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Learn Muscle Anatomy: Bursae

The other day, I let my ten-year old niece play with Muscle Premium on my phone (while I watched cartoons). She kept making the model spin—around and around and around, like a ballerina, until she abandoned it for the muscle actions.

"Auntie," she said, and then didn't follow up with anything, completely engrossed in Elbow Flexion. When I prompted her, she looked up at me and whispered, as if confessing a secret, "Is it supposed to hurt?"

I asked her what she meant. She pointed to the moving model.

"Do bones rub against each other like that all the time? Because I feel like it should hurt."

I watched the animation for a bit—the olecranon of the ulna slid against the humerus's olecranon fossa, back and forth, flexed in a continuous loop. It did look a bit like they were rubbing together. Actually, if there wasn't something there acting as a cushion, moving the joints would be incredibly painful. Luckily, we're not that bad off.

"It doesn't hurt," I began, settling back to finish The Legend of Korra, "Because you have little pillows called bursae inside you that stop your bones from rubbing like that."

She turned her attention back to the TV and waited until the end of the episode to ask more. (My niece is polite like that.)

That was an actual exchange between me and my niece Em, who has an affinity for all things science and interesting. She's awesome.

Anyway, what I told her was the truth: bursae prevent our bones and muscles, particularly in the joints, from rubbing together and creating painful friction. Imagine trying to bend your knee without something to cushion the movement. Talk about ouch, right?

Muscle patella superficial subcutaneous prepatellar bursa knee synovial resized 600 

See those purple lumps in the picture? Those are bursae. They live between bones and bones, or bones and muscles, or muscles and skin, serving to prevent friction at points of stress throughout the body. In the picture, you can see the bursae are either prominently displayed (on top of the patella) or partially hidden between bone and muscle. Think of how often you move and bend your knees—I'm doing it right now, and I'm just sitting! It would be a much more painful action without the bursae there to cushion things.

Bursae come in three packages: synovial, subcutaneous, and adventitious. 



Most of the bursae in the body are synovial: thin-walled sacs interposed between bones, muscles, and tendons. The lining of a bursa contains a capillary layer of synovial fluid, which provides two lubricated surfaces that enable freedom of movement. Synovial bursae tend to be located in your joints, like your knees, feet, and shoulders.

 Muscle bursae shoulder joint subacromial bursa synovial resized 600


There are also adventitious, or accidental, bursae. These occur in soft tissue over bony prominences, usually because of repeated pressure or shearing.

An example of an adventitious bursae is a bunion, which is a deformity of the big toe. Wearing ill-fitting shoes can sometimes force the big toe inward towards the other toes. The bursa at the metatarsophalangeal joint becomes swollen, but the biggest issue is the normal part of the head of the first metatarsal bone is tilting sideways and sticks out at its top. This creates a large bump or prominence.

 Muscle metatarsophalangeal bursa hallux big toe joint metatarsal resized 600

Subcutaneous bursae lie between the skin and a bony process, like the aforementioned olecranon of the elbow.

Muscle olecranon subcutaneous bursa elbow joint resized 600  


You've probably spent enough time on this blog to know what the suffix –itis means, so you won't be surprised when I say that bursitis is the inflammation of a bursa. When the bursae become inflamed, their gliding ability is lost, which can be painful. An inflamed bursa is usually the result of trauma, overuse, or infection. Even something as simple as lifting something heavy can bring it on.

The joints of the hips, elbows, and shoulders are normally the areas affected by bursitis, but it can occur anywhere (inflammation of the bursae in the knee is known as Washmaid's Knee).




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3D Skeletal System: 5 Cool Facts about the Hip Bones

I know, I know, it's another blog post about the pelvic girdle. Well, sort of. Sometimes it's a thankless job, holding the weight of our upper half, so this post is going to be dedicated to my favorite pair of jutting wings—the hip bones!

So sit down and take a load off, and let's dive in!


1. The proper term for the hip bones is "os coxae."

Os coxae hipbones ilia ischium pubis pelvic girdle

"Os coxae" comes from the Latin words "os" meaning bone and "coxae" from the old Latin word for hip.


2. Each hip bone is actually made up of three bones.

It may look like one bone, but each hip bone is made up of the ilium, pubis, and ischium, which are completely fused.


3. There's a cavity in each hip bone.

Acetabulum hip socket hipbones os coxae

No, seriously! The concave cavity in the hip socket is known as the acetabulum, which is where the head of the femur articulates.


4. There are noticeable differences between the male and female hip bones.

The female hip bones are more delicate and shallow than the male's, with less sloped ilia. However, the superior aperture of the female pelvis is larger and more circular than the male's.

5. There is a giant hole between the hip bones.

Superior aperture pelvic girdle os coxae hipbones

Well, technically it's a space more than a hole. The superior aperture is the space that divides the abdominal cavity from the pelvic cavity. If you look at the pelvic area from a superior view, you will see the superior aperture is circular and formed by the hip bones, sacrum, and pubic symphysis.


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3D Skeletal System: 5 Awesome Ligaments

How much do you enjoy moving? A lot, I bet. While many attribute our ability to move to the muscular system, it actually goes a little deeper than that! Your skeletal system is a rigid organ (yep, it’s living tissue!) that, without your awesome ligaments, wouldn't be able to move normally!

Ligaments are fibrous swathes of connective tissue that connect bones and help prevent your joints from flapping around willy-nilly. They also help to hold organs in place.

Let’s take a look at five of the coolest ligaments in your body!


5. Linea alba

Linea alba abdomen ligament skeleton

I love the linea alba. Do you know why? Because it’s exactly what its namesake says it is—a white line. The linea alba is a thin stretch of connective tissue that runs between the xiphoid process of the sternum and the pubic symphysis of the pelvic girdle. It also acts to divide the two rectus abdominis muscles.


4. Pubocervical fascia

Pubocervical fascia pelvis uterus bladder skeleton ligaments

Stretched across the pelvic cavity is the pubocervical fascia, which connect the cervix to the pelvic walls and posteriorly blend with the perineal membrane. Think of the pubocervical fascia as a sort of hammock for the uterus and bladder.


3. Flexor retinaculum

Flexor retinaculum ligament wrist carpals bones

The flexor retinaculum and I have a complicated relationship. On the one hand (ha!), it helps to keep the flexor tendons together; on the other, it’s to blame for my increasingly rough ride through the carpal tunnel.

The flexor retinaculum is an arch of tough, fibrous tissue attached to the pisiform, hamate, trapezium, and scaphoid, and—with the carpal bones—forms a tunnel through which the flexor tendons of the hand and median nerve pass. Carpal tunnel syndrome occurs when one of the tendons becomes inflamed; since the retinaculum is so tough, there’s not enough stretch in it to accommodate the swollen tendon, so the tendon presses against the median nerve, resulting in numbness and/or pain.



2. Nuchal ligament

Nuchal ligament skull vertebrae skeleton bones

You know what’s awesome about the nuchal ligament? If you look at it laterally, it looks like a shark fin (yes, I’m one of those people who watches “Shark Week” religiously).

The nuchal ligament extends from the external occipital protuberance and median nuchal line to the spinous process of the seventh cervical vertebra. It stabilizes the head and neck, as muscles that would otherwise attach to the spinous processes of the vertebrae instead attach to the nuchal ligament.


1. All the ligaments of the skull

Skull jaw temporomandibular joint capsule ligaments

How could I pick just one ligament in the skull when all of them are so important? The ligaments of the skull comprise the ligaments that surround the temporomandibular joint, reinforcing the area where the cranium articulates with the mandible. The temporomandibular joint is a synovial joint and allows not only for flexion and extension, but also small movements of rotation and gliding.

The main “hinge” of the joint is the sphenomandibular ligament—a flat, thin band that connects the spina angularis of the sphenoid to the lingual of the mandibular foramen. The other ligaments connect the mandible to various bones of the skull.



All the images and most of the content were taken from Human Anatomy Atlas, our best-selling human anatomy reference app. Get a free preview eBook now!
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3D Skeletal System: 5 Cool Facts about the Femur

The femur is an awesome bone—and not just because it has a cool name. 


1. The femur is the longest bone in the body.

Femur long bone tibia trochanter condyle



2. The femur is a weight-bearing bone.

Femur weight bearing long bone strongest bone longest bone



3. The greater trochanter provides leverage for gluteal muscles and other muscles that rotate the thigh.

Trochanter femur long bone gluteal muscles



4. The medial femoral condyle bears more weight due to the center of gravity being medial to the knee.

Medial condyle femur bone



5. Being a long bone, the femur contains both red and yellow marrow.

Femur marrow medullary cavity cancellous bone


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3D Skeletal System: 7 Interesting Facts about the Thoracic Cage

We spend a lot of time kicking around the office, discussing anatomy and fighting about which structures are the coolest. We all know how I feel about the pelvic girdle, but the thoracic cage comes in at a close second.

The thoracic cage is an interesting structure designed to protect some of the most important organs in your body. Keep reading to find out just how the thoracic cage is structured to make you keep on keepin' on!


1. The thoracic cage is conical in shapenarrow above and broad below. This is what helps give your upper body shape.

thoracic cage ribs


2. Run your hands down the back of your ribs. Can you feel the shape of the cage? It is relatively flat; the anterior part of the cage is noticeably curved to accommodate organs within it.

Ribs lateral


3. If you look at the cage in the transverse inferior, you will notice it is shaped somewhat like a kidney. 

Thoracic cage rib shape kidney


4. Its kidney-like shape is ideal for protecting the heart and lungs. Look how neatly they fit inside. 

Lungs heart thoracic cage


5. The costal cartilage are fibrous tissues that allow for the expansion of the thoracic cage. When air comes into the lungs, the lungs inflate and the thoracic cage expands to accommodate them. 

Costal cartilage ribs thoracic cage


6. The 12 ribs that form the cage are uniquely shaped. Most have a head (articulates with the bodies of the vertebrae), a neck (flattened section of bone), and a shaft (serves as the attachment site for several muscles).

Ribs thoracic


7. The shape of the head is ideal for articulating with the vertebrae. 

ribs vertebrae spinous process

Explore the thoracic cage.

Watch the video to see the thoracic cage from all angles and explore a rib! 


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3D Skeletal System: The Shoulder Girdle

Think of how many things you do during the day; you stretch first thing in the morning, lift your coffee cup, wave at people, type, write, text, play Angry Birds—no matter what it is you do, chances are you're using your arms to do them. And you know what's incredible about that? You wouldn't be able to use your arms without the shoulder girdle!

I'm fascinated by the shoulder girdle, to be honest. It's such an interesting structure and presents an even more interesting joint. Let's take a quick look, shall we?

As always, if you own Skeleton Premium 2, feel free to follow along!

Shoulder girdle axial skeleton

Bones of the Shoulder Girdle

The shoulder girdle is made up of two posterior scapulae and two anterior clavicles. These bones make an incomplete ring around the upper thoracic cage. The medial end of each clavicle articulates with the manubrium and scapulae; each scapula connects to the thoracic cage by muscle only. How crazy is that? The only thing that connects the upper limbs and the scapula to the axial skeleton are the clavicles. That's a pretty big job, wouldn't you say?

The scapulae are flat, triangular bones that serve as the attachment sites of many muscles, including the deltoids. Muscles of the back and thorax connect the scapulae to the thoracic cage.

Here's a quick run-down of the girdle's articulations:




Humerus, clavicle


Scapula, manubrium, cartilage of the first rib


Manubrium clavicles articulation


Glenoid Cavity and the Shoulder Joint

Hold one of your arms out and rotate it. Your arms have more motility than your legs, despite the fact that the shoulder and hip joints are the same type of joint!

Capsular ligament shoulder

Each scapula has a concave, articular surface called the glenoid cavity (or glenoid fossa), which articulates with the head of the humerus. The cavity is covered with cartilage. The ball-like head of the humerus articulates with the glenoid cavity, creating a ball-and-socket joint that allows the upper limb great motility. A capsular ligament is part of the articular capsule that surrounds a synovial joint (freely moving joint); capsular ligaments reinforce joints and provide stability. The capsular ligament of the shoulder joint provides more motility than the hip joint—in fact, it allows the scapula and the humerus to separate more than 2.5 cm, which means your upper limbs have a fantastic range of motion!

Watch the range of motion of a ball-and-socket joint here:


Landmarks of the Scapula

The scapula is such an interesting bone. I mean, look at it! It's such a cool shape that it's also known as the shoulder blade. You are walking around with two knives permanently strapped to your back. How awesome is that?

Landmarks, or characteristics of the bone, have different functions: muscle attachment sites, bone and ligament articulation sites, passage for nerves and vessels, and more. In the image, all bone landmarks are highlighted in different colors (a nifty tool in Skeleton Premium 2).

scapula bone landmarks acromion scapular spine coracoid

For example, the acromion and the coracoid process (in light pinkish purple) are the two major projections of the scapula. The acromion is an oblong process that hangs over the glenoid cavity, while the coracoid process is a thick, curved process that runs almost parallel to the acromion. They are connected by the coracoacromial ligament. This ligament supports proper movement of the joint by arching over the superior portion of the shoulder, protecting it.

The scapular spine (in pale orange-red) is a prominent plate of bone that serves as the attachment site for portions of the trapezius and the deltoid. At the distal end of the spine is the acromion. 


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3D Skeletal System: Compact Bone, Spongy Bone, and Osteons -- Oh My!

long bones femur compact bones osteon marrow

Your skeleton's awesome—no bones about it!

I couldn't resist.

The point still stands! Your skeleton is an incredible structure. It gives your body shape, it protects vital organs, and it's alive. That's right! When you think of the skeleton, what comes to mind? Hard, dry bones, right? You can thank artistic conditioning in kindergarten for that one. The thing is, bones may be hard on the outside, but on the inside they're a smorgasbord of vessels, nerves, and other things. I may just blow your mind with this post.


osteon compact bone cortical bone cancellous bone spongy bone bone matrix

Compact Bone (cortical bone)

Compact bone is dense bone tissue found on the outside of a bone. Basically, in kindergarten when you drew skeletons, you were drawing compact bone. Compact bone is enclosed, except where it's covered by articular cartilage, and is covered by the periosteum. The periosteum is a thick fibrous membrane covering the entire surface of a bone and serving as an attachment for muscles and tendons. Vessels pass from the periosteum through pores into the compact bone and run through canals found throughout the tissue.


Spongy Bone (cancellous bone)

"Cancellous" makes it sound so negative, doesn't it? Spongy bone is on the interior of a bone and consists of slender fibers and lamellae—layers of bony tissue—that join to form a reticular structure. Spongy bone is supplied by fewer and larger vessels than compact bone. These vessels perforate the outer compact layer and are distributed into the spongy portion of bone, which is filled with marrow. Bone marrow is tissue found in long bones, like the femur, that contains stem cells.


Osteons (Haversian system)

osteon bone structure compact bone

Osteons are interesting little things. Osteons are structural units of compact bone. Each osteon consists of a central canal, which contains nerve filaments  and one or two blood vessels, surrounded by lamellae. Lacunae, small chambers containing osteocytes, are arranged concentrically around the central canal.


Femur Bone

femur bone medullary cavity bone marrow red marrow

Bone marrow fills the cavities of long bones and occupies the spaces of spongy bone. Yellow marrow, consisting mostly of fat, is found in the central cavities of long bones. Red marrow is found in the medullary cavities of flat and short bones, articular ends of long bones, vertebral bodies, spongy bone of the cranium, sternum, ribs, and scapulae.

The femur is famous for being the longest bone in the body, as well as one of the strongest. Your femurs support a great deal of weight—your entire upper body, in fact! The femur is also known for its marrow-filled medullary cavity, which is present in all of the long bones of the limbs. The sternum and hip bone are the sites from which marrow is usually extracted, however the femur is also used.

Bone marrow biopsies are done usually to determine or diagnose certain conditions, such as leukemia, anemia, abnormal number of white blood cells, and whether cancer has spread to the bones. 


Bone Marrow and Stem Cells

You've been bombarded by headlines about stem cells over the past decade, but what do they actually do? Well, stem cells can become red blood cells (which provide oxygen to tissue), white blood cells (which fight infections), or platelets (which help in blood clotting). They have the potential to become specialized cells, which can help in treating certain diseases.



Osteoporotic bone osteoporosis bone damage brittle bones compact bone

I drink seven or eight thousand glasses of milk a day. I love it, and my love for it helps protect me against osteoporosis. I'm sure you've seen dozens of ads telling you to drink milk or take vitamins to supplement bone health (Sally Field does one), but they never quite explain what osteoporosis is.

Osteoporosis is a disorder, more common with increased age, in which bone tissue becomes thinner, resulting in brittle bones that are more susceptible to breaking. Sex hormones are particularly important in stimulating bone cell growth; after middle age, these hormones decrease in production, which means bone cell growth decreases. Bones become noticeably thinner, especially for women after menopause when estrogen levels are very low.

Compact bone becomes more brittle, particularly in the long bones, which is why breaks and fractures in the tibia and femur are things to keep note of when diagnosed with osteoporosis. The femur and tibia already bear the weight of most of the body; small accidents, like simple falls, can result in injury. In spongy bone, trabeculae—small struts that are separated by cavities filled with marrow—become thinner and the spaces between them become wider, causing an overall weakening of bone structure.

Milk is rich in calcium and vitamin D, the two most important nutrients for bone health. About 99% of the calcium in our bodies is in our bones and teeth. Our bodies require vitamin D to absorb calcium. So the next time you're watching TV and Sally Field tells you that you need to eat things rich in calcium and vitamin D, you should listen. You should listen to her anyway, because she's awesome.

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Download our free eBook about the sphenoid! It's just as cool as the pelvic girdle—and it's shaped like the Bat symbol!



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3D Skeletal System: The Pelvic Girdle

I must put my hand on my hip at least a thousand times a day. I don't know what it is—it's a habit that I can't seem to break. Mostly, I think it's because I never quite know what to do with my hands if I'm just standing. But there's something to be said for resting your hand on something sturdy—even when the sturdy thing is you!

The pelvic girdle may look like one giant bone, but it's actually made of three. The pelvic girdle connects the bones of the lower limbs to the axial skeleton by different attachment sites, and supports the vertebral column.

And I'm going to be honest: I find the pelvic girdle to be one of the most beautiful structures in the entire body. I mean, look at it!

Pelvic girdle posterior anterior pelvis femur resized 600

Okay, let's take a look at the pelvic girdle, its relationship with the femur, and its responsibilities in the body.


Bones of the Pelvic Girdle

When we're born, we have over 300 bones. As we grow, some bones fuse together, leaving us with 206 by the time we're adults. The pelvic girdle, as I said above, is made up of three fused bones: the ischium, the ilium, and the pubis.

Pelvic girdle pelvis ischium ilium pubic bones cavity

The pubis forms the anterior part of the pelvic girdle. It is a flattened, irregular-shaped bone that articulates with the pubic symphysis, a cartilaginous joint.

The ilium is the largest and most recognizable part of the pelvis: it looks like the top of a wing. If your hip bones "stick out" (are visible through your skin), it's usually the ilium you're seeing; they protrude outwards. The ilium articulates with the sacrum, forming the posterior wall of the pelvic cavity. The iliac crest, the superior-most area of bone, is the attachment site for several muscles, including the latissimus dorsi and gluteus maximus.

The ischium, located directly below the pubis, forms the bottom of the "wing." Together with the pubis, the ischium forms the obturator canal, the large hole in the bone. The obturator canal is the attachment site for the obturator muscles, as well as nerves and blood vessels.


Acetabulum and the Pelvic Girdle

The femur and the pelvic girdle have a very special relationship—it's special in that it allows you to walk. Take a look at the femur (see image). The helmet-like structure highlighted in gold is the head. Many bones have a head.  What's special about this particular head is its role as the ball in the ball-and-socket hip joint.

Femur head bone acetabulum joint pelvic girdle resized 600

The pubis, ischium, and ilium have a hand in creating the acetabulum, the deep, hemispherical, cup-shaped depression (the socket) with which the head of the femur articulates. The head of the femur fits perfectly in this cavity; its ball shape allows it to roll the femur, rotating it in a way that facilitates walking. Ligaments hold the bones together.

pelvic girdle acetabulum femur ball and socket joint resized 600

For a better idea of how a ball-and-socket joint works, take a look at this quick video.


Male and Female Pelvic Girdle

In addition to supporting the vertebral column and allowing you to walk, the pelvic girdle—along with the sacrum and the coccyx—forms the walls of the pelvic cavity, which contains and protects some of the reproductive, digestive, and urinary organs.

The pelvic girdle is delicate and shallower in the female pelvis than the male—the ilia are less sloped, and the superior aperture is more circular.


Want to learn more?

Download our free eBook about the sphenoid! It's just as cool as the pelvic girdle—and it's shaped like the Bat symbol!

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3D Skeletal System: Five Things to Know about the Ethmoid Bone

In my previous bone post, I talked about how much I loved the sphenoid and its Batman symbol-esque shape. That still holds true. Coming in at a close second is the ethmoid. Once, someone asked me to describe the shape of the ethmoid bone, and the only thing I could come up with was "hot mess." Look at it—how else would you describe it?

Ethmoid bone skull axial skeleton

The ethmoid (sphenoid, ethmoid—brain bone names are so cool) is an interesting little bone with some very important functions.  Let's take a look!

(And, as always, if you have Skeleton Premium 2, feel free to follow along!)


1. The Ethmoid Articulates with 13 Bones

Ethmoid bone articulations axial sphenoid frontal 1

Talk about being the belle of the ball! The ethmoid articulates with more than half of the bones in the skull: the frontal bone, sphenoid bone, nasal bones, maxillae, lacrimal bones, palatine bones, inferior conchae, and the vomer.



2. The Ethmoid Helps Form the Orbits and Nasal Cavities

Ethmoid nasal cavity orbits eyes skull

It's pretty much impossible to feel where the ethmoid is in the skull; it sits at the roof of the nose and helps form the inner portions of the orbits (eye sockets). The middle conchae, located beneath the cribiform plate, form part of the lateral walls of the nasal cavities. The orbital plates help form the interior of the orbits.



3. Cool Structures Attach to the Ethmoid 

Crista galli ethmoid bone skull

At the top of the ethmoid is a long, thin triangular process called the crista galli. This projection of bone serves as the attachment site of the falx cerebri. The falx cerebri is the larger of the two folds of dura mater separating the two hemispheres of the brain that lies between the cerebral hemispheres and contains the sagittal sinuses.

(Check out our sweet coronal cross-section!)



4. The Ethmoid Contributes To Your Sense of Smell

Ethmoid sagittal cross skull axial

The olfactory bulb, which transmits smell information to the brain, rests on top of and is protected by the cribiform plate. As you can see in the picture, the plate is perforated by numerous foramina, which give passage to the branches of the olfactory nerves.

(Look at that awesome sagittal cross-section! We're pulling out all the stops.)



5. The Labyrinths of the Ethmoid (do not contain a minotaur)

Ethmoid labyrinth bone skull axial

The labyrinths of the ethmoid contain several ethmoidal sinuses. These sinuses are composed of air cells that receive sensory information from the ethmoidal nerves. The sinuses drain into the nasal meatuses.



Download our free eBook about the sphenoid! It's just as cool as the ethmoid—and it's shaped like the Bat symbol!




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- Chico State University, CA, Anthropology Dept.

3D Skeletal System: Atlas, Axis, and the Atlanto-Axial Relationship

There are 33 vertebrae in your vertebral column. Or is it 24? Doesn't matter—bothcraniovertebral joint, atlanto-axial relationship, atlanto-axial articulation, atlas vertebrae, axis vertebrae, axis bone, atlas bone numbers are correct. You're born with 33, but the sacrum and coccyx fuse to the rest of the spine, making it 24 by the time you're an adult.

Of those 24 (not counting the sacrum and coccyx), two vertebrae are fortunate enough to have names. The atlas (C01) and axis (C02) are two of the most important vertebrae in the spine. Without them, head and neck movement would be impossible.

The atlas and axis vertebrae are the two most superior bones in the vertebral column. They are part of the seven cervical vertebrae. The atlas is the top-most bone, sitting just below the skull; it is followed by the axis. Together, they support the skull, facilitate neck movement, and protect the spinal cord. (Think of them as BFFs—you won't find one without the other.)

Unlike the other vertebrae, the atlas does not have a spinous process. Instead, it is ringlike and consists of an anterior and posterior arch, as well as two lateral masses. The transverse processes (the protrusions of bone on either side of the ring) serve as the attachment sites of muscles that assist in rotating the head. The foramina (the holes) give passage to the vertebral artery and vertebral vein.

The axis is somewhat analogous to the other cervical vertebrae in shape, but it differsdens, odontoid process, atlas vertebrae, axis vertebrae, axis bone, atlas bone
slightly for two reasons: its spinous process isn't as obviously bifid, and the presence of the dens. The spinous process serves as the attachment site for many muscles of the spine, particularly those close to the skull, as well as the nuchal ligament.

The dens, or odontoid process, is a toothlike projection of bone that rises perpendicularly from the upper surface of the body of the axis. Its purpose is very important, but I'll get to that later.

Let's talk about joints.



There are many types of vertebral joints, but the atlas and axis form the only craniovertebral joints in the body. A craniovertebral joint is exactly what it sounds like: a joint that permits movement between the vertebral column and the skull.

The ligaments in the spine support and reinforce the joints between the vertebrae. The atlas and axis in particular work with the ligaments to move the neck. The atlas and the occipital bone form the atlanto-occipital joint, which allows neck flexion. When you nod your head as if to say "yes," that is neck flexion. The atlas and axis form the atlanto-axial joint, which allows head rotation. If you shake your head as if to say "no," that is head rotation.Atlanto axial pivot joint

The atlanto-axial joint is a compound synovial joint. A synovial joint is a freely moveable joint, differing from other types of joints due to the presence of synovial fluid, which lubricates the joint. Most of the main joints (hands, feet, and other regions in the appendicular skeleton) are synovial joints.  

It is also a pivot joint. A pivot joint is made by the end of one articulating bone rotating in a ring formed by another bone and its ligaments. Think of a metal washer twisting around a bolt. The dens articulates with the facet on the atlas, as well as the transverse ligament, and this articulation provides the head with approximately 50% of its movement.



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