly detailed, and you can dismantle them to expose the hidden bits. Given that younger children are told about dental hygiene, the lower-jaw model could be useful in primary school - perhaps as a loan item between several schools - but the other two are really only for older children. The human ear is a miracle of complexity, and the concepts that underlie its function, let alone the visible mechanics of it all, are at the outer limits of human understanding.

Much the same app-lies to the eye, although its physical structures are perhaps a little easier to understand, and because the front view - the iris, pupil and cornea - is so familiar, it is natural for children to want to know what lies behind. The model of the heart is life size, and pupils will be surprised to see how small it is - the size of an adolescent’s clen-ched fist. Again, it comes to pieces so that you can see how it works.

Each of these models is beautifully made. The ear, for example, has more than 30 labelled parts, and the valves of the heart are made from what looks like paper-thin plastic.

Among the larger models, one of the most interesting is the mini torso, which comes with or without a head. This is, in effect, a limbless body, open at the front and filled with the main internal organs, each of which can be removed. There are 12 parts (nine if you do without a head), so each lung can be removed, as can the heart, stomach, liver and intestine. This is good for showing relationships between the various organs, and their relative sizes.

I should say, though, that when I took it to pieces I should have paid more attention to where everything came from, because when I came to put it together, I found myself making things that looked like some of Dr Frankenstein’s early, failed attempts.

This is a good model, well made and very clear. It would be suitable for a secondary specialist human biology course. The model is, incidentally, sexless. Male and female models (and a bi-sexual one) with removal genital organs are available from NES Arnold’s specialist Science catalogue. At Pounds 199. 95, this is an expensive resource, and one that could usefully be shared between a group of schools.

The Torso model looks much the same as the Mini torso at first glance, but is actually very different. The outside shows a small boy wearing a T-shirt. It opens up and you can see the respiratory, urinary, digestive and nervous systems. Some organs are detachable. The model comes with a 54-page teacher’s guide.

The Torso model comes from America and though aimed at the equivalent of upper primary, the teacher’s guide actually covers far more detail than is required by our primary curriculum.

Although it professes to show a lot of detail, much of it is embossed or painted on to the inside of the torso, and only a very few structures are really three dimensional. The effect, I feel, is not very realistic. This model falls between two stools, being completely satisfactory for neither primary, or secondary schools. It is also expensive.

A better bet than this, and considerably cheaper, is the Complete Anatomy which consists essentially of a one-third scale skeleton that you can fill up with internal organs. It is all then held together by a transparent plastic body shell. This is not a top-price item, and some roughness is apparent in, for example, the finish of the plastic skeleton. Nevertheless, this is good value for money and will fascinate children. As with the Mini torso, though, you need to practise putting it together before you do it with a class.

The basic problem about these torso models, which show soft tissue, is that they are not, in the last analysis all that realistic. The book with the American Torso claims that the child can say, “I’m just like that inside. ” This, though, is manifestly not so, because you are a lot more squidgy and ill-defined inside. The body is very moist and sloppy under the skin, and the organs simply do not lift out cleanly - they slip from the grasp like butcher’s offal and leave glistening stringy bits behind.

What this means, of course, is that children need to be taught that what they are seeing is, in effect, a good diagram - a three-dimensional, and helpful, but still a diagram.

Bones, though, are a bit easier to model. At both primary and secondary levels, some knowledge of the skeleton is called for - starting with the fact that it exists and working on towards the nature and function of various kinds of joints. There is no denying, of course, that children are fascinated by representations of the skeleton (“Look, Miss! An Skellington!”) and there is something to be said for allowing them to look at it closely and see that it is a real structure with a vital function.

The cheapest way of doing this is to buy the Bones Book and Skeleton.

The book is well written and lively (did you know you were born with 450 bones, and yet you end up with only just over 200) and the skeleton comes as a plastic kit for you to put together.

It can then be displayed under a transparent plastic dome. This is a good, inexpensive primary school resource.

A little further up the range is the Human skeleton one-third scale. There are 20 parts to assemble (fortunately, there is a picture to help) and the result is adequate for the price, although if you want real detail of the joints you would want something better. Again, this is very suitable for the primary school.

At the top of the range, though, is the full-size skeleton. The one on offer here, mounted on a mobile stand, is a cast of a real male skeleton, and is, to all intents and purposes, indistinguishable from the genuine skeleton that I remember from my own schooldays. It is a relatively expensive item, but it will last indefinitely, and a secondary school with a good science department would undoubtedly find good use for it.

All the above resources were supplied by NES and are available in the current catalogue. Details: NES Arnold, Ludlow HIll Road, West Bridgford, Nottingham NG2 6HD. Tel: 0115 945 2200. The same items may be available elsewhere for different prices

Ear Pounds 89.95 Eye Pounds 52 Lower Jaw Pounds 133 Heart Pounds 61 Mini torso Pounds 199.95 (without head Pounds 184) Torso model Pounds 199.95 Complete anatomy Pounds 84.95. Bones book and skeleton Pounds 13.50

Human skeleton one-third scale Pounds 52.95 Human skeleton Pounds 271