Dissected: Getting under your skin

foot dissection2Broadcaster: BBC4

Year: 2016 (originally 2014)

Genre: Documentary

URLs:
Hand (episode 1) https://learningonscreen.ac.uk/ondemand/index.php/clip/95314
Foot (episode 2) https://learningonscreen.ac.uk/ondemand/index.php/clip/91909

Review by Eunice Muruako

In two hour-long episodes, presenter George McGavin (Glasgow University) delves beneath the skin to expose the anatomy of the human hand and foot. The series sheds light on the various functions of the hand and foot by identifying the muscles, tendons and ligaments responsible for movement, and how the human anatomy compares with that of other animals.

Episode 1: Hand The dissection of the donor arm begins in the forearm to expose the muscles which give power to the hand for gripping and the tendons which attach each muscle to the bones in the fingers. The surgeon demonstrates how the tendons motion in the hand by tugging on the tendons to cur the fingers. The structure of the hand can be changed with heavy use – the X-ray of a frequent rock climber shows that the cortical bone (the hard outer layer of the skeleton) is thicker than in a non-climber the tendons and pulleys are also thicker, this allows their hands to maintain a firmer grip.

A feature of the human hand which is distinguished from most other animals is the opposable thumb; it is the most mobile finger and the saddle joint the base allows it to be positioned directly opposite each of the other fingers. Chimpanzees also have opposable thumbs, however there are differences between a human and chimpanzee hands: chimps have shorter thumbs relative to their other fingers making it more difficult to use precision grips but allowing them to hang from trees. Anthropologist Dr Tracy Kivell suggests in the programme that human hand evolved from a chimpanzee-like structure because of our use of tools and no longer needing to walk with our hands. However, research published in 2015 by Alméjica and colleagues found that the human hand may actually be more primitive than a chimp’s. The researchers compared the thumb and finger proportions of living apes and monkeys with extinct species and found that early ancestors had hands more like humans today. Commentary in the journal Science summarises the study and opinions on the study by other experts in the field.

The final part of the dissection reveals the median and ulnar nerves which convey sensory information from the hand, the fingertips contain the highest concentration of the smallest nerve branches resulting in greater sensation than almost anywhere else in the body. The sensitivity of the fingertips is shown by McGavin attempting to identify two objects, first with his forearm and then with his fingertips. Although he is able to gain some information about the objects’ size and texture with his forearm McGavin is unable to identify the banana and coin until he uses his fingertips, he is even able to identify which coin it is. Scientists at Exeter University are attempting to replicate the sensation of the fingertips to create the ability to touch objects remotely. As Dr Ian Summers explains, the tactile stimulator designed by the Exeter team is able to simulate touch by recreating the vibrations caused by the skin interacting with the surface of the material.

Episode 2: Foot In the second part of the series George McGavin takes a look at a dissection of the human foot. Following the same approach as that used in the first episode, muscle groups, bones, tendons and nerve endings are gradually revealed and their function in the operation of the foot is explained.

The heel is covered in many layers of dead skin cells and fat which act as padding to cushion from impact. When McGavin meets with Professor Rami Abboud he learns that the heel absorbs 2-3 times the body weight during the heel strike when walking or running and so the cushioning is important to improve the shock absorbing ability of the heel. However the heel strike is completely eliminated when the volunteer is asked to run barefoot; all the impact is taken on the balls of the feet and the toes, this spreads the pressure over a larger surface area and reduces the shock absorbed. The weight bearing ability of the foot is distributed across the heel to the toes connected by a layer of tissue, known as the plantar fascia, which is responsible for delivering mechanical power to the foot. Power generated in the calf muscles is transferred to the heel via the Achilles tendon when the muscle contracts.

The final dissection of the foot is the big toe which bears the greatest amount of force during movement. The tendon which connects to the big toe runs along the entire length of the foot to the muscles in the calf.

As with the hand, the structure and function of the foot is compared with other animals. Tetrapod animals, including humans, have pentadactyl limbs which consist of five fingers or toes. The pentadactyl limb has evolved in many different ways allowing horses to run at speed on a single toe, and chameleons to grip onto branches. The orangutan’s foot structure is not as rigid as the human foot and is instead more similar to the human hand which allows them to curl around branches and vines, the flexibility of the orang’s foot is aided by the mid-tarsal joint.

These episodes would also be of interest to medical students.

Simple guides on the anatomy of the hand and foot, as well as other structures, can be found at teachmeanatomy.info.

Please note a version of episode 1 (the incredible human hand) with sign language is available via this link.

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