one cast from real bones, so that you can compare this to your actual body. But the

problem is that these thing are riveted so that it doesn't move like a human skeleton

moves. There is a spool in here that allows a foot to go into dorsiflexion and plantar

flexion that we can't do on this because it's stuck together. But we don't have that limitation

when we draw and when we have the 3D model.

This is an extended foot. I like to begin with a rough something to contain the proportions.

It's okay if you take some time with that. But to make it look three dimensional, even

when I do that rough something, I'm thinking of height, width, and depth. And we letter

those systems as X, Y, and Z. And with just those coordinates, we can build a Lego foot

standing non-extended. Which, even if it doesn't look like a foot, it looks 3D.

Then when we put this cylinder in there, it runs along X which helps us draw the ellipse,

and we know that that is the fulcrum of the foot. That means it's the center axle of the

foot's rotation. If I draw it from the side, you'll see that we have the depth. That's

the Z line. The height, that's the Y line. And then we're looking straight down on X

where that ankle joint is.

And when that foot moves on that hinge joint and makes an arc, goes into extension, we

can see that Z will not line up on Z. It will aim up. So when we do this refined foot, I'm

going to chisel some of the plane so it looks a little bit more like a foot. It's not all

right angles. And the big foot, we'll put into an extended position rather than a straight

up and down position. So let's get the ankle bones. That would be the tibia and the fibula.

And they come down kind of like a wrench to grip that spool bone of the talus. I'll refine

this a bit, badly. Sorry about that, I'm a bad influence with my hunt and scratch lines.

I'll offer my excuse for it later. Right now, I just apologize for it.

Let's put that spool in there. Since it's extended, the Y lines won't point straight

up and down. They'll aim off in that direction. And cross contours make us sure of the form,

and we don't have all right angles there. We've got some beveled, slanted lines. Before

I turn this into an abomination, it is so embarrassing to be seen in the privacy of

one's hesitations. I wish we could just speed it up. Segue into the ... There, that's better.

There we are, all dressed up for the debut and no embarrassing personal scratching.

Thank you very much, Marshall! That was a great little demo. Marshall's other demos

for the Joint assignment are in the Premium section of the Anatomy Course. If you'd like

to see Marshall construct these joints while explaining some perspective principles plus

all the other Anatomy premium content go to proko.com/anatomy.