Another device that Ingres and other line draftsmen use is overlapping lines. Above is the "Study for the golden age", a very beutiful and important Ingres drawing. Below I have cropped out a detail and drawn some cryptic lines and numbers on it.
If you look at the bounding line of the figure, what Ives Gammell sometimes called the arabesque you will notice it is not entirely uninterrupted. Ingres has cut some of his lines diving into the interior to express the overlapping forms of the musculature. I don't have my anatomy books with me tonight so I will spare you most of the nomenclature. I am going by memory so you artistic anatomists are welcome to correct me in the comments.
1) Here is a real good example of what I am describing, Ingres has shown the biceps diving down under the pectoralis major, and on the viewers left the triceps diving in under the teres major and minor. He has clearly expressed the way these forms meet. His overlapping forms are installed from his knowledge of anatomy as much as they are observed.
At 3) the pectoral shows itself in front of the neck, this overlap establishes dimension. That neck is obviously behind the top line of the pectoral
At 4) the underside of the chin cuts in front of the sterno-clito mastoid, which goes behind it on its way to the base of the unseen ear.
At 5) the line is darkened, I believe that is the Latisimus dorsi there, and when the line begins to represent the ribcage it goes soft. By varying his line weight here he is able to separate out the different masses.
At 6) The external oblique is depicted by a line that cuts inside the outline of the figure. Whew, I hope I got those right!
So here is the point. Rather than a dumb outline like a coathanger or a traced silhouette, Ingres uses lines that overlap and dive inwards across another line to show the overlapping of forms.
Like the other nights post showing the varying of line weight to express form without leaving the discipline of pure line, Ingres uses overlapping lines to express the form without having to resort to much shading which would clutter up the open lights of his spare drawing.