Well Freedo0 I'm not taking that course but I can tell you're drawing the rib cage. That in itself is a good sign you're on the right track. Try making the lines in front thicker and as the line sweeps to the back it gets thinner. This is a Ink illustration technique. It has nothing to do with your anatomy course. I only bring it up because by doing this I hope it will help you see a 3d object on the page. Right now Its hard to tell what part of that rib cage is in front and what is in back. By making the front lines thicker you can distinguish the object better and the "3dness" of it should automatically become more visible. Making a 2d drawing look 3d is a trick of the brain, by learning to draw you're learning that visual language. When you start to see that rib cage go from lines on paper to an object taking shape you can make better use of your contour lines. Better use of contour lines gives more realism to a line effect called overlap which also gives a drawing the feel of an object in space. practice practice practice but there is no rush.