Description

Radius (Radius)

Head of radius (Caput radii) – Articulates with capitulum of humerus and radial notch of ulna.
Articular facet (Fovea articularis) – Shallow depression on top of the head.
Articular circumference (Circumferentia articularis) – Lateral surface of head, contacts ulna.
Neck of radius (Collum radii) – Narrow section between head and shaft.
Radial tuberosity (Tuberositas radii) – Biceps brachii insertion site.
Anterior border (Margo anterior) – Border facing front.
Interosseous border (Margo interosseus) – Medial margin; attachment for interosseous membrane.
Posterior border (Margo posterior) – Less distinct, on posterior aspect.
Anterior surface (Facies anterior) – Between anterior and interosseous borders.
Posterior surface (Facies posterior) – Between posterior and interosseous borders.
Lateral surface (Facies lateralis) – Outer surface of shaft.
Styloid process (Processus styloideus radii) – Lateral wrist projection; attachment for radial collateral ligament.
Suprastyloid crest (Crista suprastyloidea) – Just above styloid; brachioradialis insertion.
Dorsal radial tubercle (Tuberculum dorsale radii) – Lister’s tubercle; pulley for extensor pollicis longus.
Ulnar notch of radius (Incisura ulnaris) – Medial indentation; articulates with ulna.
Carpal articular surface (Facies articularis carpea) – Inferior surface; articulates with scaphoid and lunate.

Ulna (Ulna)

Olecranon (Olecranon) – Proximal posterior projection; fits into olecranon fossa.
Trochlear notch (Incisura trochlearis) – Large anterior C-shaped articular surface.
Coronoid process (Processus coronoideus) – Anterior projection; fits into coronoid fossa.
Sublime tubercle (Tuberculum subliminale) – Medial side of coronoid; ligament/muscle attachment.
Radial notch (Incisura radialis) – Lateral surface; articulates with radius.
Ulnar tuberosity (Tuberositas ulnae) – Just below coronoid; brachialis insertion site.
Anterior border (Margo anterior) – Front margin of shaft.
Interosseous border (Margo interosseus) – Lateral edge; attachment for interosseous membrane.
Supinator crest (Crista m. supinatoris) – Near top of interosseous border; supinator origin.
Posterior border (Margo posterior) – Subcutaneous ridge down posterior side.
Anterior surface (Facies anterior)
Posterior surface (Facies posterior)
Medial surface (Facies medialis)
Head of ulna (Caput ulnae) – Rounded distal end.
Styloid process (Processus styloideus ulnae) – Medial wrist projection.
Articular circumference (Circumferentia articularis ulnae) – Surface around head; fits into radius.
Articular facet (Facies articularis) – Lateral part of ulnar head; part of TFCC.

Joints

Proximal radioulnar joint (Articulatio radioulnaris proximalis) – Pivot joint between radial head and radial notch of ulna.
Distal radioulnar joint (Articulatio radioulnaris distalis) – Between ulnar head and ulnar notch of radius.
Radiocarpal joint (Articulatio radiocarpalis) – Between distal radius and scaphoid/lunate.
Humeroradial joint (Articulatio humeroradialis) – Capitulum with radial head.
Humeroulnar joint (Articulatio humeroulnaris) – Trochlea with trochlear notch.

Other Structures

Interosseous membrane – Strong fibrous sheet between shafts.
Oblique cord – Small band from ulna to radius, near radial tuberosity.
Palmar & dorsal radioulnar ligaments – Stabilize distal radioulnar joint.

Sources:
Memorix Anatomy
Gray’s Anatomy for Students (41st ed.)
Acland’s Video Atlas of Human Anatomy
Atlas of Human Anatomy – Frank H. Netter
Essential Clinical Anatomy – Moore
Inderbir Singh’s Textbook of Human Osteology
Kenhub (as secondary source)

Programs used:
Complete Anatomy, 3D visuals, PowerPoint, Camtasia, Canva.

Transcript

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This is a video about
the bones of the forearm,

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more specifically about the radius, and the Ulna.

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The ulna helps flexing and extending your lower
arm at the elbow joint, while the radius is the

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one that rotates — turning your palm down
in pronation, and back up in supination.

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So what we are going to do in this video is to
go through the parts of each of these bones,

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we’ll go through their surface landmarks,
and then talk a little bit about the

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joints that hold these two bones together.
What’s up everyone, my name is Taim. I’m a

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medical doctor, and I make animated medical
lectures to make different topics in medicine

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visually easier to understand. If you’d like a PDF
version or a quiz of this presentation, you can

0:00:36.640,0:00:40.400
find it on my website, along with organized
video lectures to help with your studies.

0:00:40.400,0:00:43.680
Alright, let’s get started.
We’ll talk through the radius,

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then the ulna. Now the radius is interesting.
This is the bone that’s on the side of your thumb.

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Let’s isolate it and zoom in a little.
The radius is divided into three parts.

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The proximal end, the body, and the distal
end. Let’s start with the proximal part.

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At the very top, we’ve got the head of the
radius. It’s shaped like a shallow cylinder,

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and it’s designed to articulate in two different
ways. First, on the very top surface of the head,

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we’ve got a shallow depression, that’s called
the articular facet. This is the surface that

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makes contact with the capitulum of the
humerus to form the humeroradial joint.

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Then, wrapping around the edge of the head,
we have the articular circumference. This part

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doesn’t interact with the humerus, but instead
spins against the radial notch of the ulna,

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to form the proximal radioulnar joint.
Let me show you that visually. First, here’s

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the capitulum of the humerus, then we’ve got the
radial notch of the ulna. Now if we remove those

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labels and take a step back, we can highlight the
humeroradial joint, then the proximal radioulnar

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joint, and also the humeroulnar joint beside it,
just to show all three in context. These are the

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three joints that together form the elbow joint,
which we went through in detail in the last video.

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So, that was the head of the radius.
Just below it, we have a narrowed region

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known as the neck of the radius. It’s like a
little transition zone between the wider head

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above and the rest of the shaft below.
Before we continue, let’s go ahead and

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orient ourselves a little bit to really
understand the surfaces of this bone.

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Here is the radius again, now placed in anatomical
position, and here it is in context, showing the

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anterior view, so you’re looking at the front
of the forearm. And here’s the posterior view,

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showing the back of the forearm. Let’s now
isolate the anterior side, and the posterior side.

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With that context, let’s continue by looking
at the structures along the body of the radius.

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So the first thing to note on the shaft is the
radial tuberosity. This is a rough projection

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just below the neck of the radius, on the
medial side of the anterior surface. It’s

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a really important bony landmark, because this
is where the biceps brachii tendon inserts. So

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when the biceps contracts, it pulls on this
point and flexes the forearm at the elbow.

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Now, if we look further down along the shaft, we
can identify three borders or margins. These help

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define the surfaces of the radius. First, we have
the anterior border, which runs down the front.

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Then there’s the interosseous border, which is on
the medial side, facing the ulna. This is where

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the interosseous membrane attaches, basically a
fibrous sheet that connects the radius and ulna

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together, helping transmit forces and stabilizing
the forearm. And on the back, we’ve got the

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posterior border, which is less distinct but still
visible along the posterior aspect of the shaft.

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Now between these borders, we can see three
surfaces. There’s the anterior surface,

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which lies between the anterior and interosseous
borders. Then the posterior surface,

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found between the posterior and interosseous
borders. And finally, the lateral surface,

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which lies between the anterior and posterior
borders on the outer side of the bone.

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That’s it for the body of the radius.
Let’s now zoom into the distal end of this bone.

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The first structure we notice here is the styloid
process of the radius. This one is really easy

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to feel, on the lateral side of the wrist.
This styloid process acts as an important

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attachment point. We’ve got the radial collateral
ligament of the wrist joint anchoring here,

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helping stabilize the lateral side
of the wrist. And just next to it,

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there’s the palmar radiocarpal ligament, which
supports the palmar side of the wrist capsule.

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Now, just above the styloid process, there’s
a little ridge called the suprastyloid

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crest. This is where the brachioradialis muscle
inserts. This muscle travels down from the arm,

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along the lateral side of the forearm, and it
grabs onto this area, helping flex the elbow.

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On the posterior view we can see a series
of grooves for the extensor tendons.

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These grooves guide the tendons of muscles like
the extensor pollicis longus and extensor carpi

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radialis as they pass into the hand.
Right next to those grooves, you can

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see the dorsal radial tubercle, also known
as Lister’s tubercle. This is a small but

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sharp bony ridge that acts like a pulley for
the tendon of the extensor pollicis longus.

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On the medial side we’ve got the ulnar notch of
the radius. This area accommodates the head of

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the ulna, forming the distal radioulnar
joint, which allows the radius to pivot

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around the ulna during pronation and supination.
On the inferior surface of the distal radius,

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we’ve got the articular surface
for the carpal bones. It’s divided

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into two main articular areas: one for the
scaphoid, and one for the lunate. Together,

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these make up the radiocarpal articulation,
which is the main joint of the wrist.

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And that’s it for the radius.
Let’s now turn our attention to the ulna.

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The ulna, just like the radius, is divided
into three parts: a proximal end, the body,

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and a distal end. Let’s start by
zooming in on the proximal part.

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Here we can see the olecranon, that big curved
projection at the top that forms the bony tip

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of your elbow. Another thing we can see is the
trochlear notch, which is that large C-shaped

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surface on the anterior aspect, designed to grip
the trochlea of the humerus. Just below that,

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there’s the coronoid process that fits into
the coronoid fossa of the humerus during elbow

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flexion. And on the medial side of the coronoid,
there’s a subtle elevation called the sublime

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tubercle, which serves as an attachment
for part of the ulnar collateral ligament

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and the flexor digitorum superficialis muscle.
Now let’s look at the elbow joint again to see

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how all these structures fit together.
So here’s the trochlea of the humerus,

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and here’s the radial notch on the lateral side
of the ulna. Then we bring in the articular

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circumference of the radial head. When we place
these elements together, we can clearly see the

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humeroulnar joint, where the ulna meets the
humerus, and the proximal radioulnar joint,

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where the radius spins against the ulna.
Alright, awesome.

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Let’s now take a moment and orientate again. The
ulnar bone we just highlighted some structures on,

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we actually were seeing it from the medial side.
And here it is now from the lateral view. Notice

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how the olecranon fits right into the olecranon
fossa of the humerus when your arm is extended.

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Let’s now isolate the ulna from both the medial
and lateral views so we can see it more clearly.

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And before we move on from the proximal end,
there’s one more structure we need to look at,

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and that is the tuberosity of the ulna. You’ll
find this just below the coronoid process,

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and it serves as the insertion point
for the brachialis muscle, which is

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one of the main flexors of the elbow joint.
Okay, let’s now talk about the body of the ulna.

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Just like we did with the radius, we’re going to
go through its borders and surfaces. We got an

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anterior border, running down the front of the
bone. Then we’ve got the interosseous border,

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which is on the lateral side, facing the radius.
This is where the interosseous membrane attaches.

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Up near the top of that border is a small ridge,
this is called the supinator crest, which is

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the origin point for the supinator muscle.
On the back of the ulna, there’s the posterior

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border, and this one you can actually feel under
the skin, it runs down the subcutaneous edge of

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your forearm. Now in between those borders, we’ve
got the medial surface, the anterior surface,

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and the posterior surface, each providing
attachment points for different muscles.

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Alright. Let’s go ahead and move
on to the distal end of the ulna.

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This part’s a little interesting,
even though it’s at the wrist level,

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it doesn’t actually participate in the radiocarpal
joint. Instead, the distal end of the ulna has a

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rounded area called the head of the ulna. And
coming off that is the ulnar styloid process,

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which is that little bump you can feel on
the back and medial side of your wrist.

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Wrapping around the head is the articular
circumference, which contacts the ulnar

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notch of the radius to form the distal radioulnar
joint. And on the lateral side of the ulnar head,

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we can see the articular facet, that’s where part
of the triangular fibrocartilage complex sits.

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So that’s it for the ulna.
Now just for orientation,

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we can see the ulnar styloid
process here on the medial side,

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and over here we’ve got the radial
styloid process on the lateral side.

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And one of the coolest things about the radius
and ulna is, thanks to muscles like the pronator

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quadratus and pronator teres, they give us the
ability to rotate our palm down in pronation,

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or up into supination. You can see how the
radius rotates over the ulna to pronate the

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hand, and then back up again to supinate it.
So, how are the radius and ulna held together?

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Well, first, proximally, they’re
held together by the elbow joint,

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as we covered earlier. Then distally, we’ve got
the distal radioulnar joint. Along the shafts is

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the interosseous membrane, that strong fibrous
sheet connecting the bones. And near the top,

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there’s also a small additional structure called
the oblique cord, a short ligament that reinforces

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the connection a bit further.
Let’s go ahead and zoom into

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the distal radioulnar joint now.
This is a synovial pivot joint,

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made up of the ulnar notch of the radius
articulating with the articular circumference

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of the ulna. It’s surrounded by a fibrous
capsule, and lined by a synovial membrane

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to allow for smooth rotation. Supporting it on
the front is the palmar radioulnar ligament,

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and on the back, we’ve got the dorsal radioulnar
ligament, both of which help stabilize the joint.

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Now, on the shaft we’ve got the interosseous
membrane running between the radius and ulna.

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This isn’t just a passive sheet, it helps
distribute forces from the radius to the ulna,

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especially during weight bearing. And it also
serves as a passage for small vessels and nerves,

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like the anterior interosseous artery.
Now, finally, let’s take a closer look at

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that oblique cord. It’s a small fibrous band
that stretches from the ulna to the radius,

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just below the radial tuberosity. It’s
not always mentioned in textbooks,

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but it plays a role in limiting separation
between the bones and adding a bit of stability.

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Alright, and that’s it for this one.
We’ve now covered the radius, the ulna,

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and the joints and connective
structures that hold them together.

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I really hope this video helped you
visualize and understand this topic better.

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In the next video, we’re going to continue
down the upper limb and look at the bones

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of the wrist and hand, all of the carpal
bones, the metacarpals, and the phalanges.

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Click the next video, and I’ll see you there.
If you want a handmade PDF version of this

0:11:32.320,0:11:36.640
lecture, take a quiz to test your knowledge,
or access an organized list of all my videos,

0:11:36.640,0:11:40.172
you can find everything on my website.
Thanks for watching! See you in the next one.