Phalanx bone

"Phalange" redirects here. For other uses, seePhalanx (disambiguation).

Phalanx bone
Bones of the hand
Bones of the foot
Details
Articulations	Metacarpophalangeal,metatarsophalangeal,interphalangeal
Identifiers
Latin	phalanx pl. phalanges
Anatomical terms of bone

Thephalanges/fəˈlændʒiːz/(sg.:phalanx/ˈfælæŋks/) aredigitalbonesin thehandsandfeetof mostvertebrates.Inprimates,the thumbs and big toes have two phalanges while the otherdigitshave three phalanges. The phalanges are classed aslong bones.

Structure

The phalanges in a human hand

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Toe bones or phalanges of the foot. Note the big toe has no middle phalanx.
People vary; sometimes the smallest toe also has none (not shown).

Distal phalangesof the foot

Middle phalangesof the foot

Proximal phalangesof the foot

The phalanges are the bones that make up the fingers of the hand and the toes of the foot. There are 56 phalanges in the human body, with fourteen on each hand and foot. Three phalanges are present on each finger and toe, with the exception of thethumbandbig toe,which possess only two. The middle and far phalanges of thefourth andfifth toes are often fused together (symphalangism).The phalanges of thehandare commonly known as the finger bones. The phalanges of the foot differ from the hand in that they are often shorter and more compressed, especially in the proximal phalanges, those closest to the torso.

A phalanx is named according to whether it isproximal,middle, ordistaland its associated finger or toe. The proximal phalanges are those that are closest to the hand or foot. In the hand, the prominent, knobby ends of the phalanges are known asknuckles.The proximal phalanges join with themetacarpalsof the hand ormetatarsalsof the foot at themetacarpophalangeal jointormetatarsophalangeal joint.The intermediate phalanx is not only intermediate in location, but usually also in size. The thumb and large toe do not possess a middle phalanx. The distal phalanges are the bones at the tips of the fingers or toes. The proximal, intermediate, and distal phalanges articulate with one another throughinterphalangeal joints of handandinterphalangeal joints of the foot.

Bone anatomy

Each phalanx consists of a central part, called thebody,and two extremities.

• Thebodyis flat on either side, concave on the palmar surface, and convex on the dorsal surface.Its sides are marked with rough areas giving attachment to fibrous sheaths of flexor tendons. It tapers from above downwards.
• Theproximal extremitiesof the bones of the first row present oval, concave articular surfaces, broader from side to side than from front to back. The proximal extremity of each of the bones of the second and third rows presents a double concavity separated by a median ridge.
• Thedistal extremitiesare smaller than the proximal, and each ends in twocondyles(knuckles) separated by a shallow groove; the articular surface extends farther on the palmar than on the dorsal surface, a condition best marked in the bones of the first row.

In the foot, the proximal phalanges have a body that is compressed from side to side, convex above, and concave below. The base is concave, and the head presents a trochlear surface for articulation with the second phalanx.The middle are remarkably small and short, but rather broader than the proximal. The distal phalanges, as compared with the distal phalanges of the finger, are smaller and are flattened from above downward; each presents a broad base for articulation with the corresponding bone of the second row, and an expanded distal extremity for the support of the nail and end of the toe.

Distal phalanx

In the hand, the distal phalanges are flat on their palmar surface, small, and with a roughened, elevated surface of horseshoe form on the palmar surface, supporting the finger pulp.The flat, wide expansions found at the tips of the distal phalanges are called apical tufts. They support the fingertip pads and nails.The phalanx of the thumb has a pronounced insertion for theflexor pollicis longus(asymmetric towards the radial side), an ungual fossa, and a pair of unequal ungual spines (the ulnar being more prominent). This asymmetry is necessary to ensure that the thumb pulp is always facing the pulps of the other digits, an osteological configuration which provides the maximum contact surface with held objects.

In the foot, the distal phalanges are flat on their dorsal surface. It is largest proximally and tapers to the distal end. The proximal part of the phalanx presents a broad base for articulation with the middle phalanx, and an expanded distal extremity for the support of the nail and end of the toe.The phalanx ends in a crescent-shaped rough cap of boneepiphysis— the apical tuft (or ungual tuberosity/process) which covers a larger portion of the phalanx on the volar side than on the dorsal side. Two lateral ungual spines project proximally from the apical tuft. Near the base of the shaft are two lateral tubercles. Between these a V-shaped ridge extending proximally serves for the insertion of theflexor pollicis longus.Another ridge at the base serves for the insertion of the extensoraponeurosis.The flexor insertion is sided by twofossae— the ungual fossa distally and the proximopalmar fossa proximally.

Development

The number of phalanges in animals is often expressed as a "phalangeal formula" that indicates the numbers of phalanges in digits, beginning from the innermost medial or proximal. For example, humans have a 2-3-3-3-3 formula for thehand,meaning that the thumb has two phalanges, whilst the other fingers each have three.

In the distal phalanges of the hand the centres for the bodies appear at the distal extremities of the phalanges, instead of at the middle of the bodies, as in the other phalanges. Moreover, of all the bones of the hand, the distal phalanges are the first to ossify.

Function

Thumb and index finger of right hand during pad-to-pad precision grasping in ulnar view.

The distal phalanges ofungulatescarry and shapenailsandclawsand these in primates are referred to as theungualphalanges.

History of phalanges

Etymology

The term phalanx or phalanges refers to anancient Greek army formationin which soldiers stand side by side, several rows deep, like an arrangement of fingers or toes.

Other animals

Most landmammalsincluding humans have a 2-3-3-3-3 formula in both thehands(orpaws) andfeet.Primitivereptilesusually had the formula 2-3-4-4-5, and this pattern, with some modification, remained in many later reptiles and in themammal-like reptiles.The phalangeal formula in theflippersofcetaceans(marine mammals) varies widely due to hyperphalangy (the increase in number ofphalanx bonesin the digits). Inhumpback whales,for example, the phalangeal formula is 0/2/7/7/3; inpilot whalesthe formula is 1/10/7/2/1.

In vertebrates, proximal phalanges have a similar placement in the corresponding limbs, be theypaw,wingorfin.In many species, they are the longest and thickest phalanx ( "finger" bone). The middle phalanx also has a corresponding place in their limbs, whether they bepaw,wing,hooforfin.

The distal phalanges are cone-shaped in most mammals, including most primates, but relatively wide and flat in humans.

Primates

Morphological comparisons of pollical distal phalanges in African apes, extant humans and selected hominins. Although with several morphological differences, all the features related to refined manipulation in modern humans are already present in the late MioceneOrrorin.

The morphology of the distal phalanges of human thumbs closely reflects an adaptation for a refined precision grip with pad-to-pad contact. This has traditionally been associated with the advent of stone tool-making. However, the intrinsic hand proportions ofaustralopithsand the resemblance between human hands and the short hands ofMioceneapes, suggest that human hand proportions are largelyplesiomorphic(as found in ancestral species) — in contrast to the derived elongated hand pattern and poorly developed thumb musculature of other extanthominoids.

InNeanderthals,the apical tufts were expanded and more robust than in modern and early upper Paleolithic humans. A proposal that Neanderthal distal phalanges was an adaptation to colder climate (than in Africa) is not supported by a recent comparison showing that inhominins,cold-adapted populations possessed smaller apical tufts than do warm-adapted populations.

In non-human, living primates the apical tufts vary in size, but they are never larger than in humans. Enlarged apical tufts, to the extent they actually reflect expanded digital pulps, may have played a significant role in enhancing friction between the hand and held objects during Neolithic toolmaking.

Among non-human primatesphylogenesisand style of locomotion appear to play a role in apical tuft size. Suspensory primates andNew World monkeyshave the smallest apical tufts, while terrestrial quadrupeds andStrepsirrhineshave the largest. A study of the fingertip morphology of four small-bodied New World monkey species, indicated a correlation between increasing small-branch foraging and reduced flexor and extensor tubercles in distal phalanges and broadened distal parts of distal phalanges, coupled with expanded apical pads and developed epidermal ridges. This suggests that widened distal phalanges were developed in arboreal primates, rather than in quadrupedal terrestrial primates.

Cetaceans

Whales exhibit hyperphalangy. Hyperphalangy is an increase in the number of phalanges beyond theplesiomorphicmammal condition of three phalanges-per-digit.Hyperphalangy was present among extinctmarine reptiles--ichthyosaurs,plesiosaurs,andmosasaurs-- but not other marine mammals, leaving whales as the onlymarine mammalsto develop this characteristic.The evolutionary process continued over time, and a very derived form of hyperphalangy, with six or more phalanges per digit, evolved convergently inrorqual whalesandoceanic dolphins,and was likely associated with another wave of signaling within the interdigital tissues.

Other mammals

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  Comparison of the phalanges of an orangutan, dog, pig, cow, tapir and horse
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  Distal phalanges of aMasai giraffe

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  Three-fingered sloth
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  Terminal phalanx of aScelidotheriumground sloth

Inungulates(hoofed mammals) the forelimb is optimized for speed and endurance by a combination of length of stride and rapid step; the proximal forelimb segments are short with large muscles, while the distal segments are elongated with less musculature. In two of the major groups of ungulates,odd-toedandeven-toedungulates, what remain of the "hands" — themetacarpaland phalangeal bones — are elongated to the extent that they serve little use beyond locomotion. Thegiraffe,the largest even-toed ungulate, has large terminal phalanges and fused metacarpal bones able to absorb the stress from running.

Theslothspends its life hanging upside-down from branches, and has highly specialized third and fourth digits for the purpose. They have short and squat proximal phalanges with much longer terminal phalanges. They havevestigialsecond and fifth metacarpals, and their palm extends to the distalinterphalangeal joints.The arboreal specialization of these terminal phalanges makes it impossible for the sloth to walk on the ground where the animal has to drag its body with its claws.

Additional images

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  Phalanges.
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  Phalanges.
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  Unilateral extra phalangeal crease
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  Joints of the hand in an X-ray image
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  Movement of the three finger phalanges; Distal, middle and proximal

See also

This article usesanatomical terminology.

• Hand
• Foot

External links

• MedTerms Medical Dictionary