This post is going to be a bit longer than usual, but hopefully, it'll go quickly. So. There are five types of bone: long, short, flat, irregular, and sesamoid.
Long bones are, unsurprisingly, long. As in, they’re longer than they are wide. These are the bones that bear weight and they’re typically pretty mobile - your arms, your legs, your collar bones.
Their structure is tubular, and therefor end to end, they’re incredibly strong, but karate chop them in the middle and they’re prone to breaking. See this for yourself by taking a handful of paper straw and taping them together. If you clench your fist around them, they’ll crumple under your (surely formidable) strength, but if you have enough of them so they balance upright on their end, you can balance a book on them.
Long bones have three parts (that are relevant to us) to them, the epiphysis, the metaphysis, and the diaphysis. The epiphysis of a bone is the head - distal or proximal - that connects with another bone to form a joint. The diaphysis is the majority of the bone, the length of the shaft. The metaphysis is a teeny tiny section that joins the epiphysis and the diaphysis together
Short bones, generally speaking, are cuboid-ish - roughly equal in length, width, and height. The bones in your wrist and some of the bones in your feet are examples of short bones. They don’t do much - they allow for a little limited motion - but they’re super strong and durable, and they’re pretty tough to break.
Flat bones are, well, flat. Seems obvious, but it’s not necessarily as straight forward as you might think, as they’re not all flat, in fact, they’re quite curved, so imagine them like a sheet of bone that is moulded to fit around your important soft organs - your skull around your brain, your ribs around you lungs and heart, your pelvis around your reproductive system.
As well as providing a relatively sturdy barrier between your insides and the outside world, they also provide some lovely large surfaces for large groups of muscles to attach to - your shoulder blades and pelvis are particularly densely populated with muscle origins for a whole range of motions, both affecting the torso and the arms and legs.
Irregular bones don’t have a common shape, they’re just completely random.
Your facial bones and your vertebra are examples of irregular bones, and they’re completely fascinating. It’s a topic for another post (or two or three), but the bones of the face are particularly fantastic to study because you’ll never meet an ugly person again once you start looking at faces as well dressed skulls. They’re also very useful if you’re drawing from your imagination without reference, as you’ll be able to analyse your own drawings and see if you’re falling into the trap of ‘same face’ characters!
Lastly, we come to sesamoid bones. They’re almost not worth mentioning, really. There’s only one sesamoid bone that’s found common to every person, and they’re the patellae - the kneecap. Sesamoid bones are small little bones, almost like shards, that are embedded in tendons and help to protect them against compression. Typically in found in the hands and feet, there are a standard few that are included in anatomical diagrams that very few people don't have, and there're loads that some people may or may not have that aren't included because, textbooks, you know?
That’s it. Simple, no? Well now that we know what the bones are, let’s think about how they move, and in order to do that, we need to think about how they connect. Joints start off with three broad categories. Fixed (also known as fibrous), semi movable (AKA cartilaginous), and movable (AKA synovial). These categories have subcategories. Of course they do. Why would it be simple?
Fixed or fibrous joints don’t move. At least, they’re not supposed to. The bones of your skull which start off as very separate, very soft things (which is how babies heads are able to be literally squeezed through the cervix) over time harden and fuse together. They’re held together by a series of so-called sutures, like tiny little bone stitches that keep your brain from falling out. These function because the plates of the skull are already very close together and so there’s not a huge distance for them to reach. Drawn in red in the image is the coronal suture.
Where there is a greater distance, like between the two bones in your lower leg, a different type of fibrous joint is required - a syndesmosis. A syndesmosis can be formed of ligaments or a thick membrane that stops the bones from drifting apart from each other, which would be, frankly, the opposite of what you’d want going on inside your body.
The last kind of fibrous joint is one that I only learnt today while double checking my English terminology, and it’s a gomphosis, which is the joint between your teeth and your jawbones, and I'm not even including a picture, because I have LITERALLY NEVER needed to know this information. It is cool though. Google it, you'll see.
In terms of anatomy for drawing, the only one of these that you’ll probably need to know of are the sutures on the skull, and even then, you won’t really be drawing them, just, vaguely aware of their existence.
Semi movable, cartilaginous, or amphiarthrosis joints are pretty cool. There are two types - symphysis, and synchondrosis - but the difference between them is the specific type of cartilage, and that starts to get into a realm of detail that we really don’t need, so let’s think of them in an overview kind of way, and if you’re interested I might make a very short separate post to talk about them in more depth later.
Think about the cartilage in your ear - it’s pretty flexible, it can be squashed and folded, and generally pushed around a bit, but not too much, or it starts to hurt, right? Well essentially, a semi movable joint is two bones that are held together by a thick sheet of flexible - but not too flexible - cartilage.
Between each vertebra in your spine is a disk of cartilage and, disk to disk, the movement is relatively limited - think two dinner plates with a dish sponge between them - but when you have thirty-three of them stacked up on top of each other, each one with this spongy little cushion between them, the combined range of motion is pretty huge.
We can touch our toes, we can bend over backwards to accommodate annoying relatives, we can curve sideways to allow the cat the biggest possible patch of bed so she gets a good night’s sleep. If the joints were more mobile, we’d have to have much stronger musculature to keep ourselves upright, and we’d lose a lot of the protection of our insides that our skeletons provide. It’s not just the spine that has these joint, the ribs as they connect to the sternum have a type of semi movable joint, and the bones of the pelvis too.
Freely Movable, or Synovial joints, are the real moneymakers in the body. These are your elbows, your knees, your shoulders, and pretty my anything else you can lift, turn, stretch, bend, or twist. They’re called synovial joints because of the synovial fluid, a thick, lubricating ‘joint juice’, that stops your bones rubbing on each other when they move.*
There are six of these types of joints, and they’re quite easy to get your head around, so we’ll blast through them, and we’ll talk next time about terms that describe movement in the body.
Your hinge joint is the basic joint. Your elbow, your knee, your second and third knuckles in your fingers and toes. They go up, they go down, but they don’t go side to side. Give your knee a wiggle, if you think it goes side to side, what you’re witnessing is multiple joints acting on the same bit of the body. And if it really does go side to side, definitely go see a doctor. (b) on the diagram.
The second type of joint is a ball and socket joint - your arm to shoulder, and your leg to hip joints. If you’re wiggling your lower leg side to side right now, you’re actually rotating the upper leg in the hip socket. It’s important to have a strong musculature around these joints, as these bad boys dislocate pretty easily - it’s the payoff to having such a huge range of motion within them - and over time with usage they can wear down and become fragile, which is why people often need to have hips replaced. (f) on the diagram.
Thirdly, we have pivot joints, where one bone rotates or pivots around another. The bones in your forearm and your skull sitting atop your spine are examples, and they’re a bit more difficult to get your head (or forearm haha) around when drawing, as you really have to think about them in 3D space, but once you’ve got it you’ll find your ability to draw hands just skyrocket overnight. (a) on the diagram.
The fourth kind is a condyloid joint. They move in two planes - up and down like a hinge, but with a generous amount of rotation too. The first knuckle of your index finger, for example, can hinge up and down to mouse click but also reach side to side to hit the ‘t’ and ‘y’ keys on the keyboard. (e) on the diagram.
The penultimate joint is the saddle joint, which is one of the most mobile joints. Two saddle-shaped joint bones sit upon each other criss-cross, and they can rock back and forth and side to side, and all of the angles in between. Give your thumb a wiggle, rotate it around a bit. Do you know how it sort of reaches a point and then ‘clicks’ backwards? That’s the saddles. (c) on the diagram.
The final joints are gliding or plane Joints ((d) on the diagram). Your wrist is made of a bunch of little bones (carpals) that glide over each other (like dragon scales) and allow the wrist to move in multiple directions with varying levels of flexibility If you bend your wrist backwards or forwards, your muscles will only move it so far. But if you apply extra pressure, by doing a handstand, or leaning your hand on your hand, the wrist will bend further. The limit to how far a gliding joint can bend isn’t in the joint itself but in the other material around the joint. Eventually, the bone, muscle, skin, fat, ligaments, and tendons get in the way and stop you from being able to bend any further (unless you want to break something). That’s why with training, ballerinas are able to get their feet to have huge bends in them when they go on point.
That's all for today, art nerds, I'll see you next time.
*Fun fact, when I was a teenager I had a condition called Osgood-Schlatter disease, where all of the synovial fluid in my knees drained away, and so I was in fact bone on bone. I now have these big lumps on the head of both tibias that stick out beyond the depth of my kneecaps (which makes it quite painful to kneel down) where the bone grew extra bone in bone scar tissue from the constant trauma. How cool is the human body?!