This is a tutorial on the intrinsic muscles of the hand. The intrinsic muscles of the hand are those muscles that are responsible for position grip. The muscles that originate in the forearm and insert onto the hand are responsible for power grip and the muscles that originate in the hand and are responsible for position grip are the intrinsic muscles. This is a tutorial on those muscles.
I’ll just rid of these palmar aponeuroses and we’ll take a look at the muscles underneath.
On the thumb side, you’ve got the thenar muscles. On the side of the little finger, you’ve got the hypothenar muscles. We’re looking at a palmar view. This is the anterior view of the hands.
The thenar eminence is this eminence here on the thumb side. You can see this bulge on the hand of the thumb muscles. This is the thenar eminence. And you’ve got the hypothenar eminence on the little finger side.
Between the hypothenar and thenar muscles, you’ve got this fibrous band of connective tissue, which arches over the carpal bones. This is called the flexor retinaculum. This blends with the muscles of the thenar and the hypothenar muscles.
Beneath the flexor retinaculum, you’ve got flexor tendons and the median nerves that pass through. This is the carpal tunnel. The carpal tunnel is the space between the carpal bones and the flexor retinaculum and you’ve got these flexor tendons and the median nerve which pass through.
In the intrinsic muscles of the hand, you’ve got the hypothenar muscles, the thenar muscles, lumbricals, interosseous muscles, the palmaris brevis and the adductor pollicis. I’ll just talk you through those in this tutorial.
We’re just looking at a diagram of the left hand, anterior view. The reason that I’m showing you this is because there’s this little muscle here, the palmar brevis which isn’t shown in the 3D model. This muscle originates on the flexor retinaculum, so this thing here, which interconnects the hypothenar and thenar muscles. It inserts medially on the dermis of the skin. That’s the palmaris brevis and it overlies the hypothenar muscles, which I’ll come on to talk about.
Next we’ve got the lumbrical muscles. These muscles flex the metacarpophalangal joints. You’ve got four lumbrical muscles and they originate on the tendons of the flexor digitorum profundus. You can see these tendons here. These muscles originate on the sides of these tendons and they insert onto the extensor hoods of the four fingers – so the index, middle, ring and little fingers. They flex the metacarpophalangeal joints while extending the interphalangeal joints.
I’ll just flick over to a diagram because this diagram just makes it a bit more clear. You can see the tendons of the flexor digitorum profundus and you can see the lumbricals attaching to them and inserting onto the extensor hoods of the four fingers.
The extensor hoods are these things on the fingers, which extend from the tendons of the extensor tendons and they wrap around the phalanges in a triangular shape. And you’ve got the intrinsic muscles and various other muscles which inserts onto these extensor hoods.
We’re just looking at a diagram with an extensor hood here. It just shows things more clearly. We’re looking at the dorsal surfaces of a metacarpal and the phalanges. As the extensor digitorum passes over the dorsal surface of this fingers, the tendon flattens out and wraps around these fingers. This is what the extensor hood is. It’s this triangular shape. It extends up to the distal phalanx and it covers the middle phalanx and it wraps around the sides of the metacarpophalangeal joint. You’ve got these intrinsic muscles which insert onto the extensor hood.
That’s just something to be aware of. It’s like a flat tendon, an aponeurosis, triangular shaped, which serves as the point of attachment for several muscles. It’s important to be aware of that.
Just coming back to this diagram looking at the muscles of the thenar eminence, these muscles that are responsible for the movements of the thumb, you’ve got three muscles in this group. You’ve got the opponens pollicis, which is the most deep muscles and you’ve got two muscles which lie above this, the abductor pollicis brevis and the flexor pollicis brevis.
Again, the names of the muscles give away some idea of the function. The word ‘pollicis’ is Latin referring to thumb. It actually means ‘of the thumb’. Opponens pollicis is the muscle which opposes the thumb. Abductor pollicis brevis abducts the thumb. The ‘brevis’ means ‘short’ in Latin. There are two muscles which abduct the thumb, one of which originates in the forearm, so the abductor pollicis longus. This is the abductor pollicis brevis and it’s an intrinsic muscle which lies in the hand. And then you’ve got the flexor pollicis brevis, which flexes the thumb.
These two muscles, the abductor pollicis brevis and flexor pollicis brevis are superficial. They lie above the opponens pollicis. These muscles originate on the flexor retinaculum and also on the carpal bones and they insert onto the thumb.
You can see that the flexor pollicis brevis is distal and the abductor pollicis brevis is more proximal. You’ve got the opponens pollicis lying deep to these muscles.
These muscles together with the lateral two lumbricals (so the first two lumbricals on the index and middle fingers) are innervated by the median nerve. A way of remembering this is the mnemonic MeatLOAF. Meat refers to median to help you remember. And then loaf stands for lateral lumbricals (these two lumbricals here) and then you’ve got the opponens pollicis, the abductor pollicis brevis and the flexor pollicis brevis. LOAF, lateral two lumbricals (l), opponens pollicis (o), abductor pollicis brevis (a) and (f) for flexor pollicis brevis. MeatLOAF, the median nerve innervates these muscles here.
Now that you know the thenar muscles, it’s quite easy to learn the hypothenar muscles. You just replace the word pollicis with digiti minimi, which in Latin means the ‘littlest finger’, the ‘smallest finger’.
You’ve got the opponens digiti minimi, which lies deep just like on the other side and you’ve got the abductor and flexor, so you’ve got abductor digiti minimi and the flexor digiti minimi brevis. And again, the abductor lies proximally and the flexor lies distally. These two muscles lie above the opponens digiti minimi, which lies deep.
The abductor digiti minimi originates on the pisiform bone. This is that little bone that sits on the triquetral bone. It also originates on the pisohamate ligament. There’s a little ligament which attaches from the pisiform bone to the hook of the hamate, which I showed you in the carpal bone tutorial. It inserts onto the proximal phalanx of the little finger. What this muscle does as the name suggests is that it abducts the little finger at the MCP joint.
The flexor digiti minimi brevis, this muscle originates on the hook of the hamate and also on the flexor retinaculum. It inserts onto the proximal phalanx. This muscle flexes at the MPC joint.
Just like the other side, you’ve got the abductor and the flexor lying side by side above the opponens muscle.
We’ll just take a look at the opponens muscle. Here, we’re just looking at the 3D model, left hand again. You’ve got these hypothenar muscles here. You’ve got the abductor digiti minimi and the flexor digiti minimi brevis. This actually originates on the pisiform bone, so it’s a little bit out of place. That’s the abductor.
And you’ve got the flexor digiti minimi brevis. This muscle originates on the hook of the hamate. This hamate bone here is called ‘hamate’ because the Latin ‘hamus’ refers to hook. It means ‘hook’, so this little hook process here. And you’ve got the pisohamate ligament, so that’s that ligament which connects the pisiform bone and the hook of the hamate.
The flexor digiti minimi brevis originates on the hook of the hamate and inserts onto the proximal phalanx of the little finger.
Just underneath it, you’ve got this muscle here. This is the opponens digiti minimi. This also originates on the hook of the hamate and the flexor retinaculum and then it inserts onto the medial aspect. You can see it inserting medially on this fifth metacarpal. What this muscle does is it laterally rotates the fifth metacarpal, so the little finger. Those are the three muscles of the hypothenar group.
I mentioned before the mnemonic MeatLOAF, which helps you remember that the median nerve innervates the lateral two lumbricals and the muscles of the thenar eminence. The rest of the muscles, intrinsic muscles of the hand are actually innervated by the ulnar nerve. The hypothenar muscles are innervated by the deep branch of the ulnar nerve.
We’ve talked about the palmaris brevis muscle, the muscles of the hypothenar and thenar eminence. We talked about the lumbricals. Next, we’ve got this little muscle here, the adductor pollicis. This muscle, as the name suggests, adducts the thumb. ‘Adductor’ and ‘pollicis’ referring to thumb, so it’s the adductor of the thumb. It inserts onto the base of the proximal phalanx and also the extensor hood of the thumb.
It’s got two heads. It’s got a transverse head and an oblique head. The oblique head has its fibers angled obliquely and the transverse head has horizontal fibers, which gives rise to the names.
This diagram here just shows it a bit more clearly. We’re looking at a view of the left hand still. You’ve got the thumb here and the rest is kind of flexed. We’ve removed the superficial muscles here. We got rid of the flexor and abductor pollicis and we’re looking at the adductor muscle. You can see the transverse head and the oblique head with its oblique fibers.
This muscle originates – well, it has different origins depending on the head. The adductor pollicis, the oblique head originates on the bases of metacarpals 2 and 3 and also, the capitate bone. You’ve got the transverse head here. This originates, as you can see, on the third metacarpal. This muscle inserts on the base of the proximal phalanx of the thumb and it adducts the thumb.
Okay! Just coming on to the last group of muscles, the interossei muscles. You’ve got dorsal and palmar interossei. We’re looking at the back of the left hand here. We’re looking at the dorsal surface. You can see these muscles in between the metacarpal bones. These are the dorsal interossei muscle. They originate on the sides of these metacarpal bones, the adjacent sides and they insert onto the extensor hoods. Do you remember that diagram that I showed you earlier? These muscles insert onto the extensor hood.
The dorsal interossei abduct at the index, middle and ring fingers at the metacarpal joints. The palmar interossei, you can see from the palmar surface and they again, originate on the side of the metacarpals and again insert into extensor hoods, but these muscles adducts. The palmar interossei adduct.
A way of remembering the functions of the dorsal and the palmar interossei is the mnemonic PAD and DAB. Palmar adduct and dorsal abduct. PAD and DAB.
Those are the intrinsic muscles of the hand. You’ve got the thenar group, the hypothenar group, you’ve got the lumbricals, the dorsal and palmar interossei muscles and then you’ve got these little muscles, the palmaris brevis and the adductor pollicis. Those are the muscles of the hand.