Dissect a chicken from the supermarket to discover the unusual pulley system that enables birds to fly.
Many biologists are able to look at an animal, living or dead, and learn how it works. In previous decades, this ability was acquired in whole dissections of rats, frogs and earthworms.
These days, students have legitimate concerns about whether it’s right to kill animals solely to take them apart and study them. Fewer dissections are performed in schools, and they rarely involve whole animals – instead, individual organs such as the heart are used. At the same time, meat is increasingly sold pre-prepared and packaged to minimise handling or processing in the kitchen. Many children don’t grow up watching their parents handling meat or preparing whole chickens, rabbits or other animals in the kitchen. Consequently, today’s students approach dissection with less experience and a more profound feeling of squeamishness than ever before.
Nevertheless, the land vertebrate most commonly sold for food in a nearly complete state is probably the chicken, and it is therefore the livestock most familiar to students. Even without its internal organs, the chicken provides an excellent introduction to dissection.
The flight of birds has always fascinated humans. Our arms have the same sets of bones and we share many of the same muscles as birds, yet we can only flap our arms and imagine ourselves flying. Leonardo da Vinci, for example, was fascinated by the possibility of human mechanical flight and had a lifelong passion for studying the flight of birds and the mechanical workings of the human body. He produced countless annotated sketches of the shoulders and arms of humans and how they attach to the torso. Yet when he studied the structure of a bird wing, he drew it only as far as the shoulder joint. By not investigating the connections of the wing to the muscles that make it flap, he seems to have stopped just when things got interesting.
The primary purpose of this activity is to show the two main muscles of the wing-flapping mechanism of birds and demonstrate how they work. It is perhaps surprising that although the structure of chicken wings and the muscles that make them flap are familiar to almost anyone who is not a vegetarian, people rarely make the connection between the two because the wings and the breast are often eaten separately. The secondary purpose of this activity is to familiarise students with the process and scientific value of dissections in general – that dissections are key to understanding how living things function.
The activity could be used in a biology lesson on bird flight or animal locomotion or related to a physics lesson on forces. It could also support a lesson on evolution – the reason birds’ wings are so similar in structure to our arms – or on model organisms, including how chickens can be used to investigate human limb developmentw1.
Depending on the age or ability of the students, teachers can either perform the dissection or encourage the students to do it for themselves.
The more complete the chicken, the more interesting the demonstration. In some European countries, plucked chickens are readily available from supermarkets with the head, neck, feet and internal organs (giblets) intact. Teachers can extend this demonstration to other organ systems as desired. Alternatively, a wrapped supermarket chicken as sold in the UK without feet, neck or giblets is perfectly adequate for the demonstration as described here. Teachers may choose to buy organic or free-range chickens.
This demonstration carries risks associated with the bacteria found on raw poultry such as salmonella and listeria, as well as with sharp knives or scalpels. Teachers should follow their local health and safety rules. Gloves should be worn to reduce the risk of contamination but are not a substitute for thorough washing of hands afterwards. See also the general safety note.
Students will see the similarities between the features in humans – the single bone (humerus) in the upper arm, the elbow, the two bones (radius and ulna) in the forearm, the multi-boned wrist, and the hand with the thumb – and their counterparts in the chicken wing (figure 1).
Explain to the students that in humans, the muscles moving the arms in a ‘downstroke’ motion are the pectoralis (or chest) muscles, while the ‘upstroke’ muscles are on the back. In a bird, both sets of muscles are in the chest. The muscles are easily identified, even in supermarket chicken breast fillets – the downstroke muscle is the larger pectoralis muscle on the outside, while the upstroke muscle is the supracoracoideus on the inside (figure 3). This smaller muscle is often sold as a ‘mini fillet’ in the UK or ‘aiguillete’ in France (figure 4). The antagonistic action of these two muscles on the wing (one muscle opposes the action of the other) will be demonstrated later.
The students should be given an opportunity to try this – by pulling on the pectoralis and supracoracoideus muscles, they will appreciate the amount of force that is needed to flap the wings during flight. It’s also remarkable to see that the muscles that move the wing upwards and downwards are both located below the wing. This is counter-intuitive because our human ‘upstroke’ muscles are on our back, not our chest.
The dissection can be extended to any other parts of the chicken, depending on the completeness of the bird. For example:
We would expect the ratio to reflect the flight habits of the respective species, with atypical birds that generate considerable lift on the upstroke – such as hummingbirds– having smaller ratios; and faster flying migratory birds that use aerodynamic forces to aid the upstroke – such as waterfowl – to have higher ratios.
Point out that there is a continuous seal around the supracoracoideus – it is in a cavity enclosed entirely by the pectoralis and sternum. Usually, antagonistic muscles are located on opposite sides of the joint that they act upon. The arrangement of both muscles being on the same side in birds, with one muscle wrapped around and fully enclosing its antagonist, is unusual. It would be interesting to discover whether this morphology exists elsewhere in the animal kingdom.
This demonstration is an opportunity to achieve three objectives with one bird.