I think I know what you are referring to. Can you feel the cord more from the underside, or from the top?
The septum is the portion of the tunica that is enclosing both of the spongy bodies, however it has both deep and superficial structure. Wikipedia describes the superficial structure as a single tube of fibers (imagine a sheath and oriented longitudinally) enclosing both spongy bodies. The deeper structure is fibers that lay at right angles, if you will, to that sheath, like little ringlets within the sheath. Where they meet, one would assume there is a natural ridge or prominence.
Wiki says this : “the deep fibers are arranged circularly around each corpus, and form by their junction in the median plane the septum of the penis.”
Which makes sense since “septum” is literally a divider , a “hedge” in latin. Just like the septum of your nose can be described as the longitudinally oriented structure dividing the sinuses, the penis septum forms that division down the middle that is so common in mammalian structures due to the bilateral symmetry.
I have an extender, and like you I have noticed the sensation of a rigid internal structure, it is certainly part of the tunica , it might in fact be due to the septum structure itself. But the answer I would give is that technically, when using an extender you are pulling on the very shaft itself, (the tunica), so some portion of the pulling force must be applied to it. This is the only real theory behind hanging as well. Growth or stretching through time and weight.
As for targeting sub structures of the tunica, I have to confess I don’t know how that internal anatomy responds to this kind of force. I know that in general these fibrous sheaths and that type of tissue don’t stretch much, by design. Enough to accommodate erection, obviously. What the capability beyond that is, I wouldn’t venture to guess. The success stories of hangers are testimonials to the fact that stretches can produce gains, so there must be *some* additional room for stretching or growth.
I think you should investigate the role of heat too, many who hang and stretch swear this is the secret to gains.
Thanks for your detailed reply, Fantom.
I felt again, and it does seem to be located more towards the top. Is that what you have as well?