Distal phalanges
Encyclopedia
The distal or terminal phalanges (singular phalanx) are the terminal limb bone
s located at the tip of the digits (i.e. finger
s and toe
s). In human anatomy
, the distal phalanges of the four ulnar fingers and lesser toes articulate proximally with the intermediate phalanges
at the distal interphalangeal joint
s (DIP); in the thumb and big toe, with only two phalanges, the distal phalanges articulate proximally with the proximal phalanges
. The distal phalanges carry and shape nails
and claw
s and are therefore occasionally referred to as the ungual phalanges. The distal phalanges are cone-shaped in most mammals, including most primates, but relatively wide and flat in humans.
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.
In the hand, the distal ends of the distal phalanges possess flat and wide expansions called apical tufts. They serve to support the fleshy pad or pulp on the volar side of the fingertips and the nails on the dorsal side.
The shaft of the distal phalanx is expanded proximally at the base and "waisted" distally. It ends in a crescent-shaped rough cap of bone epiphysis
— 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 the flexor pollicis longus
. Another ridge at the base serves for the insertion of the extensor aponeurosis
.
The flexor insertion is sided by two fossa
e — the ungual fossa distally and the proximopalmar fossa proximally.
(asymmetric towards the radial side), an ungual fossa, and a pair of dissymmetric 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 Neanderthal
s, 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 in hominins, 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 primates phylogeny and style of locomotion appear to play a role in apical tuft size. Suspensory primates and New World monkey
s have the smallest apical tufts, while terrestrial quadrupeds and Strepsirrhines
have 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.
In ungulates (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-toed and even-toed ungulates, what remain of the "hands" — the metacarpal and phalangeal bones — are elongated to the extent that they serve little use beyond locomotion. The giraffe
, the largest even-toed ungulate, has large terminal phalanges and fused metacarpal bones able to absorb the stress from running.
The sloth
spend 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 have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joint
s. 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.
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s located at the tip of the digits (i.e. finger
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....
s and toe
Toe
Toes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being digitigrade. Humans, and other animals that walk on the soles of their feet, are described as being plantigrade; unguligrade animals are those that walk on hooves at the tips of...
s). In human anatomy
Human anatomy
Human anatomy is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross anatomy is the study of anatomical structures that can be seen by the naked eye...
, the distal phalanges of the four ulnar fingers and lesser toes articulate proximally with the intermediate phalanges
Intermediate phalanges
Intermediate phalanges are bones found in the limbs of most vertebrates. In humans, they are the bones of the finger and toe which lie in the middle, between the two wrinkly joints. The thumb and big toe do not have intermediate phalanges...
at the distal interphalangeal joint
Distal interphalangeal joint
Distal interphalangeal joint can refer to:* Interphalangeal articulations of hand* Interphalangeal articulations of foot...
s (DIP); in the thumb and big toe, with only two phalanges, the distal phalanges articulate proximally with the proximal phalanges
Proximal phalanges
Proximal phalanges are bones found in the limbs of most vertebrates. In humans, they are the bones at the base of a toe or finger, the prominent, knobby ends of which are often called the knuckles....
. The distal phalanges carry and shape nails
Nail (anatomy)
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....
and claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...
s and are therefore occasionally referred to as the ungual phalanges. The distal phalanges are cone-shaped in most mammals, including most primates, but relatively wide and flat in humans.
Human anatomy
Fingers
The distal phalanges of the fingers are convex on their dorsal and flat on their volar surfaces; they are recognized by their small size, and by a roughened, elevated surface of a horseshoe form on the volar surface of the distal extremity of each which serves to support the sensitive pulp of the finger.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.
In the hand, the distal ends of the distal phalanges possess flat and wide expansions called apical tufts. They serve to support the fleshy pad or pulp on the volar side of the fingertips and the nails on the dorsal side.
The shaft of the distal phalanx is expanded proximally at the base and "waisted" distally. It ends in a crescent-shaped rough cap of bone epiphysis
Epiphysis
The epiphysis is the rounded end of a long bone, at its joint with adjacent bone. Between the epiphysis and diaphysis lies the metaphysis, including the epiphyseal plate...
— 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 the flexor pollicis longus
Flexor pollicis longus muscle
The flexor pollicis longus is a muscle in the forearm and hand that flexes the thumb...
. Another ridge at the base serves for the insertion of the extensor aponeurosis
Aponeurosis
Aponeuroses are layers of flat broad tendons. They have a shiny, whitish-silvery color, are histologically similar to tendons, and are very sparingly supplied with blood vessels and nerves. When dissected, aponeuroses are papery, and peel off by sections...
.
The flexor insertion is sided by two fossa
Fossa
Fossa may refer to:*Fossa dei Leoni, an Italian football supporters association of Milan*La Fossa, an Italian rap group*Formaggio di fossa, a sheep's milk cheese from Sogliano al Rubicone, Italy*Cacio di Fossa, a hard, sharp sheep's milk Italian cheese...
e — the ungual fossa distally and the proximopalmar fossa proximally.
Thumb
The human pollical distal phalanx (PDP) has a pronounced insertion for the flexor pollicis longusFlexor pollicis longus muscle
The flexor pollicis longus is a muscle in the forearm and hand that flexes the thumb...
(asymmetric towards the radial side), an ungual fossa, and a pair of dissymmetric 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.
Toes
The distal phalanges of the toes, in form, resemble those of the fingers; but they 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.Evolutionary variation
Primates
The morphology of the distal phalanges of human thumbs closely reflects an adaptation for a refined precision grip with pad-to-pad contact. While this has traditionally been associated with the advent of stone tool-making, the intrinsic hand proportions of australopiths and the resemblance between human hands and the short hands of Miocene apes, suggest that human hand proportions are largely plesiomorphic — in contrast to the derived elongated hand pattern and poorly developed thumb musculature of extant hominoids. The capability of a pad-to-pad precision grip in human hands is reflected in the morphology of the distal phalanges, especially in the pollical distal phalanges (PDP).In Neanderthal
Neanderthal
The Neanderthal is an extinct member of the Homo genus known from Pleistocene specimens found in Europe and parts of western and central Asia...
s, 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 in hominins, 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 primates phylogeny and style of locomotion appear to play a role in apical tuft size. Suspensory primates and New World monkey
New World monkey
New World monkeys are the five families of primates that are found in Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Platyrrhini parvorder and the Ceboidea superfamily, which are essentially synonymous since...
s have the smallest apical tufts, while terrestrial quadrupeds and Strepsirrhines
Strepsirrhini
The clade Strepsirrhini is one of the two suborders of primates. Madagascar's only non-human primates are strepsirrhines, and others can be found in southeast Asia and Africa...
have 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.
Other mammals
Giraffe
The giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant...
, the largest even-toed ungulate, has large terminal phalanges and fused metacarpal bones able to absorb the stress from running.
The sloth
Sloth
Sloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae , part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.They are arboreal residents of the jungles of Central and South...
spend 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 have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joint
Interphalangeal joint
Interphalangeal joint may refer to:*Interphalangeal articulations of hand*Interphalangeal articulations of foot...
s. 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.