The tunica is smooth muscle tissue, very similar to vein tissue. In fact, an Italian surgeon has perfected an operation where the Saphenous Vein in the leg is removed, and slit so that it is flat. Then the tunica on each corpus cavernosa is slit lengthwise and the vein tissue grafted into the gap. The flaccid penis looks the same, but upon erection, the penis girth expands to 7" or 8" because the vein graft allows significantly greater expansion of the corpus cavernosa. The graft eliminates the built in limitations created by the tunica. Ones tunica can be 1, 2, or 3 layers, depending on genetics. The thicker the tunica, the more difficult it is to get girth gains. I don't believe the above surgery is yet available in the U.S. The success rate in Italy (I believe Milan) has been very good. Google Saphenous Vein Graft.