The lumbricals are intrinsic muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s of the hand

Hand

A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

 that flex

Flexion

In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...

 the metacarpophalangeal joint

Metacarpophalangeal joint

The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

s and extend

Extension (kinesiology)

In kinesiology, extension is a movement of a joint that results in increased angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extending the flexed elbow. Straightening of...

 the interphalangeal joints.

Structure

There are four of these small, worm-like muscles on each hand. These muscles are unusual in that they do not attach to bone. Instead they attach proximally to the tendons of flexor digitorum profundus and distally to the extensor expansion

Extensor expansion

A extensor expansion is an anatomical term that refers to the special connective attachments by which the extensor tendons insert into the phalanges....

s.

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Form

Origin

Insertion >- | first

unipennate

It originates from the radial side of the most radial tendon of the flexor digitorum profundus (corresponding to the index finger).

extensor expansion Extensor expansion A extensor expansion is an anatomical term that refers to the special connective attachments by which the extensor tendons insert into the phalanges....  near the metacarpophalangeal joint Metacarpophalangeal joint The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint... . >- | second

unipennate

It originates from the radial side of the second most radial tendon of the flexor digitorum profundus (which corresponds to the middle finger).

>- | third

bipennate

One head originates on the radial side of the flexor digitorum profundus tendon corresponding to the ring finger, while the other originates on the ulnar side of the tendon for the middle finger.

>- | fourth

bipennate

One head originates on the radial side of the flexor digitorum profundus tendon corresponding to the little finger, while the other originates on the ulnar side of the tendon for the ring finger.

The muscle passes posteriorly along the radial side of the little finger to insert on its extensor expansion.

Innervation

The first and second lumbricals (the most radial two) are innervated by the median nerve

Median nerve

The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

.
The third and fourth lumbricals (most ulnar two) are innervated by the deep branch of the ulnar nerve.

Blood supply

There are four separate sources of blood supply for these muscles: the superficial palmar arch

Superficial palmar arch

The superficial palmar arch is formed predominantly by the ulnar artery, with a contribution from the superficial palmar branch of the radial artery...

, the common palmar digital artery, the deep palmar arch

Deep palmar arch

The deep palmar arch is an arterial network found in the palm. It is usually formed mainly from the terminal part of the radial artery, with the ulnar artery contributing via its deep palmar branch...

, and the dorsal digital artery.

Actions

The lumbrical muscles, with the help of the interosseous muscles, simultaneously flex the metacarpophalangeal joint

Metacarpophalangeal joint

The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

s while extending both interphalangeal joints of the digit on which it inserts. The lumbricals are used during an upstroke in writing.

Other lumbricals

There are also lumbrical muscles of the foot that have a similar action, though these are of less clinical concern.

Additional images