The “Ribs” are the veins that return the blood to the body. During erection, most of the return flow is stopped to stay hard. You should really see the ribs when you block off all the return blood to the body, ie. Clamping.
The “Ribs” are enlarged arteries that bring blood into the penis, the blood goes out through the veins you see in the skin.
Actually, the ribs are not arteries nor veins. i have always had them to some extent but i have been jelqing off and on for years mostly to keep the plumbing in shape (i am mid 50’s) and to increase head size and as a result they are very pronounced when i am very hard. It is either the cavernosa that have expanded in this fashion and now are pushing against the tunica or it is weaker areas of the tunica that allow the cavernosa to expand into them forming ribs. an analogy is that for anyone who works out an starts to notice more striations in the muscle fibers that give the muscle that pumped, somewhat lumpy look due to the enlargement of the tissue that is not uniform like blowing up a balloon (check out any bodybuilder mag). I also have raised places as well as depressions that form irregular patterns. i have had comments that my dick looks “muscular” as a result.
I agree. Veins would burst if they were ever pressurized like mine can be. My impression is they are either ribs of collagen like bigjim says, or compartments like bubble wrap. You would think any doctor or urologist at least would know what’s in there that produces such texture from increased pressure. My guess is “smooth” guys are just not under as much pressure or haven’t achieved enough expansion to see a difference between the hardened areas and elastic areas.