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The Muscles and Fasciæ of the Foot


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The Muscles and Fasciæ of the Foot
1. The Dorsal Muscle of the Foot
 
Extensor digitorum brevis—The fascia on the dorsum of the foot is a thin membranous layer, continuous above with the transverse and cruciate crural ligaments; on either side it blends with the plantar aponeurosis; anteriorly it forms a sheath for the tendons on the dorsum of the foot.   1
The Extensor digitorum brevis (Fig. 441) is a broad, thin muscle, which arises from the forepart of the upper and lateral surfaces of the calcaneus, in front of the groove for the Peronæus brevis; from the lateral talocalcanean ligament; and from the common limb of the cruciate crural ligament. It passes obliquely across the dorsum of the foot, and ends in four tendons. The most medial, which is the largest, is inserted into the dorsal surface of the base of the first phalanx of the great toe, crossing the dorsalis pedis artery; it is frequently described as a separate muscle—the Extensor hallucis brevis. The other three are inserted into the lateral sides of the tendons of the Extensor digitorum longus of the second, third, and fourth toes.   2
 
Variations.—Accessory slips of origin from the talus and navicular, or from the external cunei-form and third metatarsal bones to the second slip of the muscle, and one from the cuboid to the third slip have been observed. The tendons vary in number and position; they may be reduced to two, or one of them may be doubled, or an additional slip may pass to the little toe. A supernumerary slip ending on one of the metatarsophalangeal articulations, or joining a dorsal interosseous muscle is not uncommon. Deep slips between this muscle and the Dorsal interossei occur.   3
 
Nerves.—It is supplied by the deep peroneal nerve.   4
 
Actions.—The Extensor digitorum brevis extends the phalanges of the four toes into which it is inserted, but in the great toe acts only on the first phalanx. The obliquity of its direction counteracts the oblique movement given to the toes by the long Extensor, so that when both muscles act, the toes are evenly extended.   5
 
2. The Plantar Muscles of the Foot
 
Plantar Aponeurosis (aponeurosis plantaris; plantar fascia).—The plantar aponeurosis is of great strength, and consists of pearly white glistening fibers, disposed, for the most part, longitudinally: it is divided into central, lateral, and medial portions.   6
  The central portion, the thickest, is narrow behind and attached to the medial process of the tuberosity of the calcaneus, posterior to the origin of the Flexor digitorum brevis; and becoming broader and thinner in front, divides near the heads of the metatarsal bones into five processes, one for each of the toes. Each of these processes divides opposite the metatarsophalangeal articulation into two strata, superficial and deep. The superficial stratum is inserted into the skin of the transverse sulcus which separates the toes from the sole. The deeper stratum divides into two slips which embrace the side of the Flexor tendons of the toes, and blend with the sheaths of the tendons, and with the transverse metatarsal ligament, thus forming a series of arches through which the tendons of the short and long Flexors pass to the toes. The intervals left between the five processes allow the digital vessels and nerves and the tendons of the Lumbricales to become superficial. At the point of division of the aponeurosis, numerous transverse fasciculi are superadded; these serve to increase the strength of the aponeurosis at this part by binding the processes together, and connecting them with the integument. The central portion of the plantar aponeurosis is continuous with the lateral and medial portions and sends upward into the foot, at the lines of junction, two strong vertical intermuscular septa, broader in front than behind, which separate the intermediate from the lateral and medial plantar groups of muscles; from these again are derived thinner transverse septa which separate the various layers of muscles in this region. The upper surface of this aponeurosis gives origin behind to the Flexor digitorum brevis.   7
  The lateral and medial portions of the plantar aponeurosis are thinner than the central piece, and cover the sides of the sole of the foot.   8
  The lateral portion covers the under surface of the Abductor digiti quinti; it is thin in front and thick behind, where it forms a strong band between the lateral process of the tuberosity of the calcaneus and the base of the fifth metatarsal bone; it is continuous medially with the central portion of the plantar aponeurosis, and laterally with the dorsal fascia.   9
  The medial portion is thin, and covers the under surface of the Abductor hallucis; it is attached behind to the laciniate ligament, and is continuous around the side of the foot with the dorsal fascia, and laterally with the central portion of the plantar aponeurosis.   10
  The muscles in the plantar region of the foot may be divided into three groups, in a similar manner to those in the hand. Those of the medial plantar region are connected with the great toe, and corrrespond with those of the thumb; those of the lateral plantar region are connected with the little toe, and correspond with those of the little finger; and those of the intermediate plantar region are connected with the tendons intervening between the two former groups. But in order to facilitate the description of these muscles, it is more convenient to divide them into four layers, in the order in which they are successively exposed.   11
 
The First Layer (Fig. 443).
Abductor hallucis.
Flexor digitorum brevis.
Abductor digiti quinti.
  The Abductor hallucis lies along the medial border of the foot and covers the origins of the plantar vessels and nerves. It arises from the medial process of the tuberosity of the calcaneus, from the laciniate ligament, from the plantar aponeurosis, and from the intermuscular septum between it and the Flexor digitorum brevis. The fibers end in a tendon, which is inserted, together with the medial tendon of the Flexor hallucis brevis, into the tibial side of the base of the first phalanx of the great toe.   12
 
Variations.—Slip to the base of the first phalanx of the second toe.   13
  The Flexor digitorum brevis lies in the middle of the sole of the foot, immediately above the central part of the plantar aponeurosis, with which it is firmly united. Its deep surface is separated from the lateral plantar vessels and nerves by a thin layer of fascia. It arises by a narrow tendon, from the medial process of the tuberosity of the calcaneus, from the central part of the plantar aponeurosis, and from the intermuscular septa between it and the adjacent muscles. It passes forward, and divides into four tendons, one for each of the four lesser toes. Opposite the bases of the first phalanges, each tendon divides into two slips, to allow of the passage of the corresponding tendon of the Flexor digitorum longus; the two portions of the tendon then unite and form a grooved channel for the reception of the accompanying long Flexor tendon. Finally, it divides a second time, and is inserted into the sides of the second phalanx about its middle. The mode of division of the tendons of the Flexor digitorum brevis, and of their insertion into the phalanges, is analogous to that of the tendons of the Flexor digitorum sublimis in the hand.   14
 
Variations.—Slip to the little toe frequently wanting, 23 per cent.; or it may be replaced by a small fusiform muscle arising from the long flexor tendon or from the Quadratus plantæ.   15
 
Fibrous Sheaths of the Flexor Tendons.—The terminal portions of the tendons of the long and short Flexor muscles are contained in osseoaponeurotic canals similar in their arrangement to those in the fingers. These canals are formed above by the phalanges and below by fibrous bands, which arch across the tendons, and are attached on either side to the margins of the phalanges. Opposite the bodies of the proximal and second phalanges the fibrous bands are strong, and the fibers are transverse; but opposite the joints they are much thinner, and the fibers are directed obliquely. Each canal contains a mucous sheath, which is reflected on the contained tendons.   16


FIG. 443– Muscles of the sole of the foot. First layer. (See enlarged image)
 
  The Abductor digiti quinti (Abductor minimi digiti) lies along the lateral border of the foot, and is in relation by its medial margin with the lateral plantar vessels and nerves. It arises, by a broad origin, from the lateral process of the tuberosity of the calcaneus, from the under surface of the calcaneus between the two processes of the tuberosity, from the forepart of the medial process, from the plantar aponeurosis, and from the intermuscular septum between it and the Flexor digitorum brevis. Its tendon, after gliding over a smooth facet on the under surface of the base of the fifth metatarsal bone, is inserted, with the Flexor digiti quinti brevis, into the fibular side of the base of the first phalanx of the fifth toe.   17
 
Variations.—Slips of origin from the tuberosity at the base of the fifth metatarsal. Abductor ossis metatarsi quinti, origin external tubercle of the calcaneus, insertion into tuberosity of the fifth metatarsal bone in common with or beneath the outer margin of the plantar fascia.   18
 
The Second Layer (Fig. 444).
Quadratus plantæ.
Lumbricales.
  The Quadratus plantæ (Flexor accessorius) is separated from the muscles of the first layer by the lateral plantar vessels and nerve. It arises by two heads, which are separated from each other by the long plantar ligament: the medial or larger head is muscular, and is attached to the medial concave surface of the calcaneus, below the groove which lodges the tendon of the Flexor hallucis longus; the lateral head, flat and tendinous, arises from the lateral border of the inferior surface of the calcaneus, in front of the lateral process of its tuberosity, and from the long plantar ligament. The two portions join at an acute angle, and end in a flattened band which is inserted into the lateral margin and upper and under surfaces of the tendon of the Flexor digitorum longus, forming a kind of groove, in which the tendon is lodged. It usually sends slips to those tendons of the Flexor digitorum longus which pass to the second, third, and fourth toes.   19
 
Variations.—Lateral head often wanting; entire muscle absent. Variation in the number of digital tendons to which fibers can be traced. Most frequent offsets are sent to the second, third and fourth toes; in many cases to the fifth as well; occasionally to two toes only.   20
  The Lumbricales are four small muscles, accessory to the tendons of the Flexor digitorum longus and numbered from the medial side of the foot; they arise from these tendons, as far back as their angles of division, each springing from two tendons, except the first. The muscles end in tendons, which pass forward on the medial sides of the four lesser toes, and are inserted into the expansions of the tendons of the Extensor digitorum longus on the dorsal surfaces of the first phalanges.   21
 
Variations.—Absence of one or more; doubling of the third or fourth. Insertion partly or wholly into the first phalanges.   22
 
The Third Layer (Fig. 445).
Flexor hallucis brevis.
Adductor hallucis.
Flexor digiti quinti brevis.
  The Flexor hallucis brevis arises, by a pointed tendinous process, from the medial part of the under surface of the cuboid bone, from the contiguous portion of the third cuneiform, and from the prolongation of the tendon of the Tibialis posterior which is attached to that bone. It divides in front into two portions, which are inserted into the medial and lateral sides of the base of the first phalanx of the great toe, a sesamoid bone being present in each tendon at its insertion. The medial portion is blended with the Abductor hallucis previous to its insertion; the lateral portion with the Adductor hallucis; the tendon of the Flexor hallucis longus lies in a groove between them; the lateral portion is sometimes described as the first Interosseous plantaris.   23
 
Variations.—Origin subject to considerable variation; it often receives fibers from the calcaneus or long plantar ligament. Attachment to the cuboid sometimes wanting. Slip to first phalanx of the second toe.   24
  The Adductor hallucis (Adductor obliquus hallucis) arises by two heads—oblique and transverse. The oblique head is a large, thick, fleshy mass, crossing the foot obliquely and occupying the hollow space under the first second, third, and fourth metatarsal bones. It arises from the bases of the second, third, and fourth metatarsal bones, and from the sheath of the tendon of the Peronæus longus, and is inserted, together with the lateral portion of the Flexor hallucis brevis, into the lateral side of the base of the first phalanx of the great toe. The transverse head (Transversus pedis) is a narrow, flat fasciculus which arises from the plantar metatarsophalangeal ligaments of the third, fourth, and fifth toes (sometimes only from the third and fourth), and from the transverse ligament of the metatarsus. It is inserted into the lateral side of the base of the first phalanx of the great toe, its fibers blending with the tendon of insertion of the oblique head.   25


FIG. 444– Muscles of the sole of the foot. Second layer. (See enlarged image)
 


FIG. 445– Muscles of the sole of the foot. Third layer. (See enlarged image)
 
 
Variations.—Slips to the base of the first phalanx of the second toe. Opponens hallucis, occasional slips from the adductor to the metatarsal bone of the great toe.   26
  The Abductor, Flexor brevis, and Adductor of the great toe, like the similar muscles of the thumb, give off, at their insertions, fibrous expansions to blend with the tendons of the Extensor digitorum longus.   27
  The Flexor digiti quinti brevis (Flexor brevis minimi digiti) lies under the metatarsal bone of the little toe, and resembles one of the Interossei. It arises from the base of the fifth metatarsal bone, and from the sheath of the Peronæus longus; its tendon is inserted into the lateral side of the base of the first phalanx of the fifth toe. Occasionally a few of the deeper fibers are inserted into the lateral part of the distal half of the fifth metatarsal bone; these are described by some as a distinct muscle, the Opponens digiti quinti.   28
 
The Fourth Layer
 
Interossei—The Interossei in the foot are similar to those in the hand, with this exception, that they are grouped around the middle line of the second digit, instead of that of the third. They are seven in number, and consist of two groups, dorsal and plantar.   29
  The Interossei dorsales (Dorsal interossei) (Fig. 446), four in number, are situated between the metatarsal bones. They are bipenniform muscles, each arising by two heads from the adjacent sides of the metatarsal bones between which it is placed; their tendons are inserted into the bases of the first phalanges, and into the aponeurosis of the tendons of the Extensor digitorum longus. In the angular interval left between the heads of each of the three lateral muscles, one of the perforating arteries passes to the dorsum of the foot; through the space between the heads of the first muscle the deep plantar branch of the dorsalis pedis artery enters the sole of the foot. The first is inserted into the medial side of the second toe; the other three are inserted into the lateral sides of the second, third, and fourth toes.   30


FIG. 446– The Interossei dorsales. Left foot. (See enlarged image)
 


FIG. 447– The Interossei plantares. Left foot. (See enlarged image)
 
  The Interossei plantares (Plantar interossei) (Fig. 447), three in number, lie beneath rather than between the metatarsal bones, and each is connected with but one metatarsal bone. They arise from the bases and medial sides of the bodies of the third, fourth, and fifth metatarsal bones, and are inserted into the medial sides of the bases of the first phalanges of the same toes, and into the aponeuroses of the tendons of the Extensor digitorum longus.   31
 
Nerves.—The Flexor digitorum brevis, the Flexor hallucis brevis, the Abductor hallucis, and the first Lumbricalis are supplied by the medial plantar nerve; all the other muscles in the sole of the foot by the lateral plantar. The first Interosseous dorsalis frequently receives an extra filament from the medial branch of the deep peroneal nerve on the dorsum of the foot, and the second Interosseous dorsalis a twig from the lateral branch of the same nerve.   32
 
Actions.—All the muscles of the foot act upon the toes, and may be grouped as abductors, adductors, flexors, or extensors. The abductors are the Interossei dorsales, the Abductor hallucis, and the Abductor digiti quinti. The Interossei dorsales are abductors from an imaginary line passing through the axis of the second toe, so that the first muscle draws the second toe medialward, toward the great toe, the second muscle draws the same toe lateralward, and the third and fourth draw the third and fourth toes in the same direction. Like the Interossei in the hand, each assists in flexing the first phalanx and extending the second and third phalanges. The Abductor hallucis abducts the great toe from the second, and also flexes its proximal phalanx. In the same way the action of the Abductor digiti quinti is twofold, as an abductor of this toe from the fourth, and also as a flexor of its proximal phalanx. The adductors are the Interossei plantares and the Adductor hallucis. The Interossei plantares adduct the third, fourth, and fifth toes toward the imaginary line passing through the second toe, and by means of their insertions into the aponeuroses of the Extensor tendons they assist in flexing the proximal phalanges and extending the middle and terminal phalanges. The oblique head of the Adductor hallucis is chiefly concerned in adducting the great toe toward the second one, but also assists in flexing this toe; the transverse head approximates all the toes and thus increases the curve of the transverse arch of the metatarsus. The flexors are the Flexor digitorum brevis, the Quadratus plantæ, the Flexor hallucis brevis, the Flexor digiti quinti brevis, and the Lumbricales. The Flexor digitorum brevis flexes the second phalanges upon the first, and, continuing its action, flexes the first phalanges also, and brings the toes together. The Quadratus plantæ assists the Flexor digitorum longus and converts the oblique pull of the tendons of that muscle into a direct backward pull upon the toes. The Flexor digiti quinti brevis flexes the little toe and draws its metatarsal bone downward and medialward. The Lumbricales, like the corresponding muscles in the hand, assist in flexing the proximal phalanges, and by their insertions into the tendons of the Extensor digitorum longus aid that muscle in straightening the middle and terminal phalanges. The Extensor digitorum brevis extends the first phalanx of the great toe and assists the long Extensor in extending the next three toes, and at the same time gives to the toes a lateral direction when they are extended.   33