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The sole is the bottom of the
foot The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg mad ...
. In humans the sole of the foot is anatomically referred to as the plantar aspect.


Structure

The glabrous skin on the sole of the foot lacks the hair and pigmentation found elsewhere on the body, and it has a high concentration of sweat pores. The sole contains the thickest layers of skin on the body due to the weight that is continually placed on it. It is crossed by a set of creases that form during the early stages of embryonic development. Like those of the palm, the sweat pores of the sole lack
sebaceous glands A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number ...
. The sole is a sensory organ by which we can perceive the ground while standing and walking. The subcutaneous tissue in the sole has adapted to deal with the high local compressive forces on the
heel The heel is the prominence at the posterior end of the foot. It is based on the projection of one bone, the calcaneus or heel bone, behind the articulation of the bones of the lower leg. Structure To distribute the compressive forces exer ...
and the ball (between the toes and the
arch An arch is a vertical curved structure that spans an elevated space and may or may not support the weight above it, or in case of a horizontal arch like an arch dam, the hydrostatic pressure against it. Arches may be synonymous with vau ...
) by developing a system of "pressure chambers." Each chamber is composed of internal fibrofatty tissue covered by external
collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whol ...
connective tissue. The
septa The Southeastern Pennsylvania Transportation Authority (SEPTA) is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly 4 million people in five c ...
(internal walls) of these chambers are permeated by numerous
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide awa ...
s, making the sole one of the most
vascularized Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splittin ...
, or blood-enriched, regions in the human body. The sole and the longitudinal arch of the foot are supported by a thick connective tissue, the
plantar fascia The plantar fascia is the thick connective tissue (aponeurosis) which supports the arch on the bottom (plantar side) of the foot. It runs from the tuberosity of the calcaneus (heel bone) forward to the heads of the metatarsal bones (the bone b ...
. The central component of this tissue extends to the supporting bones and gives two divisions–the medial component and lateral component; thus they define the boundaries of the three muscle compartments of the sole (see below). The bones underlying the sole form the
arch An arch is a vertical curved structure that spans an elevated space and may or may not support the weight above it, or in case of a horizontal arch like an arch dam, the hydrostatic pressure against it. Arches may be synonymous with vau ...
of the foot. The arches might collapse later in life, resulting in flat feet.


Muscles


Intrinsic

The intrinsic muscles in the sole are grouped in four layers: In the first layer, the flexor digitorum brevis is the large central muscle located immediately above the plantar aponeurosis. It flexes the second to fifth toes and is flanked by
abductor hallucis The abductor hallucis muscle is an intrinsic muscle of the foot. It participates in the abduction and flexion of the great toe. Structure The abductor hallucis muscle is located in the medial border of the foot and contributes to form the promi ...
and abductor digiti minimi. In the second layer, the
quadratus plantae The quadratus plantae (flexor accessorius) is separated from the muscles of the first layer by the lateral plantar vessels and nerve. It acts to aid in flexing the 2nd to 5th toes (offsetting the oblique pull of the flexor digitorum longus) and is ...
, located below flexor digitorum brevis, inserts into the tendon of flexor digitorum longus on which the lumbricals originate. In the third layer, the oblique head of adductor hallucis joins the muscle's transversal head on the lateral side of the big toe. Medially to adductor hallucis are the two heads of
flexor hallucis brevis A flexor is a muscle that flexes a joint. In anatomy, flexion (from the Latin verb ''flectere'', to bend) is a joint movement that decreases the angle between the bones that converge at the joint. For example, one’s elbow joint flexes when on ...
, deep to the tendon of flexor hallucis longus. The considerably smaller flexor digiti minimi brevis on the lateral side can be mistaken for one of the interossei. In the fourth layer. the dorsal and
plantar interossei In human anatomy, plantar interossei muscles are three muscles located between the metatarsal bones in the foot. Structure The three plantar interosseous muscles are unipennate, as opposed to the bipennate structure of dorsal interosseous muscle ...
are located between and below the metatarsal bones and act as antagonists. The central compartment is shared by the lumbricals, quadratus plantae, flexor digitorum brevis, and adductor hallucis; the medial compartment by abductor hallucis, flexor hallucis brevis, abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi (often considered part of the former muscle); whilst the lateral compartment is occupied by extensor digitorum brevis and extensor hallucis brevis.


Extrinsic

The tendons of several extrinsic foot muscle reach the sole: * The tendons of the deep foot flexors in the posterior compartment of the leg,
tibialis posterior The tibialis posterior muscle is the most central of all the leg muscles, and is located in the deep posterior compartment of the leg. It is the key stabilizing muscle of the lower leg. Structure The tibialis posterior muscle originates on the i ...
, flexor digitorum longus, and
flexor hallucis longus The flexor hallucis longus muscle (FHL) is one of the three deep muscles of the posterior compartment of the leg that attaches to the plantar surface of the distal phalanx of the great toe. The other deep muscles are the flexor digitorum longus an ...
, passes behind the medial malleolus into the sole. * The tendon of fibularis longus similarly passes behind the lateral malleolus into the sole.


Nerve supply

The soles of the feet are extremely sensitive to touch due to a high concentration of nerve endings, with as many as 200,000 per sole. This makes them sensitive to surfaces that are walked on, ticklish and some people find them to be
erogenous zone An erogenous zone (from Greek , ''érōs'' "love"; and English ''-genous'' "producing", from Greek , ''-genḗs'' "born") is an area of the human body that has heightened sensitivity, the stimulation of which may generate a sexual response, suc ...
s. Medically, the soles are the site of the plantar reflex, the testing of which can be painful due to the sole's sensitivity. The
deep fibular nerve The deep fibular nerve (also known as deep peroneal nerve) begins at the bifurcation of the common fibular nerve between the fibula and upper part of the fibularis longus, passes infero-medially, deep to the extensor digitorum longus, to the ante ...
from the common fibular nerve provides the sensory innervation of the skin between the first and second toes and the motor innervation of the muscles of the anterior compartment of the leg and dorsal foot. Damage to the deep fibular nerve can result in
foot drop Foot drop is a gait abnormality in which the dropping of the forefoot happens due to weakness, irritation or damage to the deep fibular nerve (deep peroneal), including the sciatic nerve, or paralysis of the muscles in the anterior portion of t ...
. The plantar digital nerves from the
medial plantar nerve The medial plantar nerve (internal plantar nerve) is the larger of the two terminal divisions of the tibial nerve (medial and lateral plantar nerve), which accompanies the medial plantar artery. From its origin under the laciniate ligament it pa ...
provide sensory innervation to the skin of the plantar aspect of the toes, except for the medial part of the big toe and the lateral part of the little toe and the motor innervation of the first lumbrical. The proper plantar nerve from the common plantar digital nerve provides sensory innervation to the plantar surface of the toes as well as the dorsal aspect of the distal interphalangeal phalanges. It also provides motor innervation to flexor hallucis brevis. The superficial and deep branches of the
lateral plantar nerve The lateral plantar nerve (external plantar nerve) is a branch of the tibial nerve, in turn a branch of the sciatic nerve and supplies the skin of the fifth toe and lateral half of the fourth, as well as most of the deep muscles, its distribution ...
from the tibial nerve provide sensory innervation to the skin of the lateral side of the sole, to the fifth and half the fourth toes, and the nail bed of these toes. They also provide motor innervation to quadratus plantae, abductor digiti minimi, flexor digiti minimi brevis, lateral three lumbricals, adductor hallucis, and the dorsal and plantar interossei. The
medial plantar nerve The medial plantar nerve (internal plantar nerve) is the larger of the two terminal divisions of the tibial nerve (medial and lateral plantar nerve), which accompanies the medial plantar artery. From its origin under the laciniate ligament it pa ...
from the tibial nerve provides sensory innervation to the skin of the medial side of the sole, the skin of the medial three and a half toes, and the nail beds of these toes. It also provides motor innervation to abductor hallucis, flexor hallucis brevis, flexor digitorum brevis, and the first lumbrical. The
saphenous nerve The saphenous nerve (long or internal saphenous nerve) is the largest cutaneous branch of the femoral nerve. It is a strictly sensory nerve, and has no motor function. Structure It is purely a sensory nerve. The saphenous nerve is the larges ...
from the femoral nerve provides sensory innervation to the medial side of the foot as well as the medial side of the leg. Likewise, the sural nerve provides sensory innervation to the skin on the lateral side of the foot as well as the skin on the posterior aspect of the lower leg. The tibial nerve from the sciatic nerve provides sensory innervation to the skin of the sole and toes, and the dorsal aspect of the toes. It provides motor innervation to plantaris, tibialis posterior, flexor hallucis longus, flexor digitorum longus as well as posterior muscles in the leg.


Society and culture

The purpose of protecting the sole against uncomfortable and harmful impacts of the environment during locomotion initiated the general introduction of
footwear Footwear refers to garments worn on the feet, which typically serves the purpose of protection against adversities of the environment such as wear from ground textures and temperature. Footwear in the manner of shoes therefore primarily serves ...
in early human history. The achieved protection of the susceptible soles provided for faster and considerably more efficient movement particularly in adverse environmental terrains as against walking or running on bare feet. The sensitivity of the sole makes it an objective for a sensual touch, tickling or sexual stimulation and also a target for means of corporal punishment. The beating of the soles of a person's bare feet ( foot whipping or "bastinado") with specific objects such as rods and canes has hereby served as a means of corporal punishment and discipline in various civilizations to this day. The application of this measure is mostly found in prisons and similar institutions, such as schools and reformatories. It is also a common method of physical
torture Torture is the deliberate infliction of severe pain or suffering on a person for reasons such as punishment, extracting a confession, interrogation for information, or intimidating third parties. Some definitions are restricted to acts ...
. In Thailand, Saudi Arabia, and some Muslim countries it is considered offensive to sit raising the leg so the uncovered sole of the foot is visible and therefore taboo.


Other animals

Terrestrial animals using their soles for locomotion are called plantigrade. In
chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative t ...
s, the soles are furrowed with creases deeper and more distinct than in their palms. In the palms, the pattern density is thickest in the central part, but in the sole, the density is thickest near the big toe whilst a large part of the remaining sole is covered by thick, tight, and smooth skin almost without furrows. In
bonobo The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee and less often the dwarf chimpanzee or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus '' Pan,'' the other being the comm ...
s, the pattern intensity of the epidermal ridges (i.e. "fingerprints") of the palms and soles is considerably higher than in chimpanzees. Whilst the pattern intensity in the palm is the highest of all species of apes, in the sole, the density decreases and is comparable to other apes.


Clinical significance

The sole is subject to many skin diseases.


See also

* Barefoot * Footprint *
Gait (human) A gait is a pattern of limb movements made during locomotion. Human gaits are the various ways in which humans can move, either naturally or as a result of specialized training. Human gait is defined as bipedal, biphasic forward propulsion o ...
* Human skeletal changes due to bipedalism


Notes


References

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External links

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