The thumb has one fewer bone than the other fingers, but it makes up for this with a far greater freedom of movement. We’ll look at that, but we need to get a handle on the shapes of things around the thumb first. There are two main muscle masses associated with the thumb. If we look at the back of the hand, when the thumb is held against the body of the hand, we see a muscle mound between thumb and hand:

Dorsal interosseus

This is the muscle we’re using if we hold the thumb close to the hand, and lifted up. The mound of muscle can be seen if we look at the hand from the thumb side:

Dorsal interosseus, lateral view

This muscle goes between the base bone of the thumb and the base bone (metacarpal) of the index finger.

Dorsal interosseus III

We also see, looking at the back of the hand, that the second joint of the thumb lies in an arc with the main knuckles of the hand.

Knuckle Arc

Looking at the hand from the palm side, with the thumb held close again, we see the main thumb mound — known, for reasons I haven’t looked into, as the Mound of Venus.

Mound of Venus

There’s a lot going on, inside that mound, that I’m not going to get into here. It’s not just one muscle. It’s several muscles, connecting the base bone of the thumb to different bone and ligament places at the heel of the hand. For now, we just need to know the general outlines.

This is also a good place to look again at the important relationship between the form of the palm and thumb, and the bones of the hand. First, see again how the second joint of the thumb lies in an arc with the main knuckles:

Knuckle Arc II

Putting some flesh on the hand, look at that knuckle arc (blue) in relation to the arc of the end of the palm (green), and the arc of the first joints of the fingers (orange). The edge of the palm (green) is halfway between the main knuckles of the hand and the first joints of the fingers.

Form of Palm

It’s a little easier to see this relationship, and how it affects the shape of the palm, especially when the palm is cupped, if we look at the hand in a slightly unnatural pose — fingers straight and held in a flat group, and bent at close to a right angle from the palm.

In this position, at least on my hand, the palm gets a crease all across it (drawn dark here).

Form of palm II

If you do this with your hand, and then turn it sideways, you can see how that crease lines up with the knuckles on the back:

Form of Palm III

Turning the hand back a bit, and looking at the bones, this is what they’re doing — the main knuckles are lined up behind that big crease in the palm:

Form of Palm IV

Here’s the big muscle mound (purple), and the three arcs, again: the knuckle arc (blue), the arc of the end of the palm (green), and the arc of the first joints of the fingers (orange):

Form of Palm V

Next time I talk about drawing the hand, I’ll say a bit more about the basic masses, and a bit about The Wrist. Then I want to get to the real meat of all this — drawing the hand gesturing.