Practice 5 Elements of Shading

How many drawing books start out with the Sphere?  Have you ever really studied the sphere?  Why do they start with the Sphere?  Is it because there are certain principles that should be followed when making the illusion of art?  What are the principles of shading? There are five of them.
Last night I practiced them on the sphere first and then with an eye.  I also used a Q-Tip for blending.  I've never blended that way before.  The eye is a 3" x 2" and the sphere is a little one 1.5" x 1.5" (including the cast shadow)
.  Five Elements of Shading:
1. Cast Shadow
2. Shadow Edge
3. Half Tone
4. Reflected Light
5. Highlight

Do you see them all in the Eye?
This was drawn on simple printer paper using a 2B, H and very little 4B (for the pupil) graphite pencils along with a Q-Tip cotton swab.
Hope you like my study!
Thanks for stopping by :)