Built to Move: The Appendicular Skeleton Part 1

Dr. Stephen Flanagan

Welcome to the Flanatomy podcast. I’m your host, Dr. Stephen Flanagan. We’re diving into the wild world of anatomy with a side of crazy stories from my bizarre life. Let’s explore what makes you, you!

Keshia Rayna

And I’m Keshia Rayna, your co-host, keeping it one hundred and making sure Doc doesn’t get lost in his wild tales. I’m locked and loaded with my phone to fact-check and break down the nerdy stuff so y’all can vibe with the science.

Dr. Stephen Flanagan

Today we're finally leaving the axial skeleton behind and moving on to the appendicular skeleton. And honestly, this is where anatomy starts feeling alive. The skull protects. The vertebral column protects. The rib cage protects. The appendicular skeleton does stuff. It reaches. It lifts. It throws. It punches. It climbs. It carries. It runs. It jumps. It gets you into trouble and occasionally gets you out of trouble. And according to Wikipedia, the word appendicular is the adjective form of appendage, meaning a structure attached to something larger. So the appendicular skeleton is literally the collection of bony attachments hanging off the axial skeleton.

Keshia Rayna

Which sounds less impressive than it actually is.

Dr. Stephen Flanagan

It does. If I told you the appendicular skeleton contains 126 bones, that sounds enormous. But then you realize fifty-six of those are fingers and toes. Your fingers and toes are really padding the statistics here.

Keshia Rayna

They're inflating the numbers.

Dr. Stephen Flanagan

Exactly. The remaining seventy bones are some of the all-stars of human anatomy. The upper limb and lower limb are actually built from the same fundamental blueprint. One evolved for mobility. One evolved for stability. One lets you throw a baseball. One lets you stand upright for decades without collapsing. And that's the story we're telling today.

Keshia Rayna

Let's start with the pectoral girdle because I think students are always surprised by how little the upper limb is actually attached to the skeleton.

Dr. Stephen Flanagan

Exactly. If I asked most people how the arm connects to the body they'd probably imagine this giant rigid bony connection. But there isn't one. The pectoral girdle consists of two clavicles and two scapulae. And the entire upper limb is attached to the axial skeleton primarily through a single joint on each side. The sternoclavicular joint. That's it. One tiny bridge.

Keshia Rayna

Which sounds terrifying.

Dr. Stephen Flanagan

It really does. Your arm is basically hanging from your skeleton by a glorified strut. The clavicle. Also called the collarbone. One of the most commonly fractured bones in the body. The clavicle extends horizontally across the superior thorax. Medially it articulates with the manubrium of the sternum. Laterally it articulates with the acromion of the scapula. Hence the names: Sternal end. Acromial end. Simple. Elegant. Useful.

Keshia Rayna

The clavicle always looks fragile.

Dr. Stephen Flanagan

It does. But it's actually performing an incredibly important engineering function. It acts like a strut on a crane. Its job is to hold the shoulder away from the thorax. Without it, your shoulders would collapse inward. The clavicle keeps the upper limb positioned laterally where it can move effectively.

Keshia Rayna

So it's basically a biological spacer.

Dr. Stephen Flanagan

A surprisingly strong spacer. Though not indestructible. And many of our listeners have probably broken one. Usually through sports. Bike crashes. Football. Skateboarding. Or occasionally doing something that seemed like a good idea five minutes earlier. Next, we arrive at one of my favorite bones in the body. The scapula. The shoulder blade. A bone so important that nearly every major movement of the upper limb depends on it. Yet students often ignore it because it's hiding on their back.

Keshia Rayna

Out of sight, out of mind.

Dr. Stephen Flanagan

Yup. The scapula sits between ribs two through seven. It has three borders: Superior. Medial, also called vertebral. Lateral, also called axillary. And three angles: Superior. Inferior. Lateral. But the real stars are the processes. The spine. The acromion. And the coracoid process.

Keshia Rayna

The coracoid process has one of the coolest names in anatomy.

Dr. Stephen Flanagan

Yeah. Coracoid comes from the Greek korax. Meaning crow. Or raven. So coracoid process literally means: Crow's beak. I had a hunting buddy that always talked about bird in scientific family names rather than as common names. So it was never dove hunting, it was Columbid hunting, or he would say "those Galliformes are sure gobbling" rather than calling them turkeys. I got used to it but it was a little annoying. I hear him saying "depredating corvids!" when seeing a group of crows consuming roadkill. Corvid the scientific name for the crows and ravens is a derivative of the Greek korax. And we don't have to talk, at all, about what he called duck hunting...

Keshia Rayna

Oh, I get it! That is a solid science joke, that we will let anyone listening look up on their own. Also korax immediately sounds like something from a fantasy novel.

Dr. Stephen Flanagan

It does. Most Greek names or words do. Think about it a lot of sci-fi uses Greek words like Apollo, Icarus, Android, Prometheus, Cylons, Andromeda. Once you see the coracoid process from the side you'll never forget it. It genuinely looks like a curved bird beak projecting anteriorly. And not just any bird. A corvid. Which is the family that includes crows and ravens.

Keshia Rayna

So every time students learn the coracoid process they're accidentally learning ornithology.

Dr. Stephen Flanagan

Correct. You're welcome. Now here's where I have to tell one of my favorite real-life stories. My brother-in-law managed to fracture his scapula.

Keshia Rayna

Which should immediately raise eyebrows because scapular fractures are uncommon.

Dr. Stephen Flanagan

Very uncommon. The scapula is protected by: Trapezius. Deltoid. Rhomboids. Latissimus dorsi. Rotator cuff muscles. Ribs. It's like the VIP section of the skeleton. To break a scapula usually requires substantial trauma. Motorcycle crashes. Vehicle accidents. Major falls. Yet somehow my brother-in-law accomplished it while body surfing.

Keshia Rayna

Which sounds impossible.

Dr. Stephen Flanagan

That's what the cops said! Apparently he was tossed by a wave, lost orientation, got driven downward, and hit the sand with tremendous force. Hard enough to fracture the scapular body. When he got evaluated, the police questioned my father-in-law because they had trouble believing the mechanism. They were basically saying: "Are you sure somebody didn't hit him with a sledge hammer?" It was all good though, nobody got in trouble and he just wore a sling for a bit.

Keshia Rayna

well that's good

Dr. Stephen Flanagan

Now the most important feature of the scapula is the glenoid cavity. This is where the humerus articulates. And this joint explains one of the central themes of upper-limb anatomy. Mobility versus stability. The glenoid cavity is shallow. Very shallow. Compared to the acetabulum of the hip, it's almost laughably shallow. And that's intentional. Because the shoulder sacrifices stability for movement. The hip does the opposite. We're going to come back to that idea repeatedly today. Each upper limb contains thirty bones. Grouped into: Arm. Forearm. Hand. And yes. I am going to become annoying for a moment. The arm is not the entire upper limb. The arm specifically means the region between shoulder and elbow. Anatomists become irrationally upset when people call the whole thing an arm.

Keshia Rayna

This is one of your pet peeves.

Dr. Stephen Flanagan

Absolutely. The forearm exists. Respect the forearm. And with that, we're ready to move into one of the most recognizable bones in the human body. The humerus. And no, despite what every anatomy professor since the dawn of time has said, the humerus is only mildly humorous. The humerus is the only bone of the arm. Not the upper limb. Not the shoulder. The arm. And it is an absolutely magnificent piece of biological engineering. It's the longest and strongest bone of the upper limb. Proximally, it articulates with the scapula at the glenoid cavity. Distally, it articulates with the radius and ulna at the elbow. But what I love about the humerus is that nearly every landmark on it tells you something about movement. The greater tubercle. The lesser tubercle. The intertubercular groove. The deltoid tuberosity. Every bump is basically a receipt showing where a muscle attached.

Keshia Rayna

It's like looking at a mountain range and realizing every peak was carved by a different force.

Dr. Stephen Flanagan

Exactly. Students often look at bones and see random lumps. An anatomist looks at bones and sees a history book. Every ridge tells a story. Every tuberosity tells you what was pulling on it. One landmark students should know clinically is the surgical neck. Not because surgeons named it after themselves. Actually, yes. Kind of because surgeons named it after themselves. This is a common fracture site. And nearby runs the axillary nerve. So a fracture here can affect deltoid function. Meaning suddenly the patient struggles to abduct the arm.

Keshia Rayna

Which is one of those moments where anatomy becomes medicine very quickly.

Dr. Stephen Flanagan

Exactly. You go from "name this structure" to "why can't this person raise their arm?" That's anatomy becoming clinically useful.

Dr. Stephen Flanagan

At the distal end we find two important articular surfaces. The capitulum. And the trochlea. The capitulum articulates with the radius. The trochlea articulates with the ulna. And I always tell students that trochlea sounds fancy but comes from the Greek word for pulley. Which is fitting because this whole joint system functions like a mechanical device. The elbow is one of the best examples of biological engineering you'll ever encounter. Now we move into the forearm. The forearm consists of two bones. The radius. And the ulna .And students often struggle because they think both bones do the same thing. They absolutely do not. The ulna is the elbow bone. If you want to understand the ulna, forget the wrist for a moment. Think elbow. The ulna forms the major hinge joint with the humerus. Its olecranon process forms the point of your elbow. You can feel it right now. That bony tip sticking out behind your elbow? That's the olecranon.

Keshia Rayna

The part everyone has slammed into a table at some point.

Dr. Stephen Flanagan

Exactly. And when you hit it and get that electric shock feeling? You're irritating the ulnar nerve. Not actually breaking your funny bone. Because your funny bone isn't funny and it isn't a bone. The ulnar nerve passes behind the medial epicondyle. That's why striking the area creates that tingling sensation down into the ring finger and pinky. You're literally stimulating a nerve. Students remember this forever because they've all experienced it. Now the radius is the opposite. The radius is all about the hand. The superior surface of the radial head articulates with the capitulum. Medially, it articulates with the radial notch of the ulna. And then distally it becomes the major contributor to the wrist joint. This is why I tell students: The ulna owns the elbow. The radius owns the wrist.

Keshia Rayna

Simple.

Dr. Stephen Flanagan

Very simple. And here's the coolest part. When the radius moves, the hand moves with it. When the ulna moves, not much happens at the hand. Now let's talk about one of the most elegant movements in anatomy. Supination and pronation. The radius literally rotates around the ulna. Think about that. Your forearm contains a built-in rotational mechanism. That's remarkable engineering. Supination. Palm up. Pronation. Palm down. Classic mnemonic: You're carrying a bowl of soup. Supination. Now pour the soup out. Pronation.

Keshia Rayna

Every anatomy professor on Earth has taught that one.

Dr. Stephen Flanagan

Because it works.

Dr. Stephen Flanagan

Now we arrive at one of the greatest evolutionary achievements in vertebrate history. The human hand. Twenty-seven bones. Dozens of joints. Extraordinary dexterity. The reason humans can write, sculpt, play piano, perform surgery, and waste hours scrolling social media. Let's begin with the carpals. Eight bones. Arranged in two rows. Proximal row: Scaphoid. Lunate. Triquetrum. Pisiform. Distal row: Trapezium. Trapezoid. Capitate. Hamate. And every anatomy student learns some variation of: "Sally Left The Party To Take Carmen Home."

Keshia Rayna

Poor Sally. She's been leaving that party for decades.

Dr. Stephen Flanagan

She really has. But let's talk about some of these individually.

Dr. Stephen Flanagan

Scaphoid means boat-shaped. Well more like a small 1 person boat... like a fishing boat or skiff. It is the most commonly fractured carpal. Typically from a fall onto an outstretched hand. What's fascinating is its blood supply. The blood enters distally. Which means if the bone fractures, the proximal portion can lose blood flow. That can lead to avascular necrosis. Bone death. Which sounds like a death metal album.

Keshia Rayna

Avascular Necrosis opening for Rotator Cuff Rupture.

Dr. Stephen Flanagan

I'd buy tickets.

Dr. Stephen Flanagan

The hamate has a hook. Literally called the hook of hamate. Golfers. Baseball players. Tennis players. Anyone repeatedly swinging something can injure this region. Which makes it clinically important.

Dr. Stephen Flanagan

The thumb deserves special attention .The pollex. Our opposable thumb is one of the major reasons humans became so successful. Without it, tools become difficult. Without tools, civilization looks very different. Without civilization, we don't have podcasts.

Keshia Rayna

Or exams.

Dr. Stephen Flanagan

Which some students may view as a benefit.

Dr. Stephen Flanagan

The metacarpals form the palm .Five bones. Numbered I through V beginning with the thumb. Bases articulate proximally .Heads articulate distally. And yes. Those knuckles you see? Those are metacarpal heads.

Dr. Stephen Flanagan

Now we arrive at my favorite pop-culture anatomy reference. Phalanges. Fourteen per hand. three per finger. Except the thumb. Which only has two. And every single year somebody remembers this because of Friends.

Keshia Rayna

Regina Phalange.

Dr. Stephen Flanagan

Regina Phalange. The fake name Phoebe uses repeatedly .Which means somewhere, somehow, one of the most useful anatomy mnemonics ever accidentally emerged from a sitcom.

Keshia Rayna

Honestly that's beautiful.

Dr. Stephen Flanagan

It really is. And that's where we'll pause before heading south. Because next we're moving into the pelvic girdle. And things are about to get considerably stronger, heavier, and less mobile. The lower limb isn't built to manipulate the world. It's built to survive gravity.

Dr. Stephen Flanagan

Perfect! See you next time everybody!