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Thumb

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An image showing the human thumb

The thumb is the first and the outer most digit of the hand. Although it is similar in structure and form to the other fingers it is prehensile as well as opposable which makes it the most unique of the fingers. These features allows the thumb to be able to pick things up and grasp them as well as allows the user more manual dexterity. The thumb also allows for better fine motor skills. Opposability of the thumb allows it to be rotated in order for it to face the palm and provides the thumb to be able to touch all of the other fingers. This trait is a unique feature that is seen both in human beings and primates. From skills such as picking up an object to the invention and development of tools, the opposability is responsible for most of the hand functions[1]. The nerves, muscles and bones of the thumb work together brilliantly to control the thumb's movements[2].

Human Anatomy

The muscle of the thumb

Muscles

Muscles of the thumb are divided into two groups, intrinsic and extrinsic. Intrinsic are the deep muscles, whereas the extrinsic lie superficially on the surface on the surface of a muscle. On the intrinsic side, there are the abductor pollicis brevis, abductor pollicis, first dorsal interosseus, flexor pollicis brevis, and opponens pollicis. The extrinsic muscles include the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and flexor pollicis longus[1].

Bones

Lateral view of the thumb

There are 27 bones in the human hand which include the metacarpals. The skeleton of the thumb consists of the distal phalanx, proximal phalanx,and the first metacarpal. On the end of the thumb, the distal bone is the distal phalanx. The proximal phalanx is shorter and is between the other two bones. The bone closest to where the thumb is attached to the body is the metacarpal bone[1].

Movement

The Thumb is controlled by nine individual muscles which are in turn, controlled by all three hand nerves. The three hand nerves that control the thumb muscles are the median, ulnar, and median nerves.[2]. There are three joints that help articulate the thumb and each joint contains several movements. They are the carpometacarpal, metacarpophalangeal, and the interphalangeal joint[3].

The carpometacarpal joints allow an individual to be able to perform the following motions:

  • Flexion
  • Extension
  • Adduction
  • Abduction
  • Opposition[3]

Flexion allows the thumb to bend the joint and move the bone below the thumb toward the hand and slightly forward. Extension results in an increase of angle when straightening the joint, and moves the bone below the thumb away from the hand and slightly back. The adduction movement moves the bone below the thumb toward the back of the wrist. On the other hand, abduction moves the bone below the thumb toward the front of the wrist. Finally, opposition allows the thumb to diagonally reach across the palm of the hand touching the fingers[3].

The metacarpophalangeal joints allow an individual to be able to perform these set of movements:

  • Flexion
  • Extension
  • Adduction
  • Abduction[3]

Flexion moves the base of the thumb toward the heel of the hand which results in a decrease of angle. Extension straightens the joint and moves the base of the thumb away from the heel of the hand, resulting in an increase of angle. The adduction movement allows the thumb to move toward the back of the hand . Lastly, the abduction moves the thumb away from the back of the hand[3].

Interphalangeal joints are only able to do two motions. They are:

  • Flexion
  • Extension/Hyperextension[3]

The flexion moves the distal segments of the thumb toward the base of the thumb resulting in a decrease of angle. While the extension/hyperextension motion allows the thumb to move away from its base[3].

Animals

In the animal kingdom, there are many mammals who posses some sort of a opposable thumb. If their thumb is capable of touching other digits of the hand, it is considered to be an opposable thumb. Although the majority of species do not have opposable thumb, most primates, marsupials, and others do. This thumb plays a significant role in their lives. However, not all primates have opposable thumbs like humans do. Primates such as tarsiers and marmosets have nonoppsable thumbs, while Old World monkeys and all great apes have them. The types of thumbs fall into four catagories: nonopposable, psuedo-opposable, opposable, and opposable with comparitively long thumbs. The strepsirrhines and Cebidae possess pseudo-opposable thumbs and gibbons also have opposable thumbs but with comparatively longer thumbs. However, some primates are virtually thumbless, such as the spider monkey and colobus monkey. To compensate for this, the spider monkey uses the hairless part of its long, prehensile tail for grabbing objects. Another interesting note, is that the thumbs of the apes and Old World Monkeys can rotate. In addition to mammals, dinosaurs are thought to have had partial opposable thumbs. Several of the dinosaurs who possessed these thumbs are the Troodon, a bird-like dinosaur who used its thumb to manipulate ground objects or moving undergrowth branches when searching for prey. Bamiraptor, a small, predatory dinosaur used its thumb to easily latch on to prey. It is interesting to note that flying birds have at least one opposable digit on their foot, but they are usually called halluxes instead of a thumb[4].

Opposable Thumb and Prehensile Thumb

The opposable thumb can be rotated so that it comes to face the palm and can touch all the other fingers. It is a very unique feature, not only to humans but also to primates, a few other mammals, and most marsupials. A thumb that is opposable is also prehensile, which is the property that makes an organ or appendage suitable for grasping. Organs like hands, claws, tails and even tongues and noses are prehensile in various species.[1]

Thumb Sucking

Thumb sucking is a normal and common behavior in babies and young children. However when time progresses and the infant gets older, they naturally stop on their own between the age of three to six years. Babies have a natural tendency to suck, but this begins to decline at the age of six months. However, even after six months, many babies will continue to suck on their thumbs in order to calm themselves. Babies and young children also use this method to comfort themselves when they feel hungry, afraid, restless, quiet, sleepy, or bored[5].

Thumb sucking does not cause emotional or physical damage to a very young child. But, when they still continue to suck on their thumb at the age of six, they are at a risk for dental as well as speech problems. Continuous thumb-sucking may cause the teeth to become improperly aligned or push the teeth outward. This problem usually corrects its self when the child stops thumb sucking early in life, but when they don't, an orthodontic treatment may be needed[5].

Stopping a child from sucking his or her thumb is usually simple and can be accomplished by the parents at home. This treatment includes parents setting rules and providing distractions. Examples include: putting away items that the child associates with thumb sucking, or putting gloves on their hands, or wrapping the thumb with large amounts of bandages or cloth to help the child stop this sucking behavior. Furthermore, offering praise and rewards for abstinence from thumb sucking may motivate the child to break their habit. Scolding is not recommended, as it only lowers the child's self-esteem[5].

Fun Facts

The thumb is very unique in its own way. It has many interesting uses. It can be used for hand signals, games and other unique idioms. One of the most recognizable signals is the thumbs up sign. In most cultures, it literally means yes, good, okay, or anything that is on the positive side. On the other hand, the thumbs down obviously means the opposite. It stands for no, no good or anything close to the negative. However, in some countries, a thumbs up sign is considered an insult. In most Middle Eastern nations as well as some parts of South America, the thumbs up sign is seen as savage and rude. Although the origin of this belief is not certain, a few historians believe that it comes from a Medieval English custom where business deals were sealed with the act of licking, displaying and pressing together of the parties' thumbs[6].

Another common use of the thumb is a popular game called thumb war. This is when you and your opponent grasp each others hands and chant "one, two, three, four, I declare a thumb war!" When the chant is finished, you and your opponent attempt to pin each other's thumb for a specific period of time, usually around three to five seconds[6].

In America, the quote "the rule of thumb" came to existence in 16th century England. It first came into use when a judge made a ruling that a man is allowed to beat his wife as long as the stick that was used was no thicker than his thumb. The rule of thumb is not based on science or exact calculations, but on information coming from general assumptions[6].

Video

A video showing the anatomical movement of the thumb.

References

[6]

[5]

[2]

[3]


[4]


[7]

[1]

  1. 1.0 1.1 1.2 1.3 1.4 Romanes. The Amazing Thumb Oxford University. Web. Accessed 26 February 2013.
  2. 2.0 2.1 2.2 Author unknown. Thumb Movement Hand facts and trivia. Web. Accessed 25 February 2013.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Author unknown. Thumb Articulation ExRx.net. Web. Accessed 26 February 2013.
  4. 4.0 4.1 Stephen Roberts. Other animals with opposable thumb Foundation. Web. .
  5. 5.0 5.1 5.2 5.3 Linda. The Thumb Sucking Facts The Probiotics Tooth Fairy. Web. Accessed 25 February 2013.
  6. 6.0 6.1 6.2 6.3 Author unknown. Fun Trivia Thumb Thematic 20 Questions Publishing site unknown. Web. Accessed 25 February 2013.
  7. Dr. Jay L. Wile and Marilyn M. [The Human Body Fearfully and Wonderfully made page 117] A+P. Book. 2001.