The Lumbrical muscles - of which there are 4 - are muscles found on the palm side of your hand - located between your fingers.
There's a few interesting things about the lumbricals.
They don't attach to bones, which is weird for muscles.....instead they attach to tendons found on the palm side (called ventral) and back of (called dorsal) surface of your fingers.
On the palmar side, they originate from the tendons called the flexor digitorum profundus - these help you bend your fingers into a fist. On the back side, they insert into a cool structure called the dorsal digital expansion - which is kind of like a hood that goes over each of your fingers.
They have split innervation, going to the medial 2 fingers (i.e. the pinky and the ring finger) is the ulna nerve, and to the lateral 2 fingers (your middle and index finger) is the median nerve. If you get carpal tunnel syndrome, it'll be the middle and index finger affected (+ your thumb) as the median nerve passes through the carpal tunnel.
As for what they do, Leighton Hewitt - the Aussie tennis player of all people - is instructive.
See how his fingers are all bent at the palm? Well that's the lumbricals. They flex what is called the metacaropphalangeal joint, AND, see how all his fingers are straight? Well the lumbricals do that too, the fingers are in what is called the extended position.
So that's it, the lubricals flex your 4 fingers at the palm, and allow you to straighten (extend) your 4 fingers....all without attaching to any bones whatsoever!
Cheers till next time
Ash
PS: If you'd like any anatomy stuff described in detail lemme know, I'll be happy to oblige