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  • I am so glad that you guys are not five years old!

  • If you were all in, like, kindergarten, the only way I could teach you about your skeletal

  • system would be to sing you that stupid song.

  • You know, “The toe bone’s connected to the foot bone, the foot bone’s connected

  • to the ankle bone...the ankle bone’s connected to the leg bone.”

  • Like apparently there’s one leg bone.

  • For the purposes of teaching college-level anatomy and physiology, let’s just say that

  • that song isreductive.

  • Insufficient.

  • For that song to be at all useful for remembering all 206 bones that make up your skeletal structure,

  • it would have to begin with something like: “The distal phalanx is connected to the

  • middle phalanx …” and then end with something like: “the frontal bone meets the parietal

  • bone along the coronal suture.”

  • And it would probably take like two and a half hours to sing the whole thing.

  • But there is way more to know about your skeletal system than just the names of the bones. In

  • addition to bones, there’s cartilage and fibrous connective tissue that allows them to work together.

  • And maybe more importantly, if youre going to gonna talk about the physiology of your

  • skeleton, you have to talk about joints.

  • Joints are the meeting places between two or more bones -- and even though it might not

  • sound mathematically possible, you actually have more joints in your body than you have bones.

  • In a lot of places -- like your hands and feet -- each individual bone is part of more

  • than one or two joints.

  • And then there’s the matter of what all that bone, cartilage, connective tissue -- and

  • the joints that they form -- actually work together to do.

  • And that ismove.

  • Body movements happen when muscles contract across joints, moving one bone toward another.

  • And studying the different types of movements your body is capable of, is not only pretty

  • fun, it’s also one of the best practical ways to understand your bones and joints.

  • If you ask me, all of that is infinitely more interesting than memorizing what goes where

  • and pretending like a “backboneis really an anatomical structure.

  • So. Aren’t you glad youre a grown-up?

  • Dont worry, it’s not anyone you knew. It’s plastic, and it’s a little bit smaller than the average adult skeleton.

  • But -- other than having me stand in front of a giant x-ray machine for 10 straight minutes

  • -- I can’t think of a better way to walk you through the major anatomical structures of your skeleton.

  • Now, if you managed to retain our introduction to human bones last week, youll recall

  • that anatomists typically divide the skeleton into two major parts: the axial and the appendicular.

  • The axial structure includes all 80 midline bones that form your skull, vertebral column, and thoracic cage.

  • Starting at the top here, your skull’s eight cranial and fourteen facial bones form your

  • body’s most complex bony structure, coming together to do some pretty great things, like

  • protect your brain, help you see, smell, hear, eat, and you know, have a face.

  • The 33 irregular bones in your vertebral column start with your atlas vertebra -- the one that holds

  • up your skull, and is named for the Greek god Atlas, who held the world on his shoulders as a punishment.

  • I’m not saying youve got a huge head or anything. I’m just saying that’s what it’s called.

  • The vertebrae run down from your skull to your pelvis, providing the central support

  • for your upper body and completing the enormously important job of protecting your spinal cord

  • -- the main communication line between nearly all of your body, and your brain.

  • Most of your vital organs are protected by the 12 pairs of ribs and dagger-like sternum

  • that together comprise your thoracic cage, which also provides attachment points for

  • your back, chest, shoulder, and neck muscles.

  • Now, the appendicular skeleton includes your upper and lower limb appendages and pectoral

  • and pelvic girdles that attach to the axial skeleton at the shoulder and the thigh.

  • And even though your arms and legs clearly serve different functions -- unless you spend

  • a lot of time walking around on your hands or picking stuff up with your feet -- both

  • sets of limbs share a similar set-up.

  • Theyre each composed of three major segments -- a common arrangement we see in loads of

  • animals all the way back to fish, which anatomists refer to asone bone, two bones, lots of

  • bones and digits.”

  • We can’t all be Dr. Seuss.

  • And that’s just about enough anatomy for this plastic skeleton to pay for itself. I’d

  • rather focus on the physiology of the skeleton, and in order to do that, weve got to talk about joints.

  • As is often the case in anatomy, we classify joints both by what theyre made of, and by

  • what they do. Because form follows function, we can't really talk about one without talking about the other.

  • So, the structural classification of your bones is all about what kind of material binds

  • those bones together -- like, is it a fibrous tissue or cartilage, or a special fluid-filled joint cavity.

  • But the functional classification focuses on how much that joint can move.

  • So, for example, you have joints in your body that don’t move at all. These include, say, the

  • joints between the bones that make up your cranium. These non-moving joints are called synarthroses.

  • But you also have joints that move only slightly, like the spot where your two pubic bones meet

  • to form your pelvis. That joint exists mainly to absorb shock from walking and running,

  • but it also has proven very handy during things like childbirth. These partly-moving joints

  • are called amphiarthroses.

  • Finally, there are the diarthroses, which are fully movable, like your classic knee

  • and elbow joints. Theyre mostly found in your limbs.

  • But if you eventually find yourself in the business of examining and treating people’s

  • joints -- and there are a lot of doctors who do -- then itll be especially useful to

  • know the structural classifications.

  • Structurally speaking, youve got fibrous, cartilaginous, and synovial joints.

  • Fibrous joints connect bones with dense fibrous connective tissue, and are mostly immovable

  • -- like those sutures between your skull bones.

  • As you might guess, your cartilaginous joints unite bones using cartilage, and are similar

  • to their fibrous brethren in that they don’t move very much, and they lack a joint cavity.

  • But the family of joints that make all the sports, and the entire art of breakdancing,

  • and yoga possible, are the freely movable synovial joints.

  • Most joints in your body fall into this category.

  • Although they do make use of cartilage and fibrous connective tissues like ligaments,

  • theyre different in that the bones they join are separated by a fluid-filled joint cavity.

  • The cool thing about these cavities is that they contain a bit of viscous, egg-white-like

  • synovial fluid that acts like grease on a hinge.

  • Without that lubricant, just running down the street could cause enough friction not

  • only to wear out your joint surfaces, but actually overheat your joints enough to essentially

  • cook the surrounding tissue and leave your legs smoking like a desperate Looney Tunes character.

  • These freely moveable synovial joints come in six different configurations that together

  • allow pretty much any movement your body can make, from a subtle head nod to a vigorous jumping jack.

  • All of which you will find in action at your typical dance party.

  • Seriously, though, dance parties are an excellent place to study skeletal physiology.

  • I mean let’s just take this from my perspective: I’m at my high school prom. I’m wearing

  • a tux that is really uncomfortable and I’m feeling kind of shy, standing on the edge of the crowd.

  • Maybe I want to test the waters out a little bit with like a little hand dancing. Just

  • the hand going. Waving my hand from side to side -- it’s a type of gliding movement,

  • one that occurs when one flat bone surface glides over another.

  • All these motions use gliding, or plane joints, like the one between the distal ends of your

  • radius and ulna, and the carpal bones of your wrist.

  • But say after a while I feel like these gliding movements just aren’t enough to fully express

  • myself. I have to enlist the more versatile angular movements.

  • These are the types of motion that either increase or decrease the angle between two

  • bones across any plane in the body, like if I bob my head, or kick my leg up, or raise

  • the roof. If people still do that.

  • If that bending motion decreases the angle of the joint and brings those bones together,

  • like when I bend my arm together, that movement is called flexion.

  • When I bend it back, I’m increasing that angle, and it’s called extension.

  • And if I continue that motion beyond my normal anatomical positioning, and it’s somewhat

  • dangerous, that is called hyperextension.

  • These motions enlist the hinge joints. Similarly, my oval-shaped condylar knuckle joints allow

  • my fingers to bend and ball up for a good fist pump.

  • Now I’m getting super into it. I might like raise my arms up over my head. Well, the act

  • of raising my arms away from my body is called abduction, while moving them back down toward

  • the body is called adduction.

  • And this here, like, lasso movement? That’s circumduction -- a combination of all five

  • of those movements that allow the forearm to move in a circle while the elbow joint stays relatively stable.

  • Which really makes you wanna give credit to those cowboys.

  • Now if you want to get really serious about that lassoing and expand it enough to include

  • the entire arm, the move eventually will morph from an angular movement to a rotational one

  • -- a motion that turns the bone around its axis, in this case, the humerus.

  • Hip and shoulder joints use a ball-and-socket design that allow rotational movement, but

  • the more flexible a joint is, the more unstable and fragile it is -- which is one reason why

  • you see so many dislocated shoulders and hip replacements.

  • And it’s worth pointing out that some movements are just special and unique and weird, and

  • only occur in a few select joints -- especially in your hands, feet, and jaw.

  • For example, your ability to touch your thumb to your fingertips or give a thumbs up is

  • thanks to opposition movement, facilitated by your saddle joint, which makes your thumbs opposable.

  • The pivot joint between your radius and ulna allow you to rotate your palm forward or anteriorly

  • in a motion called supination, and turning it backward or posteriorly is called pronation.

  • So that’s what I like to think is the more grown-up approach to teaching you about your skeleton.

  • It did not involve any singing, but yes, a little dancing.

  • And it was worth it, because you learned about the basic structure of your skeletal system,

  • including the arrangements of both the axial and the appendicular skeleton. And we went

  • over the structural and functional classifications of your joints, along with the major types

  • of movement that they make possible.

  • Special thanks to our Headmaster of Learning Thomas Frank for his support for Crash Course

  • and free education. This episode was co-sponsored by Greg Avarbuch, Faht-Mah ikh-Bahl, and Ricky D. Shields.

  • Thank you to all of our Patreon patrons who help make Crash Course possible through their

  • monthly contributions. If you like Crash Course and want to help us make great new

  • videos like this one, you can check out patreon.com/crashcourse

  • Crash Course is filmed in the Doctor Cheryl C. Kinney Crash Course Studio. This episode

  • was written by Kathleen Yale, edited by Blake de Pastino, and our consultant, is Dr. Brandon

  • Jackson. Our director is Nicholas Jenkins, the editor and script supervisor is Nicole

  • Sweeney, our sound designer is Michael Aranda, and the graphics team is Thought Café.

I am so glad that you guys are not five years old!

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關節。速成班A&P #20 (Joints: Crash Course A&P #20)

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