First post on this topic here. Second post on this topic here.
We're getting into principles that are useful not only in drawing the contour of the form, but also in rendering interior forms.
FORMS MERGE INTO ADJACENT FORMS. Forms of the body are not discrete. Think about how the inside of the upper arm merges into the lower arm. The curve of the biceps anticipates the inside of the elbow as it approaches it. If you just draw the upper arm, then the lower arm, without observing how they merge together, the form will not look organic; it will look like a marionette. The way that form merges together must be carefully observed; it varies depending on muscle flexion, angle, and so on.
SMALLER FORMS ARE PACKED WITHIN LARGER FORMS. Think of a large form, such as the thigh. Although you can approximate it with a sort of smooth tube shape, when you look closely you see that there are smaller forms along the whole length of the larger form. Nestled within those sub-forms are even smaller forms. In order to render the thigh in such a way that it looks organic, we need to carefully observe how those sub forms interact with the main form. This becomes especially important as we try to represent how light washes across a complex array of packed forms.
FORMS KNIT TOGETHER. Think of the center of the breastbone. In that spot you can easily see where the left and right sides of the body were joined together as the body formed in the womb. The line of that joining isn’t straight; it meanders back and forth, suturing together. While the center line of the body is the easiest place to see this effect, it is also true across the whole body. The form of the wrist knits into the form of the hand. The form of the thigh is woven into the form of the hip. If we just render even gradations of light over a form, we’ll miss this interweaving and the form won’t look organic.
ALL FORMS ARE CURVED. The body is composed of rounded forms. This principle is relatively easy to understand and make use of when you are drawing the outline of the body: all of the lines you draw are curves. That's the case even though bones are relatively straight, because the sinews and muscles around the bones wrap around them and create roundness. But it also has a less obvious implication when you are using gradations of value to depict three dimensional organic form: every point on a curve is receiving a different amount of light than any point next to it. That is to say, every part of the body is part of a value gradient. If you draw or paint an area of flat color that does not change in value, it will look flat, not round, and therefore not organic. All forms on the body, large or small, must be rendered as a gradient (however subtle) from one value to another in order to look organically rounded.