Deep Layer of the Anterior Compartment of the Forearm

00:01 Now, let's move on and have a look at the deep layer of the forearm.

00:06 So, the deep layer of the forearm within this anterior compartment, we have a couple of muscles.

00:11 We have flexor digitorum profundus and we have flexor pollicis longus.

00:16 So, flexor digitorum profundus is going to attach to the four digits we mentioned previously for flexor superficialis.

00:24 Now, we're going to have flexor pollicis longus here as well which is associated with the first digit and that is going to be the thumb.

00:33 Most deep within this layer and sometimes, it's really kind of difficult to find because you need to move some of the tendons distally away. We have pronator quadratus.

00:43 And this muscle, as its name suggests, is important in pronating the forearm.

00:48 In between the two bones of the forearm here, we've got an interosseous membrane and that's highlighted here because it forms an important attachment site for some of these muscles.

00:58 So, let's have a look at some of the attachments of these muscles.

01:02 Let's start with flexor digitorum profundus. It comes from the anterior and the medial surfaces of the ulna and also, from the interosseus membrane.

01:11 So, the anterior and medial surfaces of the ulna and also, the interosseus membrane before passing deep now to flexor digitorum superficialis but still, within the carpal tunnel, it's attaching to the medial four distal phalanges.

01:26 So, not the middle ones.

01:28 This time, it's the medial four distal phalanges, the most distal tips of the fingers.

01:34 It's also innervated via the median nerve like a lot of these muscles are supplied by the median nerve except flexi carpi ulnaris.

01:43 If we have a look more closely at the nerve, we can see the median nerve is passing down through the cubital fossa into this space and it also gives rise to the anterior interosseus nerve that runs alongside the interosseus membrane.

01:56 The ulnar nerve does play an important part in supplying the most ulna aspect of this muscle.

02:02 The most substantial part is supplied by the median nerve.

02:05 But the ulnar nerve supplies the medial part of the muscle and does give a contribution.

02:11 Let's have a look at the function of flexor digitorum profundus.

02:14 So, flexor digitorum profundus is going to be important in flexing the wrist like a lot of the muscles in this space, flexion of the wrist here by flexor digitorum profundus, it's also going to help flex the distal interphalangeal joint.

02:29 That's the joint between the distal and the middle phalanges as it crosses that distal interphalangeal joint it's going to help flex the distal phalanges.

02:39 Continued contraction of that muscle will help to contribute to forming a closed grip.

02:45 Also within the deep layer, we have flexor pollicis longus.

02:49 Flexor pollicis longus is going to come from the interosseus membrane.

02:52 And it's also coming from the anterior surface of the radius.

02:56 As it passes towards the hand, it ends up going to the distal phalanx of the first digit.

03:02 So, it passes all the way to the distal phalanx of the first digit.

03:06 It's supplied by the median nerve and specifically, as it's running closely along the interosseous membrane, it's also supplied by the anterior interosseus branch coming from the median nerve.

03:18 If we look at the function of this muscle, again, its name helps to support the idea that it's supplying the thumb, flexor pollicis.

03:26 But because it also passes the wrist joint, it's also going to help flexion of the wrist.

03:32 So, we've got flexion of the wrist by flexor pollicis longus but primarily, because it passes to the distal phalanx of the thumb, it helps to flex the interphalangeal joint around the thumb and also, the metacarpophalangeal joints of the thumb where the thumb phalanges are attaching to the various carpal bones of the wrist and metacarpal bones that form the hand.

03:58 So, importantly, flexor pollicis longus is helpful in flexing the thumb, the first digit, but also, flexing the wrist as well. Pronator quadratus is the final muscle within the deep layer.

04:11 It's also the most distally located muscle belly of these muscles.

04:15 Pronator quadratus really runs between the radius and the ulna.

04:19 It originates from the distal quarter of the ulna, indicating that the bone that it actually moves is the radius.

04:26 And here, we've got the distal quarter of the ulna for its origin and it inserts onto the distal quarter of the radius.

04:33 And this muscle is important in pronating the forearm.

04:38 It's innervated via the anterior interosseous nerve which we can see here.

04:42 And as I said a moment ago, it's important in pronating the forearm.

04:47 So, that's where you're going from this supine position of your forearm into a pronated position where the radius is going to lie across the ulna.

04:56 And that's due to pronator quadratus.