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Hand

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This image shows the major parts of the hand: muscles and bones.

The Hand is one of the most unique structures in the human body. It must perform incredibly complex tasks ranging from typing on a computer, writing a paper, playing video games, to hammering objects, rock climbing, or simply scratching an itch. To do all of these incredibly different tasks, the hand must possess systems that allow it to exercise great control and vast variations in the strength the activity requires, such as peeling a grape or crushing a walnut. Despite being a very small part of the overall biological system, the hands contain almost a quarter of the bones found in the human body. IT also has the most nerve endings in the entire body, and is able to detect the slightest amounts of pressure. The human hand possess the ability to do all of the tasks that people accomplish every day, and they have proven able to do this since the time of the first man. Without hands, humans would not be able to do the majority of the tasks that they need to do in their every day lives.

Anatomy

This image depicts the superficial muscles of the hand.

Muscles

There are two types of muscles that affect the movement of the hand: extrinsic hand muscles and intrinsic hand muscles. Intrinsic hand muscles are muscles that are located in the hand, while extrinsic hand muscles are located in the forearm. The intrinsic hand muscles deal with the finer movement and control, while the extrinsic muscles are for grasping and flexing [1].

Extrinsic Hand Muscles

Extrinsic hand muscles are typically located in the forearm. They use tendons to insert themselves into the hand bones. These tendons function as cables and move the hand when the muscles where they originate contract or relax. The extrinsic hand muscles are either innervated by the medial nerve or the radial nerve. However, a small group of them are innervated by the ulnar nerve. These muscles are involved in extending and flexing the four digits, the thumb, and the wrist [1]. Extrinsic hand muscles can be divided into two groups: those that act on the thumb and those that act on the hand[2].


The extrinsic muscles acting on the hand are the:

  • Extensor digitorum cominus
  • Extensor digiti minimi
  • Extensor indicis proprius
  • Flexor digitorum superficialis
  • Flexor digitorum profundus[2]

The muscles specifically acting on the thumb are the:

  • Abductor pollicis longus
  • Extensor pollicis brevis
  • Extensor pollicis Longus
  • Flexor pollicis longus[2]


Intrinsic Hand Muscles

Intrinsic hand muscles are located in the hand itself and generally control the finer movements of the fingers and thumb. All intrinsic hand muscles are controlled either by the median or ulnar nerve. The intrinsic muscles will either abduct or adduct the phalanges or thumb[1]. These muscles are used for the sensitive muscle movement that the hands must engage in on a daily basis. Without the intrinsic hand muscles we would not be able to perform many complex tasks such as playing an instrument or video games. The intrinsic muscles can be divided into two broad groups: the muscles acting on the phalanges, and the muscles acting on the thumb[2].


The muscles that act on the second through fourth digits of the hand are the:

  • Dorsal interossei (4)
  • Palmar interossei (3)
  • Lumbricals (1-4)
  • Palmaris brevis
  • Abductor digiti minimi
  • Flexor digiti minimi brevis
  • Opponens digiti minimi[2]


The muscles that act on the thumb are the:

  • Abductor pollicis brevis
  • Opponens pollicis
  • Flexor pollicis brevis superficial head
  • Deep head
  • Adductor pollicis oblique head
  • Transverse head[2]

Bones

This figure depicts the bones that are found in the hand.

There are 206 bones in the average human adult, and 54 of those bones are located in the hands, 27 in each hand. Along with the feet, which contain 52 bones, more than half of all the bones found in the human body are either in the hands or feet[3].

The fingers make up the majority of the human hand, containing 14 of the 27 bones. There are three different types of bones in the hand, the: phalanges, carpals, and metacarpals. The four fingers each contain three phalanges (the proximal, middle, and distal). However, the thumb only contains two phalanges. The proximal phalange (closest to the wrist) is attached to the end of a metacarpal[4].

In addition to the phalanges the hand also contains metacarpals. The metacarpals attach at the end of the carpals and at the beginning of the proximal phalange. The metacarpal attached to the thumb has the most mobility, even thought, it is the shortest. The hand also contains carpals, which are located in two rows of four, and make up the wrist[5]

Fingernails

Main Article: Fingernail

There are several different structures that come together to form fingernails. The nail root, formally called the germinal matrix, extends into the finger itself, and is located underneath the skin. The lunula (the white portion at the base of the nail) is the end of the nail root. The sterile matrix, commonly called the nail bed, is located underneath the actual nail, and is very smooth. It forms a flat layer that the nail will grow over. If the nail bed has grooves then the nail will grow unevenly which may cause the nail to split. The nail plate, the actual fingernail, has a pinkish appearance because of the large amount of blood vessels located under the nail. The nail itself is formed from clear keratin. Keratin is a protein fiber that forms both the outer layer of skin, and the majority of hair[6].


In addition, the fingernail also contains a cuticle. The cuticle connects the nail plate to the skin on the finger. It provides a water-proof seal and protects the finger. Also, the perionychium, the skin surrounding the nail, overlaps the nail on all sides and helps keep the nail in place, while protecting the nail from any irritants that would otherwise come under the nail. Finally, the nail contains a hyponychium, the gap that is between the end of the finger and the end of the nail. It also provides a water-proof barrier[6].

Fingerprints

This image shows the three basic fingerprint patterns, the whorl, the arch, and the loop (it also includes the Tented Arch pattern).

Fingerprints are located at the tips of the fingers, and the toes. Every human has their own distinct fingerprint and no two people have the exact same pattern, not even identical twins. Although, there are three basic patterns for fingerprints the: arch, whorl, and loop. While most fingerprints will fall into one of these categories the individual pattern will be different from one person to the next[7]. Fingerprints, which are epidermal ridges, are basically ridges that occur in the epidermis covering the fingers. The epidermis, the outermost layer of skin, covers the dermal papillae, which is a series of bumps in the dermis, and the epidermal ridges simply are the covering over the dermal papillae[8].

Fingerprints have several uses, but the most important use it that it allows people to grasp items. Because they increase the surface area of the fingers, they allow people to have more area in contact with the item they wish to pick up. This greatly increases the friction between the human and the object, making it easier to hold onto something[8].


Fingerprinting

Fingerprinting is an identification process in which the fingerprints of someone are matched to ones taken from a crime scene. The process begins when a fingerprint is identified and placed into a search engine that will then match the print an identical print in the database. While this system is typically very accurate there are still flaws with it. If someone were to remove their fingerprints through surgery, an accident, or even a disease the police would not be able to match the fingerprint to the person who left it. Alternatively, the criminal might have worn gloves and thus they would not have left any fingerprints at the crime scene[9]. Fingerprinting was developed over a 100 years ago, and the first forensic professional organization, the International Association for Identification (IAI), which deals with fingerprints, was founded in 1915. Since that time fingerprinting has become the most widely used system to identify criminals, and it forms the basis for the majority of police investigations[9].

Nerves

There are three major nerves involved with controlling our hands and they are the: Radial, Median, and Ulnar nerves. All three of these nerves are mixed nerves, meaning they contain both sensory nerves and motor nerves. Each nerve controls a separate part or motion of the hand, while there is some overlap as each nerve carries out its different function. The radial nerve controls the motion and stability of the hands. It also provides for sensory information from the radial aspect of the dorsal part of the hand, the dorsal area of the thumb, and the dorsal area of the index finger, long finger, and also the radial half of the ring finger proximal to the distal finger joints. The Median nerve controls the finer movement of the hand, and is responsible for the pinching motion of the hand. It provides sensory information from: the long, thumb, index, and also the radial side of the ring finger. Finally, the Ulnar Nerve is responsible for the strong grasping force of the hand. The ulnar nerve also provides sensory information from the ulnar position of the dorsal part of the hand, along with the little finger, and parts of the ring finger. Because the nerves in the hand provide sensory information and conduct action potentials, they are mixed nerves. Mixed nerves mean that the nerve bundle contains both sensory and motor nerves[10].

Nerve Receptors

The hand has some of the highest concentrations of nerves receptors in any part of the body. Nerve receptors are responsible for recognizing and classifying different stimulus. There are three broad categories of nerve receptors: special, somatic, and visceral receptors. However, only the somatic receptors will be discussed in this article. Somatic receptors can be broken down into two separate categories: cutaneous receptors and proprioceptors. Cutaneous receptors are responsible for receiving various stimuli and transmitting a signal to the central nervous system. They can respond to: pressure, heat, pain, stretching of the skin, movement, incredibly light touches, heavy touches, and also discerning two separate objects touching your skin. Proprioceptors are located in the muscles and tendons, and they react to being contracted or relaxed. This allows the brains to know if a muscles needs to be contracted or relaxed. All of these nerve receptors are found in the hands, and in some of the highest concentrations anywhere in the body[11].

Movement

Movement in the hand is accomplished through the presence of three different synovial joints the: saddle, gliding, and condyloid joints. Saddle joints are relatively rare throughout the body, but the joint at the base of the thumb is one example. Saddle joints resemble condyloid joints, but they have a larger range of motion which results from one concave side and an opposing convex side. This allows for the thumb to have a wider range of motion than the rest of the joints in the hand. Gliding joints, also called plane joints, allow for significantly less movement than the other two types of joints. They do not offer as much movement because they are two flat surfaces that glide across each other. This type of joint is found between the carpals of the hand and in the wrist. Condyloid joints, also called ellipsoid joints, are the last joints that can be found in the hand. Condyloid joints are very similar to ball and socket joints. However, they offer significantly less rotational movement. These joints are found in the wrist, and also the joints between the metacarpals and phalanges[12].

Video

The video talks about the anatomy of the hand, and how the hand functions.

References

  1. 1.0 1.1 1.2 Wilhelmi, Bradon J. Muscles and Tendons Medscape. Web. June 27, 2011.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 University of Utah Department of Orthopedic Surgery Residents and Faculty.Anatomy of the hand University of Utah. Web. Accessed on February 225, 2013.
  3. MedicineNet Inc. Definition of Skeleton, bones of the Medicine.net. Web. Reviewed 6-14-2012.
  4. Gray, Henry. The Phalanges of the Hand Bartleby. Web. published 1918.
  5. Wilhelm, Bradon. Bones Medscape. Web. updated June 27, 2011.
  6. 6.0 6.1 Brannon, Heather. Nail About.com. Web. updated November 4, 2008.
  7. Frenette, Julia. Fingerprint Patterns Odec.ca. Web. Accessed February 26, 2013.
  8. 8.0 8.1 Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. Page 62. Print.
  9. 9.0 9.1 German, Ed., German, Nannette. The History of Fingerprints onin.com. Web. Updated February 2. 2013.
  10. Wilhelmi, Bradon. Nerves Medscape. Web. Updated June 27,2011
  11. Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. 258. Print.
  12. QA International. types of synovial joints ikonet.com. Web. Accessed February 27, 2013.